Merge branch 'read-only-local-store' into overlayfs-store

.github/ISSUE_TEMPLATE/missing_documentation.md

@@ -11,6 +11,10 @@ assignees: ''
 
 <!-- describe your problem -->
 
+## Proposal
+
+<!-- propose a solution -->
+
 ## Checklist
 
 <!-- make sure this issue is not redundant or obsolete -->
@@ -22,10 +26,6 @@ assignees: ''
 [source]: https://github.com/NixOS/nix/tree/master/doc/manual/src
 [open documentation issues and pull requests]: https://github.com/NixOS/nix/labels/documentation
 
-## Proposal
-
-<!-- propose a solution -->
-
 ## Priorities
 
 Add :+1: to [issues you find important](https://github.com/NixOS/nix/issues?q=is%3Aissue+is%3Aopen+sort%3Areactions-%2B1-desc).

.github/workflows/backport.yml

@@ -21,7 +21,7 @@ jobs:
           fetch-depth: 0
       - name: Create backport PRs
         # should be kept in sync with `version`
-        uses: zeebe-io/backport-action@v1.3.0
+        uses: zeebe-io/backport-action@v1.3.1
         with:
           # Config README: https://github.com/zeebe-io/backport-action#backport-action
           github_token: ${{ secrets.GITHUB_TOKEN }}

.github/workflows/ci.yml

@@ -11,6 +11,7 @@ jobs:
   tests:
     needs: [check_secrets]
     strategy:
+      fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest]
     runs-on: ${{ matrix.os }}
@@ -19,7 +20,7 @@ jobs:
     - uses: actions/checkout@v3
       with:
         fetch-depth: 0
-    - uses: cachix/install-nix-action@v21
+    - uses: cachix/install-nix-action@v22
       with:
         # The sandbox would otherwise be disabled by default on Darwin
         extra_nix_config: "sandbox = true"
@@ -61,7 +62,7 @@ jobs:
       with:
         fetch-depth: 0
     - run: echo CACHIX_NAME="$(echo $GITHUB_REPOSITORY-install-tests | tr "[A-Z]/" "[a-z]-")" >> $GITHUB_ENV
-    - uses: cachix/install-nix-action@v21
+    - uses: cachix/install-nix-action@v22
       with:
         install_url: https://releases.nixos.org/nix/nix-2.13.3/install
     - uses: cachix/cachix-action@v12
@@ -76,13 +77,14 @@ jobs:
     needs: [installer, check_secrets]
     if: github.event_name == 'push' && needs.check_secrets.outputs.cachix == 'true'
     strategy:
+      fail-fast: false
       matrix:
         os: [ubuntu-latest, macos-latest]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v3
     - run: echo CACHIX_NAME="$(echo $GITHUB_REPOSITORY-install-tests | tr "[A-Z]/" "[a-z]-")" >> $GITHUB_ENV
-    - uses: cachix/install-nix-action@v21
+    - uses: cachix/install-nix-action@v22
       with:
         install_url: '${{needs.installer.outputs.installerURL}}'
         install_options: "--tarball-url-prefix https://${{ env.CACHIX_NAME }}.cachix.org/serve"
@@ -109,7 +111,7 @@ jobs:
     - uses: actions/checkout@v3
       with:
         fetch-depth: 0
-    - uses: cachix/install-nix-action@v21
+    - uses: cachix/install-nix-action@v22
       with:
         install_url: https://releases.nixos.org/nix/nix-2.13.3/install
     - run: echo CACHIX_NAME="$(echo $GITHUB_REPOSITORY-install-tests | tr "[A-Z]/" "[a-z]-")" >> $GITHUB_ENV

.gitignore

@@ -89,6 +89,7 @@ perl/Makefile.config
 /tests/ca/config.nix
 /tests/dyn-drv/config.nix
 /tests/repl-result-out
+/tests/test-libstoreconsumer/test-libstoreconsumer
 
 # /tests/lang/
 /tests/lang/*.out

CONTRIBUTING.md

@@ -5,7 +5,6 @@ We appreciate your support.
 
 Reading and following these guidelines will help us make the contribution process easy and effective for everyone involved.
 
-
 ## Report a bug
 
 1. Check on the [GitHub issue tracker](https://github.com/NixOS/nix/issues) if your bug was already reported.
@@ -30,6 +29,8 @@ Check out the [security policy](https://github.com/NixOS/nix/security/policy).
    You can use [labels](https://github.com/NixOS/nix/labels) to filter for relevant topics.
 
 2. Search for related issues that cover what you're going to work on. It could help to mention there that you will work on the issue.
+
+   Issues labeled ["good first issue"](https://github.com/NixOS/nix/labels/good-first-issue) should be relatively easy to fix and are likely to get merged quickly.
    Pull requests addressing issues labeled ["idea approved"](https://github.com/NixOS/nix/labels/idea%20approved) are especially welcomed by maintainers and will receive prioritised review.
 
 3. Check the [Nix reference manual](https://nixos.org/manual/nix/unstable/contributing/hacking.html) for information on building Nix and running its tests.

Makefile

@@ -28,6 +28,7 @@ makefiles += \
   src/libexpr/tests/local.mk \
   tests/local.mk \
   tests/overlay-local-store/local.mk \
+  tests/test-libstoreconsumer/local.mk \
   tests/plugins/local.mk
 else
 makefiles += \

doc/manual/src/SUMMARY.md.in

@@ -97,7 +97,10 @@
       - [manifest.json](command-ref/files/manifest.json.md)
     - [Channels](command-ref/files/channels.md)
     - [Default Nix expression](command-ref/files/default-nix-expression.md)
-- [Architecture](architecture/architecture.md)
+- [Architecture and Design](architecture/architecture.md)
+  - [File System Object](architecture/file-system-object.md)
+- [Protocols](protocols/protocols.md)
+  - [Serving Tarball Flakes](protocols/tarball-fetcher.md)
 - [Glossary](glossary.md)
 - [Contributing](contributing/contributing.md)
   - [Hacking](contributing/hacking.md)

doc/manual/src/architecture/architecture.md

@@ -7,11 +7,11 @@ It should help users understand why Nix behaves as it does, and it should help d
 
 Nix consists of [hierarchical layers].
 
-[hierarchical layers]: https://en.m.wikipedia.org/wiki/Multitier_architecture#Layers
+[hierarchical layers]: https://en.wikipedia.org/wiki/Multitier_architecture#Layers
 
 The following [concept map] shows its main components (rectangles), the objects they operate on (rounded rectangles), and their interactions (connecting phrases):
 
-[concept map]: https://en.m.wikipedia.org/wiki/Concept_map
+[concept map]: https://en.wikipedia.org/wiki/Concept_map
 
 ```
 
@@ -76,7 +76,7 @@ The result of a build task can be input to another build task.
 The following [data flow diagram] shows a build plan for illustration.
 Build inputs used as instructions to a build task are marked accordingly:
 
-[data flow diagram]: https://en.m.wikipedia.org/wiki/Data-flow_diagram
+[data flow diagram]: https://en.wikipedia.org/wiki/Data-flow_diagram
 
 ```
 +--------------------------------------------------------------------+

doc/manual/src/architecture/file-system-object.md

@@ -0,0 +1,64 @@
+# File System Object
+
+Nix uses a simplified model of the file system, which consists of file system objects.
+Every file system object is one of the following:
+
+ - File
+
+   - A possibly empty sequence of bytes for contents
+   - A single boolean representing the [executable](https://en.m.wikipedia.org/wiki/File-system_permissions#Permissions) permission
+
+ - Directory
+
+   Mapping of names to child file system objects
+
+ - [Symbolic link](https://en.m.wikipedia.org/wiki/Symbolic_link)
+
+   An arbitrary string.
+   Nix does not assign any semantics to symbolic links.
+
+File system objects and their children form a tree.
+A bare file or symlink can be a root file system object.
+
+Nix does not encode any other file system notions such as [hard links](https://en.m.wikipedia.org/wiki/Hard_link), [permissions](https://en.m.wikipedia.org/wiki/File-system_permissions), timestamps, or other metadata.
+
+## Examples of file system objects
+
+A plain file:
+
+```
+50 B, executable: false
+```
+
+An executable file:
+
+```
+122 KB, executable: true
+```
+
+A symlink:
+
+```
+-> /usr/bin/sh
+```
+
+A directory with contents:
+
+```
+├── bin
+│   └── hello: 35 KB, executable: true
+└── share
+    ├── info
+    │   └── hello.info: 36 KB, executable: false
+    └── man
+        └── man1
+            └── hello.1.gz: 790 B, executable: false
+```
+
+A directory that contains a symlink and other directories:
+
+```
+├── bin -> share/go/bin
+├── nix-support/
+└── share/
+```

doc/manual/src/command-ref/nix-channel.md

@@ -4,7 +4,7 @@
 
 # Synopsis
 
-`nix-channel` {`--add` url [*name*] | `--remove` *name* | `--list` | `--update` [*names…*] | `--rollback` [*generation*] }
+`nix-channel` {`--add` url [*name*] | `--remove` *name* | `--list` | `--update` [*names…*] | `--list-generations` | `--rollback` [*generation*] }
 
 # Description
 
@@ -39,6 +39,15 @@ This command has the following operations:
     for `nix-env` operations (by symlinking them from the directory
     `~/.nix-defexpr`).
 
+  - `--list-generations`\
+    Prints a list of all the current existing generations for the
+    channel profile.
+
+    Works the same way as
+    ```
+    nix-env --profile /nix/var/nix/profiles/per-user/$USER/channels --list-generations
+    ```
+
   - `--rollback` \[*generation*\]\
     Reverts the previous call to `nix-channel
                     --update`. Optionally, you can specify a specific channel generation

doc/manual/src/command-ref/nix-collect-garbage.md

@@ -1,6 +1,6 @@
 # Name
 
-`nix-collect-garbage` - delete unreachable store paths
+`nix-collect-garbage` - delete unreachable [store objects]
 
 # Synopsis
 
@@ -8,17 +8,57 @@
 
 # Description
 
-The command `nix-collect-garbage` is mostly an alias of [`nix-store
+The command `nix-collect-garbage` is mostly an alias of [`nix-store --gc`](@docroot@/command-ref/nix-store/gc.md).
---gc`](@docroot@/command-ref/nix-store/gc.md), that is, it deletes all
+That is, it deletes all unreachable [store objects] in the Nix store to clean up your system.
-unreachable paths in the Nix store to clean up your system. However,
+
-it provides two additional options: `-d` (`--delete-old`), which
+However, it provides two additional options,
-deletes all old generations of all profiles in `/nix/var/nix/profiles`
+[`--delete-old`](#opt-delete-old) and [`--delete-older-than`](#opt-delete-older-than),
-by invoking `nix-env --delete-generations old` on all profiles (of
+which also delete old [profiles], allowing potentially more [store objects] to be deleted because profiles are also garbage collection roots.
-course, this makes rollbacks to previous configurations impossible);
+These options are the equivalent of running
-and `--delete-older-than` *period*, where period is a value such as
+[`nix-env --delete-generations`](@docroot@/command-ref/nix-env/delete-generations.md)
-`30d`, which deletes all generations older than the specified number
+with various augments on multiple profiles,
-of days in all profiles in `/nix/var/nix/profiles` (except for the
+prior to running `nix-collect-garbage` (or just `nix-store --gc`) without any flags.
-generations that were active at that point in time).
+
+> **Note**
+>
+> Deleting previous configurations makes rollbacks to them impossible.
+
+These flags should be used with care, because they potentially delete generations of profiles used by other users on the system.
+
+## Locations searched for profiles
+
+`nix-collect-garbage` cannot know about all profiles; that information doesn't exist.
+Instead, it looks in a few locations, and acts on all profiles it finds there:
+
+1. The default profile locations as specified in the [profiles] section of the manual.
+
+2. > **NOTE**
+   >
+   > Not stable; subject to change
+   >
+   > Do not rely on this functionality; it just exists for migration purposes and is may change in the future.
+   > These deprecated paths remain a private implementation detail of Nix.
+
+   `$NIX_STATE_DIR/profiles` and `$NIX_STATE_DIR/profiles/per-user`.
+
+   With the exception of `$NIX_STATE_DIR/profiles/per-user/root` and `$NIX_STATE_DIR/profiles/default`, these directories are no longer used by other commands.
+   `nix-collect-garbage` looks there anyways in order to clean up profiles from older versions of Nix.
+
+# Options
+
+These options are for deleting old [profiles] prior to deleting unreachable [store objects].
+
+- <span id="opt-delete-old">[`--delete-old`](#opt-delete-old)</span> / `-d`\
+  Delete all old generations of profiles.
+
+  This is the equivalent of invoking `nix-env --delete-generations old` on each found profile.
+
+- <span id="opt-delete-older-than">[`--delete-older-than`](#opt-delete-older-than)</span> *period*\
+  Delete all generations of profiles older than the specified amount (except for the generations that were active at that point in time).
+  *period* is a value such as `30d`, which would mean 30 days.
+
+  This is the equivalent of invoking [`nix-env --delete-generations <period>`](@docroot@/command-ref/nix-env/delete-generations.md#generations-days) on each found profile.
+  See the documentation of that command for additional information about the *period* argument.
 
