nix flake update: Recreate the lock file

This is probably what most people expect it to do. Fixes #3781.

There is a new command 'nix flake lock' that has the old behaviour of
'nix flake update', i.e. it just adds missing lock file entries unless
overriden using --update-input.

src/libutil/args.cc

@@ -19,6 +19,14 @@ void Args::addFlag(Flag && flag_)
     if (flag->shortName) shortFlags[flag->shortName] = flag;
 }
 
+void Args::removeFlag(const std::string & longName)
+{
+    auto flag = longFlags.find(longName);
+    assert(flag != longFlags.end());
+    if (flag->second->shortName) shortFlags.erase(flag->second->shortName);
+    longFlags.erase(flag);
+}
+
 void Completions::add(std::string completion, std::string description)
 {
     assert(description.find('\n') == std::string::npos);

src/libutil/args.hh

@@ -140,6 +140,8 @@ public:
 
     void addFlag(Flag && flag);
 
+    void removeFlag(const std::string & longName);
+
     void expectArgs(ExpectedArg && arg)
     {
         expectedArgs.emplace_back(std::move(arg));

src/nix/flake-lock.md

@@ -0,0 +1,38 @@
+R""(
+
+# Examples
+
+* Update the `nixpkgs` and `nix` inputs of the flake in the current
+  directory:
+
+  ```console
+  # nix flake lock --update-input nixpkgs --update-input nix
+  * Updated 'nix': 'github:NixOS/nix/9fab14adbc3810d5cc1f88672fde1eee4358405c' -> 'github:NixOS/nix/8927cba62f5afb33b01016d5c4f7f8b7d0adde3c'
+  * Updated 'nixpkgs': 'github:NixOS/nixpkgs/3d2d8f281a27d466fa54b469b5993f7dde198375' -> 'github:NixOS/nixpkgs/a3a3dda3bacf61e8a39258a0ed9c924eeca8e293'
+  ```
+
+# Description
+
+This command updates the lock file of a flake (`flake.lock`) so that
+it contains a lock for every flake input specified in
+`flake.nix`. Existing lock file entries are not updated unless
+required by a flag such as `--update-input`.
+
+Note that every command that operates on a flake will also update the
+lock file if needed, and supports the same flags. Therefore,
+
+```console
+# nix flake lock --update-input nixpkgs
+# nix build
+```
+
+is equivalent to:
+
+```console
+# nix build --update-input nixpkgs
+```
+
+Thus, this command is only useful if you want to update the lock file
+separately from any other action such as building.
+
+)""

src/nix/flake-update.md

@@ -2,52 +2,33 @@ R""(
 
 # Examples
 
-* Update the `nixpkgs` and `nix` inputs of the flake in the current
-  directory:
-
-  ```console
-  # nix flake update --update-input nixpkgs --update-input nix
-  * Updated 'nix': 'github:NixOS/nix/9fab14adbc3810d5cc1f88672fde1eee4358405c' -> 'github:NixOS/nix/8927cba62f5afb33b01016d5c4f7f8b7d0adde3c'
-  * Updated 'nixpkgs': 'github:NixOS/nixpkgs/3d2d8f281a27d466fa54b469b5993f7dde198375' -> 'github:NixOS/nixpkgs/a3a3dda3bacf61e8a39258a0ed9c924eeca8e293'
-  ```
-
 * Recreate the lock file (i.e. update all inputs) and commit the new
   lock file:
 
   ```console
-  # nix flake update --recreate-lock-file --commit-lock-file
+  # nix flake update
+  * Updated 'nix': 'github:NixOS/nix/9fab14adbc3810d5cc1f88672fde1eee4358405c' -> 'github:NixOS/nix/8927cba62f5afb33b01016d5c4f7f8b7d0adde3c'
+  * Updated 'nixpkgs': 'github:NixOS/nixpkgs/3d2d8f281a27d466fa54b469b5993f7dde198375' -> 'github:NixOS/nixpkgs/a3a3dda3bacf61e8a39258a0ed9c924eeca8e293'
   …
   warning: committed new revision '158bcbd9d6cc08ab859c0810186c1beebc982aad'
   ```
 
 # Description
 
-This command updates the lock file of a flake (`flake.lock`) so that
-it contains a lock for every flake input specified in
-`flake.nix`. Note that every command that operates on a flake will
-also update the lock file if needed, and supports the same
-flags. Therefore,
+This command recreates the lock file of a flake (`flake.lock`), thus
+updating the lock for every mutable input (like `nixpkgs`) to its
+current version. This is equivalent to passing `--recreate-lock-file`
+to any command that operates on a flake. That is,
 
 ```console
-# nix flake update --update-input nixpkgs
+# nix flake update
 # nix build
 ```
 
 is equivalent to:
 
 ```console
-# nix build --update-input nixpkgs
+# nix build --recreate-lock-file
 ```
 
-Thus, this command is only useful if you want to update the lock file
-separately from any other action such as building.
-
-> **Note**
->
-> This command does *not* update locks that are already present unless
-> you explicitly ask for it using `--update-input` or
-> `--recreate-lock-file`. Thus, if the lock file already has locks for
-> every input, then `nix flake update` (without arguments) does
-> nothing.
-
 )""

src/nix/flake.cc

@@ -104,6 +104,14 @@ struct CmdFlakeUpdate : FlakeCommand
         return "update flake lock file";
     }
 
+    CmdFlakeUpdate()
+    {
+        /* Remove flags that don't make sense. */
+        removeFlag("recreate-lock-file");
+        removeFlag("update-input");
+        removeFlag("no-update-lock-file");
+    }
+
     std::string doc() override
     {
         return
@@ -113,7 +121,30 @@ struct CmdFlakeUpdate : FlakeCommand
 
     void run(nix::ref<nix::Store> store) override
     {
-        /* Use --refresh by default for 'nix flake update'. */
+        settings.tarballTtl = 0;
+
+        lockFlags.recreateLockFile = true;
+
+        lockFlake();
+    }
+};
+
+struct CmdFlakeLock : FlakeCommand
+{
+    std::string description() override
+    {
+        return "create missing lock file entries";
+    }
+
+    std::string doc() override
+    {
+        return
+          #include "flake-lock.md"
+          ;
+    }
+
+    void run(nix::ref<nix::Store> store) override
+    {
         settings.tarballTtl = 0;
 
         lockFlake();
@@ -1006,6 +1037,7 @@ struct CmdFlake : NixMultiCommand
     CmdFlake()
         : MultiCommand({
                 {"update", []() { return make_ref<CmdFlakeUpdate>(); }},
+                {"lock", []() { return make_ref<CmdFlakeLock>(); }},
                 {"info", []() { return make_ref<CmdFlakeInfo>(); }},
                 {"list-inputs", []() { return make_ref<CmdFlakeListInputs>(); }},
                 {"check", []() { return make_ref<CmdFlakeCheck>(); }},