 {{#include ./opt-common.md}}
 
@@ -32,3 +72,6 @@ generations of each profile, do
 ```console
 $ nix-collect-garbage -d
 ```
+
+[profiles]: @docroot@/command-ref/files/profiles.md
+[store objects]: @docroot@/glossary.md#gloss-store-object

doc/manual/src/command-ref/nix-env/delete-generations.md

@@ -9,14 +9,39 @@
 # Description
 
 This operation deletes the specified generations of the current profile.
-The generations can be a list of generation numbers, the special value
+
-`old` to delete all non-current generations, a value such as `30d` to
+*generations* can be a one of the following:
-delete all generations older than the specified number of days (except
+
-for the generation that was active at that point in time), or a value
+- <span id="generations-list">`<number>...`</span>:\
-such as `+5` to keep the last `5` generations ignoring any newer than
+  A list of generation numbers, each one a separate command-line argument.
-current, e.g., if `30` is the current generation `+5` will delete
+
-generation `25` and all older generations. Periodically deleting old
+  Delete exactly the profile generations given by their generation number.
-generations is important to make garbage collection effective.
+  Deleting the current generation is not allowed.
+
+- The special value <span id="generations-old">`old`</span>
+
+  Delete all generations older than the current one.
+
+- <span id="generations-days">`<days>d`</span>:\
+  The last *days* days
+
+  *Example*: `30d`
+
+  Delete all generations older than *days* days.
+  The generation that was active at that point in time is excluded, and will not be deleted.
+
+- <span id="generations-count">`+<count>`</span>:\
+  The last *count* generations up to the present
+
+  *Example*: `+5`
+
+  Keep the last *count* generations, along with any newer than current.
+
+Periodically deleting old generations is important to make garbage collection
+effective.
+The is because profiles are also garbage collection roots — any [store object] reachable from a profile is "alive" and ineligible for deletion.
+
+[store object]: @docroot@/glossary.md#gloss-store-object
 
 {{#include ./opt-common.md}}
 
@@ -28,19 +53,35 @@ generations is important to make garbage collection effective.
 
 # Examples
 
+## Delete explicit generation numbers
+
 ```console
 $ nix-env --delete-generations 3 4 8
 ```
 
+Delete the generations numbered 3, 4, and 8, so long as the current active generation is not any of those.
+
+## Keep most-recent by count count
+
 ```console
 $ nix-env --delete-generations +5
 ```
 
+Suppose `30` is the current generation, and we currently have generations numbered `20` through `32`.
+
+Then this command will delete generations `20` through `25` (`<= 30 - 5`),
+and keep generations `26` through `31` (`> 30 - 5`).
+
+## Keep most-recent in days
+
 ```console
 $ nix-env --delete-generations 30d
 ```
 
+This command will delete all generations older than 30 days, except for the generation that was active 30 days ago (if it currently exists).
+
+## Delete all older
+
 ```console
 $ nix-env --profile other_profile --delete-generations old
 ```
-

doc/manual/src/contributing/hacking.md

@@ -378,7 +378,7 @@ rm $(git ls-files doc/manual/ -o | grep -F '.md') && rmdir doc/manual/src/comman
 [`mdbook-linkcheck`] does not implement checking [URI fragments] yet.
 
 [`mdbook-linkcheck`]: https://github.com/Michael-F-Bryan/mdbook-linkcheck
-[URI fragments]: https://en.m.wikipedia.org/wiki/URI_fragment
+[URI fragments]: https://en.wikipedia.org/wiki/URI_fragment
 
 #### `@docroot@` variable
 

doc/manual/src/glossary.md

@@ -85,12 +85,17 @@
 
     [store path]: #gloss-store-path
 
+  - [file system object]{#gloss-store-object}\
+    The Nix data model for representing simplified file system data.
+
+    See [File System Object](@docroot@/architecture/file-system-object.md) for details.
+
+    [file system object]: #gloss-file-system-object
+
   - [store object]{#gloss-store-object}\
-    A file that is an immediate child of the Nix store directory. These
+
-    can be regular files, but also entire directory trees. Store objects
+    A store object consists of a [file system object], [reference]s to other store objects, and other metadata.
-    can be sources (objects copied from outside of the store),
+    It can be referred to by a [store path].
-    derivation outputs (objects produced by running a build task), or
-    derivations (files describing a build task).
 
     [store object]: #gloss-store-object
 
@@ -112,9 +117,10 @@
     from some server.
 
   - [substituter]{#gloss-substituter}\
-    A *substituter* is an additional store from which Nix will
+    An additional [store]{#gloss-store} from which Nix can obtain store objects instead of building them.
-    copy store objects it doesn't have.  For details, see the
+    Often the substituter is a [binary cache](#gloss-binary-cache), but any store can serve as substituter.
-    [`substituters` option](./command-ref/conf-file.md#conf-substituters).
+
+    See the [`substituters` configuration option](./command-ref/conf-file.md#conf-substituters) for details.
 
     [substituter]: #gloss-substituter
 

doc/manual/src/installation/prerequisites-source.md

@@ -10,7 +10,7 @@
   - Bash Shell. The `./configure` script relies on bashisms, so Bash is
     required.
 
-  - A version of GCC or Clang that supports C++17.
+  - A version of GCC or Clang that supports C++20.
 
   - `pkg-config` to locate dependencies. If your distribution does not
     provide it, you can get it from

doc/manual/src/language/index.md

@@ -1,12 +1,11 @@
 # Nix Language
 
-The Nix language is
+The Nix language is designed for conveniently creating and composing *derivations* – precise descriptions of how contents of existing files are used to derive new files.
+It is:
 
 - *domain-specific*
 
-  It only exists for the Nix package manager:
+  It comes with [built-in functions](@docroot@/language/builtins.md) to integrate with the Nix store, which manages files and performs the derivations declared in the Nix language.
-  to describe packages and configurations as well as their variants and compositions.
-  It is not intended for general purpose use.
 
 - *declarative*
 
@@ -25,7 +24,7 @@ The Nix language is
 
 - *lazy*
 
-  Expressions are only evaluated when their value is needed.
+  Values are only computed when they are needed.
 
 - *dynamically typed*
 

doc/manual/src/protocols/protocols.md

@@ -0,0 +1,4 @@
+# Protocols
+
+This chapter documents various developer-facing interfaces provided by
+Nix.

doc/manual/src/protocols/tarball-fetcher.md

@@ -0,0 +1,42 @@
+# Lockable HTTP Tarball Protocol
+
+Tarball flakes can be served as regular tarballs via HTTP or the file
+system (for `file://` URLs). Unless the server implements the Lockable
+HTTP Tarball protocol, it is the responsibility of the user to make sure that
+the URL always produces the same tarball contents.
+
+An HTTP server can return an "immutable" HTTP URL appropriate for lock
+files. This allows users to specify a tarball flake input in
+`flake.nix` that requests the latest version of a flake
+(e.g. `https://example.org/hello/latest.tar.gz`), while `flake.lock`
+will record a URL whose contents will not change
+(e.g. `https://example.org/hello/<revision>.tar.gz`). To do so, the
+server must return an [HTTP `Link` header](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Link) with the `rel` attribute set to
+`immutable`, as follows:
+
+```
+Link: <flakeref>; rel="immutable"
+```
+
+(Note the required `<` and `>` characters around *flakeref*.)
+
+*flakeref* must be a tarball flakeref. It can contain flake attributes
+such as `narHash`, `rev` and `revCount`. If `narHash` is included, its
+value must be the NAR hash of the unpacked tarball (as computed via
+`nix hash path`). Nix checks the contents of the returned tarball
+against the `narHash` attribute. The `rev` and `revCount` attributes
+are useful when the tarball flake is a mirror of a fetcher type that
+has those attributes, such as Git or GitHub. They are not checked by
+Nix.
+
+```
+Link: <https://example.org/hello/442793d9ec0584f6a6e82fa253850c8085bb150a.tar.gz
+  ?rev=442793d9ec0584f6a6e82fa253850c8085bb150a
+  &revCount=835
+  &narHash=sha256-GUm8Uh/U74zFCwkvt9Mri4DSM%2BmHj3tYhXUkYpiv31M%3D>; rel="immutable"
+```
+
+(The linebreaks in this example are for clarity and must not be included in the actual response.)
+
+For tarball flakes, the value of the `lastModified` flake attribute is
+defined as the timestamp of the newest file inside the tarball.

doc/manual/src/release-notes/rl-next.md

@@ -1,2 +1,3 @@
 # Release X.Y (202?-??-??)
 
+- [`nix-channel`](../command-ref/nix-channel.md) now supports a `--list-generations` subcommand

flake.nix

@@ -590,6 +590,8 @@
 
         tests.sourcehutFlakes = runNixOSTestFor "x86_64-linux" ./tests/nixos/sourcehut-flakes.nix;
 
+        tests.tarballFlakes = runNixOSTestFor "x86_64-linux" ./tests/nixos/tarball-flakes.nix;
+
         tests.containers = runNixOSTestFor "x86_64-linux" ./tests/nixos/containers/containers.nix;
 
         tests.setuid = lib.genAttrs

maintainers/README.md

@@ -117,6 +117,7 @@ Pull requests in this column are reviewed together during work meetings.
 This is both for spreading implementation knowledge and for establishing common values in code reviews.
 
 When the overall direction is agreed upon, even when further changes are required, the pull request is assigned to one team member.
+If significant changes are requested or reviewers cannot come to a conclusion in reasonable time, the pull request is [marked as draft](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/changing-the-stage-of-a-pull-request#converting-a-pull-request-to-a-draft).
 
 ### Assigned
 

scripts/install-darwin-multi-user.sh

@@ -100,7 +100,7 @@ poly_extra_try_me_commands() {
 poly_configure_nix_daemon_service() {
     task "Setting up the nix-daemon LaunchDaemon"
     _sudo "to set up the nix-daemon as a LaunchDaemon" \
-          /bin/cp -f "/nix/var/nix/profiles/default$NIX_DAEMON_DEST" "$NIX_DAEMON_DEST"
+          /usr/bin/install -m -rw-r--r-- "/nix/var/nix/profiles/default$NIX_DAEMON_DEST" "$NIX_DAEMON_DEST"
 
     _sudo "to load the LaunchDaemon plist for nix-daemon" \
           launchctl load /Library/LaunchDaemons/org.nixos.nix-daemon.plist

scripts/install-multi-user.sh

@@ -700,6 +700,10 @@ EOF
 }
 
 welcome_to_nix() {
+    local -r NIX_UID_RANGES="${NIX_FIRST_BUILD_UID}..$((NIX_FIRST_BUILD_UID + NIX_USER_COUNT - 1))"
+    local -r RANGE_TEXT=$(echo -ne "${BLUE}(uids [${NIX_UID_RANGES}])${ESC}")
+    local -r GROUP_TEXT=$(echo -ne "${BLUE}(gid ${NIX_BUILD_GROUP_ID})${ESC}")
+
     ok "Welcome to the Multi-User Nix Installation"
 
     cat <<EOF
@@ -713,8 +717,8 @@ manager. This will happen in a few stages:
 2. Show you what I am going to install and where. Then I will ask
    if you are ready to continue.
 
-3. Create the system users and groups that the Nix daemon uses to run
+3. Create the system users ${RANGE_TEXT} and groups ${GROUP_TEXT}
-   builds.
+   that the Nix daemon uses to run builds.
 
 4. Perform the basic installation of the Nix files daemon.
 

src/libcmd/command.cc

@@ -239,9 +239,7 @@ void MixProfile::updateProfile(const StorePath & storePath)
     if (!store) throw Error("'--profile' is not supported for this Nix store");
     auto profile2 = absPath(*profile);
     switchLink(profile2,
-        createGeneration(
+        createGeneration(*store, profile2, storePath));
-            ref<LocalFSStore>(store),
-            profile2, storePath));
 }
 
 void MixProfile::updateProfile(const BuiltPaths & buildables)

src/libcmd/common-eval-args.cc

@@ -165,7 +165,7 @@ SourcePath lookupFileArg(EvalState & state, std::string_view s)
 {
     if (EvalSettings::isPseudoUrl(s)) {
         auto storePath = fetchers::downloadTarball(
-            state.store, EvalSettings::resolvePseudoUrl(s), "source", false).first.storePath;
+            state.store, EvalSettings::resolvePseudoUrl(s), "source", false).tree.storePath;
         return state.rootPath(CanonPath(state.store->toRealPath(storePath)));
     }
 

src/libcmd/installables.cc

@@ -701,7 +701,7 @@ RawInstallablesCommand::RawInstallablesCommand()
 {
     addFlag({
         .longName = "stdin",
-        .description = "Read installables from the standard input.",
+        .description = "Read installables from the standard input. No default installable applied.",
         .handler = {&readFromStdIn, true}
     });
 
@@ -730,9 +730,9 @@ void RawInstallablesCommand::run(ref<Store> store)
         while (std::cin >> word) {
             rawInstallables.emplace_back(std::move(word));
         }
+    } else {
+        applyDefaultInstallables(rawInstallables);
     }
-
-    applyDefaultInstallables(rawInstallables);
     run(store, std::move(rawInstallables));
 }
 

src/libexpr/eval.hh

@@ -741,7 +741,8 @@ struct EvalSettings : Config
           If set to `true`, the Nix evaluator will not allow access to any
           files outside of the Nix search path (as set via the `NIX_PATH`
           environment variable or the `-I` option), or to URIs outside of
-          `allowed-uri`. The default is `false`.
+          [`allowed-uris`](../command-ref/conf-file.md#conf-allowed-uris).
+          The default is `false`.
         )"};
 
     Setting<bool> pureEval{this, false, "pure-eval",

src/libexpr/parser.y

@@ -793,7 +793,7 @@ std::pair<bool, std::string> EvalState::resolveSearchPathElem(const SearchPathEl
     if (EvalSettings::isPseudoUrl(elem.second)) {
         try {
             auto storePath = fetchers::downloadTarball(
-                store, EvalSettings::resolvePseudoUrl(elem.second), "source", false).first.storePath;
+                store, EvalSettings::resolvePseudoUrl(elem.second), "source", false).tree.storePath;
             res = { true, store->toRealPath(storePath) };
         } catch (FileTransferError & e) {
             logWarning({

src/libexpr/primops.cc

@@ -6,7 +6,7 @@
 #include "globals.hh"
 #include "json-to-value.hh"
 #include "names.hh"
-#include "references.hh"
+#include "path-references.hh"
 #include "store-api.hh"
 #include "util.hh"
 #include "value-to-json.hh"
@@ -4058,18 +4058,6 @@ static RegisterPrimOp primop_splitVersion({
 RegisterPrimOp::PrimOps * RegisterPrimOp::primOps;
 
 
-RegisterPrimOp::RegisterPrimOp(std::string name, size_t arity, PrimOpFun fun)
-{
-    if (!primOps) primOps = new PrimOps;
-    primOps->push_back({
-        .name = name,
-        .args = {},
-        .arity = arity,
-        .fun = fun,
-    });
-}
-
-
 RegisterPrimOp::RegisterPrimOp(Info && info)
 {
     if (!primOps) primOps = new PrimOps;

src/libexpr/primops.hh

@@ -28,11 +28,6 @@ struct RegisterPrimOp
      * will get called during EvalState initialization, so there
      * may be primops not yet added and builtins is not yet sorted.
      */
-    RegisterPrimOp(
-        std::string name,
-        size_t arity,
-        PrimOpFun fun);
-
     RegisterPrimOp(Info && info);
 };
 

src/libexpr/primops/context.cc

@@ -12,7 +12,11 @@ static void prim_unsafeDiscardStringContext(EvalState & state, const PosIdx pos,
     v.mkString(*s);
 }
 
-static RegisterPrimOp primop_unsafeDiscardStringContext("__unsafeDiscardStringContext", 1, prim_unsafeDiscardStringContext);
+static RegisterPrimOp primop_unsafeDiscardStringContext({
+    .name = "__unsafeDiscardStringContext",
+    .arity = 1,
+    .fun = prim_unsafeDiscardStringContext
+});
 
 
 static void prim_hasContext(EvalState & state, const PosIdx pos, Value * * args, Value & v)
@@ -22,7 +26,16 @@ static void prim_hasContext(EvalState & state, const PosIdx pos, Value * * args,
     v.mkBool(!context.empty());
 }
 
-static RegisterPrimOp primop_hasContext("__hasContext", 1, prim_hasContext);
+static RegisterPrimOp primop_hasContext({
+    .name = "__hasContext",
+    .args = {"s"},
+    .doc = R"(
+      Return `true` if string *s* has a non-empty context. The
+      context can be obtained with
+      [`getContext`](#builtins-getContext).
+    )",
+    .fun = prim_hasContext
+});
 
 
 /* Sometimes we want to pass a derivation path (i.e. pkg.drvPath) to a
@@ -51,7 +64,11 @@ static void prim_unsafeDiscardOutputDependency(EvalState & state, const PosIdx p
     v.mkString(*s, context2);
 }
 
-static RegisterPrimOp primop_unsafeDiscardOutputDependency("__unsafeDiscardOutputDependency", 1, prim_unsafeDiscardOutputDependency);
+static RegisterPrimOp primop_unsafeDiscardOutputDependency({
+    .name = "__unsafeDiscardOutputDependency",
+    .arity = 1,
+    .fun = prim_unsafeDiscardOutputDependency
+});
 
 
 /* Extract the context of a string as a structured Nix value.
@@ -119,7 +136,30 @@ static void prim_getContext(EvalState & state, const PosIdx pos, Value * * args,
     v.mkAttrs(attrs);
 }
 
-static RegisterPrimOp primop_getContext("__getContext", 1, prim_getContext);
+static RegisterPrimOp primop_getContext({
+    .name = "__getContext",
+    .args = {"s"},
+    .doc = R"(
+      Return the string context of *s*.
+
+      The string context tracks references to derivations within a string.
+      It is represented as an attribute set of [store derivation](@docroot@/glossary.md#gloss-store-derivation) paths mapping to output names.
+
+      Using [string interpolation](@docroot@/language/string-interpolation.md) on a derivation will add that derivation to the string context.
+      For example,
+
+      ```nix
+      builtins.getContext "${derivation { name = "a"; builder = "b"; system = "c"; }}"
+      ```
+
+      evaluates to
+
+      ```
+      { "/nix/store/arhvjaf6zmlyn8vh8fgn55rpwnxq0n7l-a.drv" = { outputs = [ "out" ]; }; }
+      ```
+    )",
+    .fun = prim_getContext
+});
 
 
 /* Append the given context to a given string.
@@ -192,6 +232,10 @@ static void prim_appendContext(EvalState & state, const PosIdx pos, Value * * ar
     v.mkString(orig, context);
 }
 
-static RegisterPrimOp primop_appendContext("__appendContext", 2, prim_appendContext);
+static RegisterPrimOp primop_appendContext({
+    .name = "__appendContext",
+    .arity = 2,
+    .fun = prim_appendContext
+});
 
 }

src/libexpr/primops/fetchMercurial.cc

@@ -88,6 +88,10 @@ static void prim_fetchMercurial(EvalState & state, const PosIdx pos, Value * * a
     state.allowPath(tree.storePath);
 }
 
-static RegisterPrimOp r_fetchMercurial("fetchMercurial", 1, prim_fetchMercurial);
+static RegisterPrimOp r_fetchMercurial({
+    .name = "fetchMercurial",
+    .arity = 1,
+    .fun = prim_fetchMercurial
+});
 
 }

src/libexpr/primops/fetchTree.cc

@@ -194,7 +194,11 @@ static void prim_fetchTree(EvalState & state, const PosIdx pos, Value * * args,
 }
 
 // FIXME: document
-static RegisterPrimOp primop_fetchTree("fetchTree", 1, prim_fetchTree);
+static RegisterPrimOp primop_fetchTree({
+    .name = "fetchTree",
+    .arity = 1,
+    .fun = prim_fetchTree
+});
 
 static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v,
     const std::string & who, bool unpack, std::string name)
@@ -262,7 +266,7 @@ static void fetch(EvalState & state, const PosIdx pos, Value * * args, Value & v
     //       https://github.com/NixOS/nix/issues/4313
     auto storePath =
         unpack
-        ? fetchers::downloadTarball(state.store, *url, name, (bool) expectedHash).first.storePath
+        ? fetchers::downloadTarball(state.store, *url, name, (bool) expectedHash).tree.storePath
         : fetchers::downloadFile(state.store, *url, name, (bool) expectedHash).storePath;
 
     if (expectedHash) {

src/libexpr/primops/fromTOML.cc

@@ -90,6 +90,24 @@ static void prim_fromTOML(EvalState & state, const PosIdx pos, Value * * args, V
     }
 }
 
-static RegisterPrimOp primop_fromTOML("fromTOML", 1, prim_fromTOML);
+static RegisterPrimOp primop_fromTOML({
+    .name = "fromTOML",
+    .args = {"e"},
+    .doc = R"(
+      Convert a TOML string to a Nix value. For example,
+
+      ```nix
+      builtins.fromTOML ''
+        x=1
+        s="a"
+        [table]
+        y=2
+      ''
+      ```
+
+      returns the value `{ s = "a"; table = { y = 2; }; x = 1; }`.
+    )",
+    .fun = prim_fromTOML
+});
 
 }

src/libfetchers/attrs.hh

@@ -2,6 +2,7 @@
 ///@file
 
 #include "types.hh"
+#include "hash.hh"
 
 #include <variant>
 

src/libfetchers/fetchers.cc

@@ -159,6 +159,12 @@ std::pair<Tree, Input> Input::fetch(ref<Store> store) const
                 input.to_string(), *prevLastModified);
     }
 
+    if (auto prevRev = getRev()) {
+        if (input.getRev() != prevRev)
+            throw Error("'rev' attribute mismatch in input '%s', expected %s",
+                input.to_string(), prevRev->gitRev());
+    }
+
     if (auto prevRevCount = getRevCount()) {
         if (input.getRevCount() != prevRevCount)
             throw Error("'revCount' attribute mismatch in input '%s', expected %d",

src/libfetchers/fetchers.hh

@@ -158,6 +158,7 @@ struct DownloadFileResult
     StorePath storePath;
     std::string etag;
     std::string effectiveUrl;
+    std::optional<std::string> immutableUrl;
 };
 
 DownloadFileResult downloadFile(
@@ -167,7 +168,14 @@ DownloadFileResult downloadFile(
     bool locked,
     const Headers & headers = {});
 
-std::pair<Tree, time_t> downloadTarball(
+struct DownloadTarballResult
+{
+    Tree tree;
+    time_t lastModified;
+    std::optional<std::string> immutableUrl;
+};
+
+DownloadTarballResult downloadTarball(
     ref<Store> store,
     const std::string & url,
     const std::string & name,

src/libfetchers/github.cc

@@ -207,21 +207,21 @@ struct GitArchiveInputScheme : InputScheme
 
         auto url = getDownloadUrl(input);
 
-        auto [tree, lastModified] = downloadTarball(store, url.url, input.getName(), true, url.headers);
+        auto result = downloadTarball(store, url.url, input.getName(), true, url.headers);
 
-        input.attrs.insert_or_assign("lastModified", uint64_t(lastModified));
+        input.attrs.insert_or_assign("lastModified", uint64_t(result.lastModified));
 
         getCache()->add(
             store,
             lockedAttrs,
             {
                 {"rev", rev->gitRev()},
-                {"lastModified", uint64_t(lastModified)}
+                {"lastModified", uint64_t(result.lastModified)}
             },
-            tree.storePath,
+            result.tree.storePath,
             true);
 
-        return {std::move(tree.storePath), input};
+        return {result.tree.storePath, input};
     }
 };
 

src/libfetchers/tarball.cc

@@ -32,7 +32,8 @@ DownloadFileResult downloadFile(
         return {
             .storePath = std::move(cached->storePath),
             .etag = getStrAttr(cached->infoAttrs, "etag"),
-            .effectiveUrl = getStrAttr(cached->infoAttrs, "url")
+            .effectiveUrl = getStrAttr(cached->infoAttrs, "url"),
+            .immutableUrl = maybeGetStrAttr(cached->infoAttrs, "immutableUrl"),
         };
     };
 
@@ -55,12 +56,14 @@ DownloadFileResult downloadFile(
     }
 
     // FIXME: write to temporary file.
-
     Attrs infoAttrs({
         {"etag", res.etag},
         {"url", res.effectiveUri},
     });
 
+    if (res.immutableUrl)
+        infoAttrs.emplace("immutableUrl", *res.immutableUrl);
+
     std::optional<StorePath> storePath;
 
     if (res.cached) {
@@ -111,10 +114,11 @@ DownloadFileResult downloadFile(
         .storePath = std::move(*storePath),
         .etag = res.etag,
         .effectiveUrl = res.effectiveUri,
+        .immutableUrl = res.immutableUrl,
     };
 }
 
-std::pair<Tree, time_t> downloadTarball(
+DownloadTarballResult downloadTarball(
     ref<Store> store,
     const std::string & url,
     const std::string & name,
@@ -131,8 +135,9 @@ std::pair<Tree, time_t> downloadTarball(
 
     if (cached && !cached->expired)
         return {
-            Tree { .actualPath = store->toRealPath(cached->storePath), .storePath = std::move(cached->storePath) },
+            .tree = Tree { .actualPath = store->toRealPath(cached->storePath), .storePath = std::move(cached->storePath) },
-            getIntAttr(cached->infoAttrs, "lastModified")
+            .lastModified = (time_t) getIntAttr(cached->infoAttrs, "lastModified"),
+            .immutableUrl = maybeGetStrAttr(cached->infoAttrs, "immutableUrl"),
         };
 
     auto res = downloadFile(store, url, name, locked, headers);
@@ -160,6 +165,9 @@ std::pair<Tree, time_t> downloadTarball(
         {"etag", res.etag},
     });
 
+    if (res.immutableUrl)
+        infoAttrs.emplace("immutableUrl", *res.immutableUrl);
+
     getCache()->add(
         store,
         inAttrs,
@@ -168,8 +176,9 @@ std::pair<Tree, time_t> downloadTarball(
         locked);
 
     return {
-        Tree { .actualPath = store->toRealPath(*unpackedStorePath), .storePath = std::move(*unpackedStorePath) },
+        .tree = Tree { .actualPath = store->toRealPath(*unpackedStorePath), .storePath = std::move(*unpackedStorePath) },
-        lastModified,
+        .lastModified = lastModified,
+        .immutableUrl = res.immutableUrl,
     };
 }
 
@@ -189,21 +198,33 @@ struct CurlInputScheme : InputScheme
 
     virtual bool isValidURL(const ParsedURL & url) const = 0;
 
-    std::optional<Input> inputFromURL(const ParsedURL & url) const override
+    std::optional<Input> inputFromURL(const ParsedURL & _url) const override
     {
-        if (!isValidURL(url))
+        if (!isValidURL(_url))
             return std::nullopt;
 
         Input input;
 
-        auto urlWithoutApplicationScheme = url;
+        auto url = _url;
-        urlWithoutApplicationScheme.scheme = parseUrlScheme(url.scheme).transport;
+
+        url.scheme = parseUrlScheme(url.scheme).transport;
 
-        input.attrs.insert_or_assign("type", inputType());
-        input.attrs.insert_or_assign("url", urlWithoutApplicationScheme.to_string());
         auto narHash = url.query.find("narHash");
         if (narHash != url.query.end())
             input.attrs.insert_or_assign("narHash", narHash->second);
+
+        if (auto i = get(url.query, "rev"))
+            input.attrs.insert_or_assign("rev", *i);
+
+        if (auto i = get(url.query, "revCount"))
+            if (auto n = string2Int<uint64_t>(*i))
+                input.attrs.insert_or_assign("revCount", *n);
+
+        url.query.erase("rev");
+        url.query.erase("revCount");
+
+        input.attrs.insert_or_assign("type", inputType());
+        input.attrs.insert_or_assign("url", url.to_string());
         return input;
     }
 
@@ -212,7 +233,8 @@ struct CurlInputScheme : InputScheme
         auto type = maybeGetStrAttr(attrs, "type");
         if (type != inputType()) return {};
 
-        std::set<std::string> allowedNames = {"type", "url", "narHash", "name", "unpack"};
+        // FIXME: some of these only apply to TarballInputScheme.
+        std::set<std::string> allowedNames = {"type", "url", "narHash", "name", "unpack", "rev", "revCount"};
         for (auto & [name, value] : attrs)
             if (!allowedNames.count(name))
                 throw Error("unsupported %s input attribute '%s'", *type, name);
@@ -275,10 +297,22 @@ struct TarballInputScheme : CurlInputScheme
                     : hasTarballExtension(url.path));
     }
 
-    std::pair<StorePath, Input> fetch(ref<Store> store, const Input & input) override
+    std::pair<StorePath, Input> fetch(ref<Store> store, const Input & _input) override
     {
-        auto tree = downloadTarball(store, getStrAttr(input.attrs, "url"), input.getName(), false).first;
+        Input input(_input);
-        return {std::move(tree.storePath), input};
+        auto url = getStrAttr(input.attrs, "url");
+        auto result = downloadTarball(store, url, input.getName(), false);
+
+        if (result.immutableUrl) {
+            auto immutableInput = Input::fromURL(*result.immutableUrl);
+            // FIXME: would be nice to support arbitrary flakerefs
+            // here, e.g. git flakes.
+            if (immutableInput.getType() != "tarball")
+                throw Error("tarball 'Link' headers that redirect to non-tarball URLs are not supported");
+            input = immutableInput;
+        }
+
+        return {result.tree.storePath, std::move(input)};
     }
 };
 

src/libstore/build/local-derivation-goal.cc

@@ -4,7 +4,7 @@
 #include "worker.hh"
 #include "builtins.hh"
 #include "builtins/buildenv.hh"
-#include "references.hh"
+#include "path-references.hh"
 #include "finally.hh"
 #include "util.hh"
 #include "archive.hh"
@@ -2389,18 +2389,21 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
             continue;
         auto references = *referencesOpt;
 
-        auto rewriteOutput = [&]() {
+        auto rewriteOutput = [&](const StringMap & rewrites) {
             /* Apply hash rewriting if necessary. */
-            if (!outputRewrites.empty()) {
+            if (!rewrites.empty()) {
                 debug("rewriting hashes in '%1%'; cross fingers", actualPath);
 
-                /* FIXME: this is in-memory. */
+                /* FIXME: Is this actually streaming? */
-                StringSink sink;
+                auto source = sinkToSource([&](Sink & nextSink) {
-                dumpPath(actualPath, sink);
+                    RewritingSink rsink(rewrites, nextSink);
+                    dumpPath(actualPath, rsink);
+                    rsink.flush();
+                });
+                Path tmpPath = actualPath + ".tmp";
+                restorePath(tmpPath, *source);
                 deletePath(actualPath);
-                sink.s = rewriteStrings(sink.s, outputRewrites);
+                movePath(tmpPath, actualPath);
-                StringSource source(sink.s);
-                restorePath(actualPath, source);
 
                 /* FIXME: set proper permissions in restorePath() so
                    we don't have to do another traversal. */
@@ -2449,7 +2452,7 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
                         "since recursive hashing is not enabled (one of outputHashMode={flat,text} is true)",
                         actualPath);
             }
-            rewriteOutput();
+            rewriteOutput(outputRewrites);
             /* FIXME optimize and deduplicate with addToStore */
             std::string oldHashPart { scratchPath->hashPart() };
             HashModuloSink caSink { outputHash.hashType, oldHashPart };
@@ -2487,16 +2490,14 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
                 Hash::dummy,
             };
             if (*scratchPath != newInfo0.path) {
-                // Also rewrite the output path
+                // If the path has some self-references, we need to rewrite
-                auto source = sinkToSource([&](Sink & nextSink) {
+                // them.
-                    RewritingSink rsink2(oldHashPart, std::string(newInfo0.path.hashPart()), nextSink);
+                // (note that this doesn't invalidate the ca hash we calculated
-                    dumpPath(actualPath, rsink2);
+                // above because it's computed *modulo the self-references*, so
-                    rsink2.flush();
+                // it already takes this rewrite into account).
-                });
+                rewriteOutput(
-                Path tmpPath = actualPath + ".tmp";
+                    StringMap{{oldHashPart,
-                restorePath(tmpPath, *source);
+                               std::string(newInfo0.path.hashPart())}});
-                deletePath(actualPath);
-                movePath(tmpPath, actualPath);
             }
 
             HashResult narHashAndSize = hashPath(htSHA256, actualPath);
@@ -2518,7 +2519,7 @@ SingleDrvOutputs LocalDerivationGoal::registerOutputs()
                     outputRewrites.insert_or_assign(
                         std::string { scratchPath->hashPart() },
                         std::string { requiredFinalPath.hashPart() });
-                rewriteOutput();
+                rewriteOutput(outputRewrites);
                 auto narHashAndSize = hashPath(htSHA256, actualPath);
                 ValidPathInfo newInfo0 { requiredFinalPath, narHashAndSize.first };
                 newInfo0.narSize = narHashAndSize.second;

src/libstore/filetransfer.cc

@@ -186,9 +186,9 @@ struct curlFileTransfer : public FileTransfer
             size_t realSize = size * nmemb;
             std::string line((char *) contents, realSize);
             printMsg(lvlVomit, "got header for '%s': %s", request.uri, trim(line));
+
             static std::regex statusLine("HTTP/[^ ]+ +[0-9]+(.*)", std::regex::extended | std::regex::icase);
-            std::smatch match;
+            if (std::smatch match; std::regex_match(line, match, statusLine)) {
-            if (std::regex_match(line, match, statusLine)) {
                 result.etag = "";
                 result.data.clear();
                 result.bodySize = 0;
@@ -196,9 +196,11 @@ struct curlFileTransfer : public FileTransfer
                 acceptRanges = false;
                 encoding = "";
             } else {
+
                 auto i = line.find(':');
                 if (i != std::string::npos) {
                     std::string name = toLower(trim(line.substr(0, i)));
+
                     if (name == "etag") {
                         result.etag = trim(line.substr(i + 1));
                         /* Hack to work around a GitHub bug: it sends
@@ -212,10 +214,22 @@ struct curlFileTransfer : public FileTransfer
                             debug("shutting down on 200 HTTP response with expected ETag");
                             return 0;
                         }
-                    } else if (name == "content-encoding")
+                    }
+
+                    else if (name == "content-encoding")
                         encoding = trim(line.substr(i + 1));
+
                     else if (name == "accept-ranges" && toLower(trim(line.substr(i + 1))) == "bytes")
                         acceptRanges = true;
+
+                    else if (name == "link" || name == "x-amz-meta-link") {
+                        auto value = trim(line.substr(i + 1));
+                        static std::regex linkRegex("<([^>]*)>; rel=\"immutable\"", std::regex::extended | std::regex::icase);
+                        if (std::smatch match; std::regex_match(value, match, linkRegex))
+                            result.immutableUrl = match.str(1);
+                        else
+                            debug("got invalid link header '%s'", value);
+                    }
                 }
             }
             return realSize;
@@ -345,7 +359,7 @@ struct curlFileTransfer : public FileTransfer
         {
             auto httpStatus = getHTTPStatus();
 
-            char * effectiveUriCStr;
+            char * effectiveUriCStr = nullptr;
             curl_easy_getinfo(req, CURLINFO_EFFECTIVE_URL, &effectiveUriCStr);
             if (effectiveUriCStr)
                 result.effectiveUri = effectiveUriCStr;

src/libstore/filetransfer.hh

@@ -80,6 +80,10 @@ struct FileTransferResult
     std::string effectiveUri;
     std::string data;
     uint64_t bodySize = 0;
+    /* An "immutable" URL for this resource (i.e. one whose contents
+       will never change), as returned by the `Link: <url>;
+       rel="immutable"` header. */
+    std::optional<std::string> immutableUrl;
 };
 
 class Store;

src/libstore/globals.cc

@@ -77,7 +77,30 @@ Settings::Settings()
     allowedImpureHostPrefixes = tokenizeString<StringSet>("/System/Library /usr/lib /dev /bin/sh");
 #endif
 
-    buildHook = getSelfExe().value_or("nix") + " __build-remote";
+    /* Set the build hook location
+
+       For builds we perform a self-invocation, so Nix has to be self-aware.
+       That is, it has to know where it is installed. We don't think it's sentient.
+
+       Normally, nix is installed according to `nixBinDir`, which is set at compile time,
+       but can be overridden. This makes for a great default that works even if this
+       code is linked as a library into some other program whose main is not aware
+       that it might need to be a build remote hook.
+
+       However, it may not have been installed at all. For example, if it's a static build,
+       there's a good chance that it has been moved out of its installation directory.
+       That makes `nixBinDir` useless. Instead, we'll query the OS for the path to the
+       current executable, using `getSelfExe()`.
+
+       As a last resort, we resort to `PATH`. Hopefully we find a `nix` there that's compatible.
+       If you're porting Nix to a new platform, that might be good enough for a while, but
+       you'll want to improve `getSelfExe()` to work on your platform.
+     */
+    std::string nixExePath = nixBinDir + "/nix";
+    if (!pathExists(nixExePath)) {
+        nixExePath = getSelfExe().value_or("nix");
+    }
+    buildHook = nixExePath + " __build-remote";
 }
 
 void loadConfFile()

src/libstore/globals.hh

@@ -710,20 +710,19 @@ public:
         Strings{"https://cache.nixos.org/"},
         "substituters",
         R"(
-          A list of [URLs of Nix stores](@docroot@/command-ref/new-cli/nix3-help-stores.md#store-url-format)
+          A list of [URLs of Nix stores](@docroot@/command-ref/new-cli/nix3-help-stores.md#store-url-format) to be used as substituters, separated by whitespace.
-          to be used as substituters, separated by whitespace.
+          A substituter is an additional [store]{@docroot@/glossary.md##gloss-store} from which Nix can obtain [store objects](@docroot@/glossary.md#gloss-store-object) instead of building them.
-          Substituters are tried based on their Priority value, which each substituter can set
-          independently. Lower value means higher priority.
-          The default is `https://cache.nixos.org`, with a Priority of 40.
 
-          At least one of the following conditions must be met for Nix to use
+          Substituters are tried based on their priority value, which each substituter can set independently.
-          a substituter:
+          Lower value means higher priority.
+          The default is `https://cache.nixos.org`, which has a priority of 40.
+
+          At least one of the following conditions must be met for Nix to use a substituter:
 
           - the substituter is in the [`trusted-substituters`](#conf-trusted-substituters) list
           - the user calling Nix is in the [`trusted-users`](#conf-trusted-users) list
 
-          In addition, each store path should be trusted as described
+          In addition, each store path should be trusted as described in [`trusted-public-keys`](#conf-trusted-public-keys)
-          in [`trusted-public-keys`](#conf-trusted-public-keys)
         )",
         {"binary-caches"}};
 

src/libstore/local-store.hh

@@ -52,14 +52,15 @@ struct LocalStoreConfig : virtual LocalFSStoreConfig
         R"(
           Allow this store to be opened when its [database](@docroot@/glossary.md#gloss-nix-database) is on a read-only filesystem.
 
-          Normally Nix will attempt to open the store database in read-write mode, even for querying (when write access is not needed).
+          Normally Nix will attempt to open the store database in read-write mode, even for querying (when write access is not needed), causing it to fail if the database is on a read-only filesystem.
-          This causes it to fail if the database is on a read-only filesystem.
 
           Enable read-only mode to disable locking and open the SQLite database with the [`immutable` parameter](https://www.sqlite.org/c3ref/open.html) set.
 
-          **Warning**
+          > **Warning**
-          Do not use this unless the filesystem is read-only.
+          > Do not use this unless the filesystem is read-only.
-          Using it when the filesystem is writable can cause incorrect query results or corruption errors if the database is changed by another process.
+          >
+          > Using it when the filesystem is writable can cause incorrect query results or corruption errors if the database is changed by another process.
+          > While the filesystem the database resides on might appear to be read-only, consider whether another user or system might have write access to it.
         )"};
 
     const std::string name() override { return "Local Store"; }

src/libstore/path-references.cc

@@ -0,0 +1,73 @@
+#include "path-references.hh"
+#include "hash.hh"
+#include "util.hh"
+#include "archive.hh"
+
+#include <map>
+#include <cstdlib>
+#include <mutex>
+#include <algorithm>
+
+
+namespace nix {
+
+
+PathRefScanSink::PathRefScanSink(StringSet && hashes, std::map<std::string, StorePath> && backMap)
+    : RefScanSink(std::move(hashes))
+    , backMap(std::move(backMap))
+{ }
+
+PathRefScanSink PathRefScanSink::fromPaths(const StorePathSet & refs)
+{
+    StringSet hashes;
+    std::map<std::string, StorePath> backMap;
+
+    for (auto & i : refs) {
+        std::string hashPart(i.hashPart());
+        auto inserted = backMap.emplace(hashPart, i).second;
+        assert(inserted);
+        hashes.insert(hashPart);
+    }
+
+    return PathRefScanSink(std::move(hashes), std::move(backMap));
+}
+
+StorePathSet PathRefScanSink::getResultPaths()
+{
+    /* Map the hashes found back to their store paths. */
+    StorePathSet found;
+    for (auto & i : getResult()) {
+        auto j = backMap.find(i);
+        assert(j != backMap.end());
+        found.insert(j->second);
+    }
+
+    return found;
+}
+
+
+std::pair<StorePathSet, HashResult> scanForReferences(
+    const std::string & path,
+    const StorePathSet & refs)
+{
+    HashSink hashSink { htSHA256 };
+    auto found = scanForReferences(hashSink, path, refs);
+    auto hash = hashSink.finish();
+    return std::pair<StorePathSet, HashResult>(found, hash);
+}
+
+StorePathSet scanForReferences(
+    Sink & toTee,
+    const Path & path,
+    const StorePathSet & refs)
+{
+    PathRefScanSink refsSink = PathRefScanSink::fromPaths(refs);
+    TeeSink sink { refsSink, toTee };
+
+    /* Look for the hashes in the NAR dump of the path. */
+    dumpPath(path, sink);
+
+    return refsSink.getResultPaths();
+}
+
+}

src/libstore/path-references.hh

@@ -0,0 +1,25 @@
+#pragma once
+
+#include "references.hh"
+#include "path.hh"
+
+namespace nix {
+
+std::pair<StorePathSet, HashResult> scanForReferences(const Path & path, const StorePathSet & refs);
+
+StorePathSet scanForReferences(Sink & toTee, const Path & path, const StorePathSet & refs);
+
+class PathRefScanSink : public RefScanSink
+{
+    std::map<std::string, StorePath> backMap;
+
+    PathRefScanSink(StringSet && hashes, std::map<std::string, StorePath> && backMap);
+
+public:
+
+    static PathRefScanSink fromPaths(const StorePathSet & refs);
+
+    StorePathSet getResultPaths();
+};
+
+}

src/libstore/profiles.cc

@@ -13,8 +13,10 @@
 namespace nix {
 
 
-/* Parse a generation name of the format
+/**
-   `<profilename>-<number>-link'. */
+ * Parse a generation name of the format
+ * `<profilename>-<number>-link'.
+ */
 static std::optional<GenerationNumber> parseName(const std::string & profileName, const std::string & name)
 {
     if (name.substr(0, profileName.size() + 1) != profileName + "-") return {};
@@ -28,7 +30,6 @@ static std::optional<GenerationNumber> parseName(const std::string & profileName
 }
 
 
-
 std::pair<Generations, std::optional<GenerationNumber>> findGenerations(Path profile)
 {
     Generations gens;
@@ -61,15 +62,16 @@ std::pair<Generations, std::optional<GenerationNumber>> findGenerations(Path pro
 }
 
 
-static void makeName(const Path & profile, GenerationNumber num,
+/**
-    Path & outLink)
+ * Create a generation name that can be parsed by `parseName()`.
+ */
+static Path makeName(const Path & profile, GenerationNumber num)
 {
-    Path prefix = fmt("%1%-%2%", profile, num);
+    return fmt("%s-%s-link", profile, num);
-    outLink = prefix + "-link";
 }
 
 
-Path createGeneration(ref<LocalFSStore> store, Path profile, StorePath outPath)
+Path createGeneration(LocalFSStore & store, Path profile, StorePath outPath)
 {
     /* The new generation number should be higher than old the
        previous ones. */
@@ -79,7 +81,7 @@ Path createGeneration(ref<LocalFSStore> store, Path profile, StorePath outPath)
     if (gens.size() > 0) {
         Generation last = gens.back();
 
-        if (readLink(last.path) == store->printStorePath(outPath)) {
+        if (readLink(last.path) == store.printStorePath(outPath)) {
             /* We only create a new generation symlink if it differs
                from the last one.
 
@@ -89,7 +91,7 @@ Path createGeneration(ref<LocalFSStore> store, Path profile, StorePath outPath)
             return last.path;
         }
 
-        num = gens.back().number;
+        num = last.number;
     } else {
         num = 0;
     }
@@ -100,9 +102,8 @@ Path createGeneration(ref<LocalFSStore> store, Path profile, StorePath outPath)
        to the permanent roots (of which the GC would have a stale
        view).  If we didn't do it this way, the GC might remove the
        user environment etc. we've just built. */
-    Path generation;
+    Path generation = makeName(profile, num + 1);
-    makeName(profile, num + 1, generation);
+    store.addPermRoot(outPath, generation);
-    store->addPermRoot(outPath, generation);
 
     return generation;
 }
@@ -117,12 +118,19 @@ static void removeFile(const Path & path)
 
 void deleteGeneration(const Path & profile, GenerationNumber gen)
 {
-    Path generation;
+    Path generation = makeName(profile, gen);
-    makeName(profile, gen, generation);
     removeFile(generation);
 }
 
-
+/**
+ * Delete a generation with dry-run mode.
+ *
+ * Like `deleteGeneration()` but:
+ *
+ *  - We log what we are going to do.
+ *
+ *  - We only actually delete if `dryRun` is false.
+ */
 static void deleteGeneration2(const Path & profile, GenerationNumber gen, bool dryRun)
 {
     if (dryRun)
@@ -150,27 +158,36 @@ void deleteGenerations(const Path & profile, const std::set<GenerationNumber> &
     }
 }
 
+/**
+ * Advanced the iterator until the given predicate `cond` returns `true`.
+ */
+static inline void iterDropUntil(Generations & gens, auto && i, auto && cond)
+{
+    for (; i != gens.rend() && !cond(*i); ++i);
+}
+
 void deleteGenerationsGreaterThan(const Path & profile, GenerationNumber max, bool dryRun)
 {
+    if (max == 0)
+        throw Error("Must keep at least one generation, otherwise the current one would be deleted");
+
     PathLocks lock;
     lockProfile(lock, profile);
 
-    bool fromCurGen = false;
+    auto [gens, _curGen] = findGenerations(profile);
-    auto [gens, curGen] = findGenerations(profile);
+    auto curGen = _curGen;
-    for (auto i = gens.rbegin(); i != gens.rend(); ++i) {
+
-        if (i->number == curGen) {
+    auto i = gens.rbegin();
-            fromCurGen = true;
+
-            max--;
+    // Find the current generation
-            continue;
+    iterDropUntil(gens, i, [&](auto & g) { return g.number == curGen; });
-        }
+
-        if (fromCurGen) {
+    // Skip over `max` generations, preserving them
-            if (max) {
+    for (auto keep = 0; i != gens.rend() && keep < max; ++i, ++keep);
-                max--;
+
-                continue;
+    // Delete the rest
-            }
+    for (; i != gens.rend(); ++i)
-            deleteGeneration2(profile, i->number, dryRun);
+        deleteGeneration2(profile, i->number, dryRun);
-        }
-    }
 }
 
 void deleteOldGenerations(const Path & profile, bool dryRun)
@@ -193,23 +210,33 @@ void deleteGenerationsOlderThan(const Path & profile, time_t t, bool dryRun)
 
     auto [gens, curGen] = findGenerations(profile);
 
-    bool canDelete = false;
+    auto i = gens.rbegin();
-    for (auto i = gens.rbegin(); i != gens.rend(); ++i)
+
-        if (canDelete) {
+    // Predicate that the generation is older than the given time.
-            assert(i->creationTime < t);
+    auto older = [&](auto & g) { return g.creationTime < t; };
-            if (i->number != curGen)
+
-                deleteGeneration2(profile, i->number, dryRun);
+    // Find the first older generation, if one exists
-        } else if (i->creationTime < t) {
+    iterDropUntil(gens, i, older);
-            /* We may now start deleting generations, but we don't
+
-               delete this generation yet, because this generation was
+    /* Take the previous generation
-               still the one that was active at the requested point in
+
-               time. */
+       We don't want delete this one yet because it
-            canDelete = true;
+       existed at the requested point in time, and
-        }
+       we want to be able to roll back to it. */
+    if (i != gens.rend()) ++i;
+
+    // Delete all previous generations (unless current).
+    for (; i != gens.rend(); ++i) {
+        /* Creating date and generations should be monotonic, so lower
+           numbered derivations should also be older. */
+        assert(older(*i));
+        if (i->number != curGen)
+            deleteGeneration2(profile, i->number, dryRun);
+    }
 }
 
 
-void deleteGenerationsOlderThan(const Path & profile, std::string_view timeSpec, bool dryRun)
+time_t parseOlderThanTimeSpec(std::string_view timeSpec)
 {
     if (timeSpec.empty() || timeSpec[timeSpec.size() - 1] != 'd')
         throw UsageError("invalid number of days specifier '%1%', expected something like '14d'", timeSpec);
@@ -221,9 +248,7 @@ void deleteGenerationsOlderThan(const Path & profile, std::string_view timeSpec,
     if (!days || *days < 1)
         throw UsageError("invalid number of days specifier '%1%'", timeSpec);
 
-    time_t oldTime = curTime - *days * 24 * 3600;
+    return curTime - *days * 24 * 3600;
-
-    deleteGenerationsOlderThan(profile, oldTime, dryRun);
 }
 
 

src/libstore/profiles.hh

@@ -1,7 +1,11 @@
 #pragma once
-///@file
+/**
+ * @file Implementation of Profiles.
+ *
+ * See the manual for additional information.
+ */
 
- #include "types.hh"
+#include "types.hh"
 #include "pathlocks.hh"
 
 #include <time.h>
@@ -12,41 +16,166 @@ namespace nix {
 class StorePath;
 
 
+/**
+ * A positive number identifying a generation for a given profile.
+ *
+ * Generation numbers are assigned sequentially. Each new generation is
+ * assigned 1 + the current highest generation number.
+ */
 typedef uint64_t GenerationNumber;
 
+/**
+ * A generation is a revision of a profile.
+ *
+ * Each generation is a mapping (key-value pair) from an identifier
+ * (`number`) to a store object (specified by `path`).
+ */
 struct Generation
 {
+    /**
+     * The number of a generation is its unique identifier within the
+     * profile.
+     */
     GenerationNumber number;
+    /**
+     * The store path identifies the store object that is the contents
+     * of the generation.
+     *
+     * These store paths / objects are not unique to the generation
+     * within a profile. Nix tries to ensure successive generations have
+     * distinct contents to avoid bloat, but nothing stops two
+     * non-adjacent generations from having the same contents.
+     *
+     * @todo Use `StorePath` instead of `Path`?
+     */
     Path path;
+
+    /**
+     * When the generation was created. This is extra metadata about the
+     * generation used to make garbage collecting old generations more
+     * convenient.
+     */
     time_t creationTime;
 };
 
+/**
+ * All the generations of a profile
+ */
 typedef std::list<Generation> Generations;
 
 
 /**
- * Returns the list of currently present generations for the specified
+ * Find all generations for the given profile.
- * profile, sorted by generation number. Also returns the number of
+ *
- * the current generation.
+ * @param profile A profile specified by its name and location combined
+ * into a path. E.g. if "foo" is the name of the profile, and "/bar/baz"
+ * is the directory it is in, then the path "/bar/baz/foo" would be the
+ * argument for this parameter.
+ *
+ * @return The pair of:
+ *
+ *   - The list of currently present generations for the specified profile,
+ *     sorted by ascending generation number.
+ *
+ *   - The number of the current/active generation.
+ *
+ * Note that the current/active generation need not be the latest one.
  */
 std::pair<Generations, std::optional<GenerationNumber>> findGenerations(Path profile);
 
 class LocalFSStore;
 
-Path createGeneration(ref<LocalFSStore> store, Path profile, StorePath outPath);
+/**
+ * Create a new generation of the given profile
+ *
+ * If the previous generation (not the currently active one!) has a
+ * distinct store object, a fresh generation number is mapped to the
+ * given store object, referenced by path. Otherwise, the previous
+ * generation is assumed.
+ *
+ * The behavior of reusing existing generations like this makes this
+ * procedure idempotent. It also avoids clutter.
+ */
+Path createGeneration(LocalFSStore & store, Path profile, StorePath outPath);
 
+/**
+ * Unconditionally delete a generation
+ *
+ * @param profile A profile specified by its name and location combined into a path.
+ *
+ * @param gen The generation number specifying exactly which generation
+ * to delete.
+ *
+ * Because there is no check of whether the generation to delete is
+ * active, this is somewhat unsafe.
+ *
+ * @todo Should we expose this at all?
+ */
 void deleteGeneration(const Path & profile, GenerationNumber gen);
 
+/**
+ * Delete the given set of generations.
+ *
+ * @param profile The profile, specified by its name and location combined into a path, whose generations we want to delete.
+ *
+ * @param gensToDelete The generations to delete, specified by a set of
+ * numbers.
+ *
+ * @param dryRun Log what would be deleted instead of actually doing
+ * so.
+ *
+ * Trying to delete the currently active generation will fail, and cause
+ * no generations to be deleted.
+ */
 void deleteGenerations(const Path & profile, const std::set<GenerationNumber> & gensToDelete, bool dryRun);
 
+/**
+ * Delete generations older than `max` passed the current generation.
+ *
+ * @param profile The profile, specified by its name and location combined into a path, whose generations we want to delete.
+ *
+ * @param max How many generations to keep up to the current one. Must
+ * be at least 1 so we don't delete the current one.
+ *
+ * @param dryRun Log what would be deleted instead of actually doing
+ * so.
+ */
 void deleteGenerationsGreaterThan(const Path & profile, GenerationNumber max, bool dryRun);
 
+/**
+ * Delete all generations other than the current one
+ *
+ * @param profile The profile, specified by its name and location combined into a path, whose generations we want to delete.
+ *
+ * @param dryRun Log what would be deleted instead of actually doing
+ * so.
+ */
 void deleteOldGenerations(const Path & profile, bool dryRun);
 
+/**
+ * Delete generations older than `t`, except for the most recent one
+ * older than `t`.
+ *
+ * @param profile The profile, specified by its name and location combined into a path, whose generations we want to delete.
+ *
+ * @param dryRun Log what would be deleted instead of actually doing
+ * so.
+ */
 void deleteGenerationsOlderThan(const Path & profile, time_t t, bool dryRun);
 
-void deleteGenerationsOlderThan(const Path & profile, std::string_view timeSpec, bool dryRun);
+/**
+ * Parse a temp spec intended for `deleteGenerationsOlderThan()`.
+ *
+ * Throws an exception if `timeSpec` fails to parse.
+ */
+time_t parseOlderThanTimeSpec(std::string_view timeSpec);
 
+/**
+ * Smaller wrapper around `replaceSymlink` for replacing the current
+ * generation of a profile. Does not enforce proper structure.
+ *
+ * @todo Always use `switchGeneration()` instead, and delete this.
+ */
 void switchLink(Path link, Path target);
 
 /**

src/libstore/sqlite.hh

@@ -29,7 +29,7 @@ enum class SQLiteOpenMode {
      * Use this mode if the database is on a read-only filesystem.
      * Fails with an error if the database does not exist.
      */
-    Immutable
+    Immutable,
 };
 
 /**

src/libutil/experimental-features.cc

@@ -225,15 +225,7 @@ constexpr std::array<ExperimentalFeatureDetails, 15> xpFeatureDetails = {{
         .tag = Xp::ReadOnlyLocalStore,
         .name = "read-only-local-store",
         .description = R"(
-            Allow the use of the `read-only` parameter in local store URIs.
+            Allow the use of the `read-only` parameter in [local store](@docroot@/command-ref/new-cli/nix3-help-stores.md#local-store) URIs.
-
-            Set this parameter to `true` to allow stores with databases on read-only filesystems to be opened for querying; ordinarily Nix will refuse to do this.
-
-            This is because SQLite requires write access to the database file to perform the file locking operations necessary for safe concurrent access.
-            When `read-only` is set to `true`, the database will be opened in immutable mode.
-
-            Under this mode, SQLite will not do any locking at all, so you should be certain that the database will not be changed.
-            While the filesystem the database resides on might be read-only to this process, consider whether another user, process, or system, might have write access to it.
         )",
     },
 }};

src/libutil/filesystem.cc

@@ -63,30 +63,19 @@ std::pair<AutoCloseFD, Path> createTempFile(const Path & prefix)
     return {std::move(fd), tmpl};
 }
 
-void createSymlink(const Path & target, const Path & link,
+void createSymlink(const Path & target, const Path & link)
-    std::optional<time_t> mtime)
 {
     if (symlink(target.c_str(), link.c_str()))
         throw SysError("creating symlink from '%1%' to '%2%'", link, target);
-    if (mtime) {
-        struct timeval times[2];
-        times[0].tv_sec = *mtime;
-        times[0].tv_usec = 0;
-        times[1].tv_sec = *mtime;
-        times[1].tv_usec = 0;
-        if (lutimes(link.c_str(), times))
-            throw SysError("setting time of symlink '%s'", link);
-    }
 }
 
-void replaceSymlink(const Path & target, const Path & link,
+void replaceSymlink(const Path & target, const Path & link)
-    std::optional<time_t> mtime)
 {
     for (unsigned int n = 0; true; n++) {
         Path tmp = canonPath(fmt("%s/.%d_%s", dirOf(link), n, baseNameOf(link)));
 
         try {
-            createSymlink(target, tmp, mtime);
+            createSymlink(target, tmp);
         } catch (SysError & e) {
             if (e.errNo == EEXIST) continue;
             throw;

src/libutil/references.cc

@@ -6,6 +6,7 @@
 #include <map>
 #include <cstdlib>
 #include <mutex>
+#include <algorithm>
 
 
 namespace nix {
@@ -66,69 +67,20 @@ void RefScanSink::operator () (std::string_view data)
 }
 
 
-PathRefScanSink::PathRefScanSink(StringSet && hashes, std::map<std::string, StorePath> && backMap)
-    : RefScanSink(std::move(hashes))
-    , backMap(std::move(backMap))
-{ }
-
-PathRefScanSink PathRefScanSink::fromPaths(const StorePathSet & refs)
-{
-    StringSet hashes;
-    std::map<std::string, StorePath> backMap;
-
-    for (auto & i : refs) {
-        std::string hashPart(i.hashPart());
-        auto inserted = backMap.emplace(hashPart, i).second;
-        assert(inserted);
-        hashes.insert(hashPart);
-    }
-
-    return PathRefScanSink(std::move(hashes), std::move(backMap));
-}
-
-StorePathSet PathRefScanSink::getResultPaths()
-{
-    /* Map the hashes found back to their store paths. */
-    StorePathSet found;
-    for (auto & i : getResult()) {
-        auto j = backMap.find(i);
-        assert(j != backMap.end());
-        found.insert(j->second);
-    }
-
-    return found;
-}
-
-
-std::pair<StorePathSet, HashResult> scanForReferences(
-    const std::string & path,
-    const StorePathSet & refs)
-{
-    HashSink hashSink { htSHA256 };
-    auto found = scanForReferences(hashSink, path, refs);
-    auto hash = hashSink.finish();
-    return std::pair<StorePathSet, HashResult>(found, hash);
-}
-
-StorePathSet scanForReferences(
-    Sink & toTee,
-    const Path & path,
-    const StorePathSet & refs)
-{
-    PathRefScanSink refsSink = PathRefScanSink::fromPaths(refs);
-    TeeSink sink { refsSink, toTee };
-
-    /* Look for the hashes in the NAR dump of the path. */
-    dumpPath(path, sink);
-
-    return refsSink.getResultPaths();
-}
-
-
 RewritingSink::RewritingSink(const std::string & from, const std::string & to, Sink & nextSink)
-    : from(from), to(to), nextSink(nextSink)
+    : RewritingSink({{from, to}}, nextSink)
 {
-    assert(from.size() == to.size());
+}
+
+RewritingSink::RewritingSink(const StringMap & rewrites, Sink & nextSink)
+    : rewrites(rewrites), nextSink(nextSink)
+{
+    std::string::size_type maxRewriteSize = 0;
+    for (auto & [from, to] : rewrites) {
+        assert(from.size() == to.size());
+        maxRewriteSize = std::max(maxRewriteSize, from.size());
+    }
+    this->maxRewriteSize = maxRewriteSize;
 }
 
 void RewritingSink::operator () (std::string_view data)
@@ -136,13 +88,13 @@ void RewritingSink::operator () (std::string_view data)
     std::string s(prev);
     s.append(data);
 
-    size_t j = 0;
+    s = rewriteStrings(s, rewrites);
-    while ((j = s.find(from, j)) != std::string::npos) {
-        matches.push_back(pos + j);
-        s.replace(j, from.size(), to);
-    }
 
-    prev = s.size() < from.size() ? s : std::string(s, s.size() - from.size() + 1, from.size() - 1);
+    prev = s.size() < maxRewriteSize
+        ? s
+        : maxRewriteSize == 0
+            ? ""
+            : std::string(s, s.size() - maxRewriteSize + 1, maxRewriteSize - 1);
 
     auto consumed = s.size() - prev.size();
 

src/libutil/references.hh

@@ -2,14 +2,9 @@
 ///@file
 
 #include "hash.hh"
-#include "path.hh"
 
 namespace nix {
 
-std::pair<StorePathSet, HashResult> scanForReferences(const Path & path, const StorePathSet & refs);
-
-StorePathSet scanForReferences(Sink & toTee, const Path & path, const StorePathSet & refs);
-
 class RefScanSink : public Sink
 {
     StringSet hashes;
@@ -28,28 +23,18 @@ public:
     void operator () (std::string_view data) override;
 };
 
-class PathRefScanSink : public RefScanSink
-{
-    std::map<std::string, StorePath> backMap;
-
-    PathRefScanSink(StringSet && hashes, std::map<std::string, StorePath> && backMap);
-
-public:
-
-    static PathRefScanSink fromPaths(const StorePathSet & refs);
-
-    StorePathSet getResultPaths();
-};
-
 struct RewritingSink : Sink
 {
-    std::string from, to, prev;
+    const StringMap rewrites;
+    std::string::size_type maxRewriteSize;
+    std::string prev;
     Sink & nextSink;
     uint64_t pos = 0;
 
     std::vector<uint64_t> matches;
 
     RewritingSink(const std::string & from, const std::string & to, Sink & nextSink);
+    RewritingSink(const StringMap & rewrites, Sink & nextSink);
 
     void operator () (std::string_view data) override;
 

src/libutil/tests/references.cc

@@ -0,0 +1,46 @@
+#include "references.hh"
+#include <gtest/gtest.h>
+
+namespace nix {
+
+using std::string;
+
+struct RewriteParams {
+    string originalString, finalString;
+    StringMap rewrites;
+
+    friend std::ostream& operator<<(std::ostream& os, const RewriteParams& bar) {
+        StringSet strRewrites;
+        for (auto & [from, to] : bar.rewrites)
+            strRewrites.insert(from + "->" + to);
+        return os <<
+            "OriginalString: " << bar.originalString << std::endl <<
+            "Rewrites: " << concatStringsSep(",", strRewrites) << std::endl <<
+            "Expected result: " << bar.finalString;
+    }
+};
+
+class RewriteTest : public ::testing::TestWithParam<RewriteParams> {
+};
+
+TEST_P(RewriteTest, IdentityRewriteIsIdentity) {
+    RewriteParams param = GetParam();
+    StringSink rewritten;
+    auto rewriter = RewritingSink(param.rewrites, rewritten);
+    rewriter(param.originalString);
+    rewriter.flush();
+    ASSERT_EQ(rewritten.s, param.finalString);
+}
+
+INSTANTIATE_TEST_CASE_P(
+    references,
+    RewriteTest,
+    ::testing::Values(
+        RewriteParams{ "foooo", "baroo", {{"foo", "bar"}, {"bar", "baz"}}},
+        RewriteParams{ "foooo", "bazoo", {{"fou", "bar"}, {"foo", "baz"}}},
+        RewriteParams{ "foooo", "foooo", {}}
+    )
+);
+
+}
+

src/libutil/util.cc

@@ -1853,6 +1853,7 @@ void setStackSize(size_t stackSize)
 
 #if __linux__
 static AutoCloseFD fdSavedMountNamespace;
+static AutoCloseFD fdSavedRoot;
 #endif
 
 void saveMountNamespace()
@@ -1860,10 +1861,11 @@ void saveMountNamespace()
 #if __linux__
     static std::once_flag done;
     std::call_once(done, []() {
-        AutoCloseFD fd = open("/proc/self/ns/mnt", O_RDONLY);
+        fdSavedMountNamespace = open("/proc/self/ns/mnt", O_RDONLY);
-        if (!fd)
+        if (!fdSavedMountNamespace)
             throw SysError("saving parent mount namespace");
-        fdSavedMountNamespace = std::move(fd);
+
+        fdSavedRoot = open("/proc/self/root", O_RDONLY);
     });
 #endif
 }
@@ -1876,9 +1878,16 @@ void restoreMountNamespace()
 
         if (fdSavedMountNamespace && setns(fdSavedMountNamespace.get(), CLONE_NEWNS) == -1)
             throw SysError("restoring parent mount namespace");
-        if (chdir(savedCwd.c_str()) == -1) {
+
-            throw SysError("restoring cwd");
+        if (fdSavedRoot) {
+            if (fchdir(fdSavedRoot.get()))
+                throw SysError("chdir into saved root");
+            if (chroot("."))
+                throw SysError("chroot into saved root");
         }
+
+        if (chdir(savedCwd.c_str()) == -1)
+            throw SysError("restoring cwd");
     } catch (Error & e) {
         debug(e.msg());
     }

src/libutil/util.hh

@@ -256,14 +256,12 @@ inline Paths createDirs(PathView path)
 /**
  * Create a symlink.
  */
-void createSymlink(const Path & target, const Path & link,
+void createSymlink(const Path & target, const Path & link);
-    std::optional<time_t> mtime = {});
 
 /**
  * Atomically create or replace a symlink.
  */
-void replaceSymlink(const Path & target, const Path & link,
+void replaceSymlink(const Path & target, const Path & link);
-    std::optional<time_t> mtime = {});
 
 void renameFile(const Path & src, const Path & dst);
 

src/nix-channel/nix-channel.cc

@@ -177,6 +177,7 @@ static int main_nix_channel(int argc, char ** argv)
             cRemove,
             cList,
             cUpdate,
+            cListGenerations,
             cRollback
         } cmd = cNone;
         std::vector<std::string> args;
@@ -193,6 +194,8 @@ static int main_nix_channel(int argc, char ** argv)
                 cmd = cList;
             } else if (*arg == "--update") {
                 cmd = cUpdate;
+            } else if (*arg == "--list-generations") {
+                cmd = cListGenerations;
             } else if (*arg == "--rollback") {
                 cmd = cRollback;
             } else {
@@ -237,6 +240,11 @@ static int main_nix_channel(int argc, char ** argv)
             case cUpdate:
                 update(StringSet(args.begin(), args.end()));
                 break;
+            case cListGenerations:
+                if (!args.empty())
+                    throw UsageError("'--list-generations' expects no arguments");
+                std::cout << runProgram(settings.nixBinDir + "/nix-env", false, {"--profile", profile, "--list-generations"}) << std::flush;
+                break;
             case cRollback:
                 if (args.size() > 1)
                     throw UsageError("'--rollback' has at most one argument");

src/nix-collect-garbage/nix-collect-garbage.cc

@@ -41,9 +41,10 @@ void removeOldGenerations(std::string dir)
             }
             if (link.find("link") != std::string::npos) {
                 printInfo("removing old generations of profile %s", path);
-                if (deleteOlderThan != "")
+                if (deleteOlderThan != "") {
-                    deleteGenerationsOlderThan(path, deleteOlderThan, dryRun);
+                    auto t = parseOlderThanTimeSpec(deleteOlderThan);
-                else
+                    deleteGenerationsOlderThan(path, t, dryRun);
+                } else
                     deleteOldGenerations(path, dryRun);
             }
         } else if (type == DT_DIR) {

src/nix-env/nix-env.cc

@@ -772,7 +772,7 @@ static void opSet(Globals & globals, Strings opFlags, Strings opArgs)
 
     debug("switching to new user environment");
     Path generation = createGeneration(
-        ref<LocalFSStore>(store2),
+        *store2,
         globals.profile,
         drv.queryOutPath());
     switchLink(globals.profile, generation);
@@ -1356,13 +1356,14 @@ static void opDeleteGenerations(Globals & globals, Strings opFlags, Strings opAr
     if (opArgs.size() == 1 && opArgs.front() == "old") {
         deleteOldGenerations(globals.profile, globals.dryRun);
     } else if (opArgs.size() == 1 && opArgs.front().find('d') != std::string::npos) {
-        deleteGenerationsOlderThan(globals.profile, opArgs.front(), globals.dryRun);
+        auto t = parseOlderThanTimeSpec(opArgs.front());
+        deleteGenerationsOlderThan(globals.profile, t, globals.dryRun);
     } else if (opArgs.size() == 1 && opArgs.front().find('+') != std::string::npos) {
         if (opArgs.front().size() < 2)
             throw Error("invalid number of generations '%1%'", opArgs.front());
         auto str_max = opArgs.front().substr(1);
         auto max = string2Int<GenerationNumber>(str_max);
-        if (!max || *max == 0)
+        if (!max)
             throw Error("invalid number of generations to keep '%1%'", opArgs.front());
         deleteGenerationsGreaterThan(globals.profile, *max, globals.dryRun);
     } else {

src/nix-env/user-env.cc

@@ -158,7 +158,7 @@ bool createUserEnv(EvalState & state, DrvInfos & elems,
         }
 
         debug("switching to new user environment");
-        Path generation = createGeneration(ref<LocalFSStore>(store2), profile, topLevelOut);
+        Path generation = createGeneration(*store2, profile, topLevelOut);
         switchLink(profile, generation);
     }
 

src/nix/profile.cc

@@ -806,9 +806,10 @@ struct CmdProfileWipeHistory : virtual StoreCommand, MixDefaultProfile, MixDryRu
 
     void run(ref<Store> store) override
     {
-        if (minAge)
+        if (minAge) {
-            deleteGenerationsOlderThan(*profile, *minAge, dryRun);
+            auto t = parseOlderThanTimeSpec(*minAge);
-        else
+            deleteGenerationsOlderThan(*profile, t, dryRun);
+        } else
             deleteOldGenerations(*profile, dryRun);
     }
 };

tests/build.sh

@@ -129,3 +129,7 @@ nix build --impure -f multiple-outputs.nix --json e --no-link | jq --exit-status
     (.drvPath | match(".*multiple-outputs-e.drv")) and
     (.outputs | keys == ["a_a", "b"]))
 '
+
+# Make sure that `--stdin` works and does not apply any defaults
+printf "" | nix build --no-link --stdin --json | jq --exit-status '. == []'
+printf "%s\n" "$drv^*" | nix build --no-link --stdin --json | jq --exit-status '.[0]|has("drvPath")'

tests/fetchClosure.sh

@@ -5,6 +5,12 @@ enableFeatures "fetch-closure"
 clearStore
 clearCacheCache
 
+# Old daemons don't properly zero out the self-references when
+# calculating the CA hashes, so this breaks `nix store
+# make-content-addressed` which expects the client and the daemon to
+# compute the same hash
+requireDaemonNewerThan "2.16.0pre20230524"
+
 # Initialize binary cache.
 nonCaPath=$(nix build --json --file ./dependencies.nix --no-link | jq -r .[].outputs.out)
 caPath=$(nix store make-content-addressed --json $nonCaPath | jq -r '.rewrites | map(.) | .[]')

tests/gc.sh

@@ -50,31 +50,3 @@ if test -e $outPath/foobar; then false; fi
 # Check that the store is empty.
 rmdir $NIX_STORE_DIR/.links
 rmdir $NIX_STORE_DIR
-
-## Test `nix-collect-garbage -d`
-testCollectGarbageD () {
-    clearProfiles
-    # Run two `nix-env` commands, should create two generations of
-    # the profile
-    nix-env -f ./user-envs.nix -i foo-1.0
-    nix-env -f ./user-envs.nix -i foo-2.0pre1
-    [[ $(nix-env --list-generations | wc -l) -eq 2 ]]
-
-    # Clear the profile history. There should be only one generation
-    # left
-    nix-collect-garbage -d
-    [[ $(nix-env --list-generations | wc -l) -eq 1 ]]
-}
-# `nix-env` doesn't work with CA derivations, so let's ignore that bit if we're
-# using them
-if [[ -z "${NIX_TESTS_CA_BY_DEFAULT:-}" ]]; then
-    testCollectGarbageD
-
-    # Run the same test, but forcing the profiles at their legacy location under
-    # /nix/var/nix.
-    #
-    # Regression test for #8294
-    rm ~/.nix-profile
-    ln -s $NIX_STATE_DIR/profiles/per-user/me ~/.nix-profile
-    testCollectGarbageD
-fi

tests/local.mk

@@ -16,6 +16,7 @@ nix_tests = \
   flakes/flake-in-submodule.sh \
   ca/gc.sh \
   gc.sh \
+  nix-collect-garbage-d.sh \
   remote-store.sh \
   legacy-ssh-store.sh \
   lang.sh \
@@ -135,6 +136,7 @@ nix_tests = \
   flakes/show.sh \
   impure-derivations.sh \
   path-from-hash-part.sh \
+  test-libstoreconsumer.sh \
   toString-path.sh \
   read-only-store.sh
 
@@ -154,6 +156,7 @@ test-deps += \
   tests/common/vars-and-functions.sh \
   tests/config.nix \
   tests/ca/config.nix \
+  tests/test-libstoreconsumer/test-libstoreconsumer \
   tests/dyn-drv/config.nix
 
 ifeq ($(BUILD_SHARED_LIBS), 1)

tests/nix-channel.sh

@@ -8,6 +8,7 @@ rm -f $TEST_HOME/.nix-channels $TEST_HOME/.nix-profile
 nix-channel --add http://foo/bar xyzzy
 nix-channel --list | grepQuiet http://foo/bar
 nix-channel --remove xyzzy
+[[ $(nix-channel --list-generations | wc -l) == 1 ]]
 
 [ -e $TEST_HOME/.nix-channels ]
 [ "$(cat $TEST_HOME/.nix-channels)" = '' ]
@@ -38,6 +39,7 @@ ln -s dependencies.nix $TEST_ROOT/nixexprs/default.nix
 # Test the update action.
 nix-channel --add file://$TEST_ROOT/foo
 nix-channel --update
+[[ $(nix-channel --list-generations | wc -l) == 2 ]]
 
 # Do a query.
 nix-env -qa \* --meta --xml --out-path > $TEST_ROOT/meta.xml

tests/nix-collect-garbage-d.sh

@@ -0,0 +1,40 @@
+source common.sh
+
+clearStore
+
+## Test `nix-collect-garbage -d`
+
+# TODO make `nix-env` doesn't work with CA derivations, and make
+# `ca/nix-collect-garbage-d.sh` wrapper.
+
+testCollectGarbageD () {
+    clearProfiles
+    # Run two `nix-env` commands, should create two generations of
+    # the profile
+    nix-env -f ./user-envs.nix -i foo-1.0 "$@"
+    nix-env -f ./user-envs.nix -i foo-2.0pre1 "$@"
+    [[ $(nix-env --list-generations "$@" | wc -l) -eq 2 ]]
+
+    # Clear the profile history. There should be only one generation
+    # left
+    nix-collect-garbage -d
+    [[ $(nix-env --list-generations "$@" | wc -l) -eq 1 ]]
+}
+
+testCollectGarbageD
+
+# Run the same test, but forcing the profiles an arbitrary location.
+rm ~/.nix-profile
+ln -s $TEST_ROOT/blah ~/.nix-profile
+testCollectGarbageD
+
+# Run the same test, but forcing the profiles at their legacy location under
+# /nix/var/nix.
+#
+# Note that we *don't* use the default profile; `nix-collect-garbage` will
+# need to check the legacy conditional unconditionally not just follow
+# `~/.nix-profile` to pass this test.
+#
+# Regression test for #8294
+rm ~/.nix-profile
+testCollectGarbageD --profile "$NIX_STATE_DIR/profiles/per-user/me"

tests/nixos/tarball-flakes.nix

@@ -0,0 +1,84 @@
+{ lib, config, nixpkgs, ... }:
+
+let
+  pkgs = config.nodes.machine.nixpkgs.pkgs;
+
+  root = pkgs.runCommand "nixpkgs-flake" {}
+    ''
+      mkdir -p $out/stable
+
+      set -x
+      dir=nixpkgs-${nixpkgs.shortRev}
+      cp -prd ${nixpkgs} $dir
+      # Set the correct timestamp in the tarball.
+      find $dir -print0 | xargs -0 touch -t ${builtins.substring 0 12 nixpkgs.lastModifiedDate}.${builtins.substring 12 2 nixpkgs.lastModifiedDate} --
+      tar cfz $out/stable/${nixpkgs.rev}.tar.gz $dir --hard-dereference
+
+      echo 'Redirect "/latest.tar.gz" "/stable/${nixpkgs.rev}.tar.gz"' > $out/.htaccess
+
+      echo 'Header set Link "<http://localhost/stable/${nixpkgs.rev}.tar.gz?rev=${nixpkgs.rev}&revCount=1234>; rel=\"immutable\""' > $out/stable/.htaccess
+    '';
+in
+
+{
+  name = "tarball-flakes";
+
+  nodes =
+    {
+      machine =
+        { config, pkgs, ... }:
+        { networking.firewall.allowedTCPPorts = [ 80 ];
+
+          services.httpd.enable = true;
+          services.httpd.adminAddr = "foo@example.org";
+          services.httpd.extraConfig = ''
+            ErrorLog syslog:local6
+          '';
+          services.httpd.virtualHosts."localhost" =
+            { servedDirs =
+                [ { urlPath = "/";
+                    dir = root;
+                  }
+                ];
+            };
+
+          virtualisation.writableStore = true;
+          virtualisation.diskSize = 2048;
+          virtualisation.additionalPaths = [ pkgs.hello pkgs.fuse ];
+          virtualisation.memorySize = 4096;
+          nix.settings.substituters = lib.mkForce [ ];
+          nix.extraOptions = "experimental-features = nix-command flakes";
+        };
+    };
+
+  testScript = { nodes }: ''
+    # fmt: off
+    import json
+
+    start_all()
+
+    machine.wait_for_unit("httpd.service")
+
+    out = machine.succeed("nix flake metadata --json http://localhost/latest.tar.gz")
+    print(out)
+    info = json.loads(out)
+
+    # Check that we got redirected to the immutable URL.
+    assert info["locked"]["url"] == "http://localhost/stable/${nixpkgs.rev}.tar.gz"
+
+    # Check that we got the rev and revCount attributes.
+    assert info["revision"] == "${nixpkgs.rev}"
+    assert info["revCount"] == 1234
+
+    # Check that fetching with rev/revCount/narHash succeeds.
+    machine.succeed("nix flake metadata --json http://localhost/latest.tar.gz?rev=" + info["revision"])
+    machine.succeed("nix flake metadata --json http://localhost/latest.tar.gz?revCount=" + str(info["revCount"]))
+    machine.succeed("nix flake metadata --json http://localhost/latest.tar.gz?narHash=" + info["locked"]["narHash"])
+
+    # Check that fetching fails if we provide incorrect attributes.
+    machine.fail("nix flake metadata --json http://localhost/latest.tar.gz?rev=493300eb13ae6fb387fbd47bf54a85915acc31c0")
+    machine.fail("nix flake metadata --json http://localhost/latest.tar.gz?revCount=789")
+    machine.fail("nix flake metadata --json http://localhost/latest.tar.gz?narHash=sha256-tbudgBSg+bHWHiHnlteNzN8TUvI80ygS9IULh4rklEw=")
+  '';
+
+}

tests/plugins/plugintest.cc

@@ -21,4 +21,8 @@ static void prim_anotherNull (EvalState & state, const PosIdx pos, Value ** args
         v.mkBool(false);
 }
 
-static RegisterPrimOp rp("anotherNull", 0, prim_anotherNull);
+static RegisterPrimOp rp({
+    .name = "anotherNull",
+    .arity = 0,
+    .fun = prim_anotherNull,
+});

tests/test-libstoreconsumer.sh

@@ -0,0 +1,6 @@
+source common.sh
+
+drv="$(nix-instantiate simple.nix)"
+cat "$drv"
+out="$(./test-libstoreconsumer/test-libstoreconsumer "$drv")"
+cat "$out/hello" | grep -F "Hello World!"

tests/test-libstoreconsumer/README.md

@@ -0,0 +1,6 @@
+
+A very simple C++ consumer of the libstore library.
+
+  - Keep it simple. Library consumers expect something simple.
+  - No build hook, or any other reinvocations.
+  - No more global state than necessary.

tests/test-libstoreconsumer/local.mk

@@ -0,0 +1,12 @@
+programs += test-libstoreconsumer
+
+test-libstoreconsumer_DIR := $(d)
+
+test-libstoreconsumer_SOURCES := \
+  $(wildcard $(d)/*.cc) \
+
+test-libstoreconsumer_CXXFLAGS += -I src/libutil -I src/libstore
+
+test-libstoreconsumer_LIBS = libstore libutil
+
+test-libstoreconsumer_LDFLAGS = -pthread $(SODIUM_LIBS) $(EDITLINE_LIBS) $(BOOST_LDFLAGS) $(LOWDOWN_LIBS)

tests/test-libstoreconsumer/main.cc

@@ -0,0 +1,45 @@
+#include "globals.hh"
+#include "store-api.hh"
+#include "build-result.hh"
+#include <iostream>
+
+using namespace nix;
+
+int main (int argc, char **argv)
+{
+    try {
+        if (argc != 2) {
+            std::cerr << "Usage: " << argv[0] << " store/path/to/something.drv\n";
+            return 1;
+        }
+
+        std::string drvPath = argv[1];
+
+        initLibStore();
+
+        auto store = nix::openStore();
+
+        // build the derivation
+
+        std::vector<DerivedPath> paths {
+            DerivedPath::Built {
+                .drvPath = store->parseStorePath(drvPath),
+                .outputs = OutputsSpec::Names{"out"}
+            }
+        };
+
+        const auto results = store->buildPathsWithResults(paths, bmNormal, store);
+
+        for (const auto & result : results) {
+            for (const auto & [outputName, realisation] : result.builtOutputs) {
+                std::cout << store->printStorePath(realisation.outPath) << "\n";
+            }
+        }
+
+        return 0;
+
+    } catch (const std::exception & e) {
+        std::cerr << "Error: " << e.what() << "\n";
+        return 1;
+    }
+}