From d00682beb2fc0ba62fb87752ba429cc0f4c4345e Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Thu, 8 May 2025 19:01:34 +0200
Subject: [PATCH] Backward compatibility hack for dealing with `dir` in
 URL-style flakerefs

---
 src/libflake/flake.cc                      |  2 +-
 src/libflake/flakeref.cc                   | 49 ++++++++++++++++++
 src/libflake/include/nix/flake/flakeref.hh |  6 +++
 tests/functional/flakes/meson.build        |  1 +
 tests/functional/flakes/old-lockfiles.sh   | 60 ++++++++++++++++++++++
 5 files changed, 117 insertions(+), 1 deletion(-)
 create mode 100644 tests/functional/flakes/old-lockfiles.sh

diff --git a/src/libflake/flake.cc b/src/libflake/flake.cc
index 516b38131..987c9f610 100644
--- a/src/libflake/flake.cc
+++ b/src/libflake/flake.cc
@@ -595,7 +595,7 @@ LockedFlake lockFlake(
                                 oldLock = *oldLock3;
 
                     if (oldLock
-                        && oldLock->originalRef == *input.ref
+                        && oldLock->originalRef.canonicalize() == input.ref->canonicalize()
                         && oldLock->parentInputAttrPath == overridenParentPath
                         && !hasCliOverride)
                     {
diff --git a/src/libflake/flakeref.cc b/src/libflake/flakeref.cc
index a8b139d65..12bddf578 100644
--- a/src/libflake/flakeref.cc
+++ b/src/libflake/flakeref.cc
@@ -289,6 +289,55 @@ std::pair<ref<SourceAccessor>, FlakeRef> FlakeRef::lazyFetch(ref<Store> store) c
     return {accessor, FlakeRef(std::move(lockedInput), subdir)};
 }
 
+FlakeRef FlakeRef::canonicalize() const
+{
+    auto flakeRef(*this);
+
+    /* Backward compatibility hack: In old versions of Nix, if you had
+       a flake input like
+
+         inputs.foo.url = "git+https://foo/bar?dir=subdir";
+
+       it would result in a lock file entry like
+
+         "original": {
+           "dir": "subdir",
+           "type": "git",
+           "url": "https://foo/bar?dir=subdir"
+         }
+
+       New versions of Nix remove `?dir=subdir` from the `url` field,
+       since the subdirectory is intended for `FlakeRef`, not the
+       fetcher (and specifically the remote server), that is, the
+       flakeref is parsed into
+
+         "original": {
+           "dir": "subdir",
+           "type": "git",
+           "url": "https://foo/bar"
+         }
+
+       However, this causes new versions of Nix to consider the lock
+       file entry to be stale since the `original` ref no longer
+       matches exactly.
+
+       For this reason, we canonicalise the `original` ref by
+       filtering the `dir` query parameter from the URL. */
+    if (auto url = fetchers::maybeGetStrAttr(flakeRef.input.attrs, "url")) {
+        try {
+            auto parsed = parseURL(*url);
+            if (auto dir2 = get(parsed.query, "dir")) {
+                if (flakeRef.subdir != "" && flakeRef.subdir == *dir2)
+                    parsed.query.erase("dir");
+            }
+            flakeRef.input.attrs.insert_or_assign("url", parsed.to_string());
+        } catch (BadURL &) {
+        }
+    }
+
+    return flakeRef;
+}
+
 std::tuple<FlakeRef, std::string, ExtendedOutputsSpec> parseFlakeRefWithFragmentAndExtendedOutputsSpec(
     const fetchers::Settings & fetchSettings,
     const std::string & url,
diff --git a/src/libflake/include/nix/flake/flakeref.hh b/src/libflake/include/nix/flake/flakeref.hh
index 8c15f9d95..6184d2363 100644
--- a/src/libflake/include/nix/flake/flakeref.hh
+++ b/src/libflake/include/nix/flake/flakeref.hh
@@ -72,6 +72,12 @@ struct FlakeRef
         const fetchers::Attrs & attrs);
 
     std::pair<ref<SourceAccessor>, FlakeRef> lazyFetch(ref<Store> store) const;
+
+    /**
+     * Canonicalize a flakeref for the purpose of comparing "old" and
+     * "new" `original` fields in lock files.
+     */
+    FlakeRef canonicalize() const;
 };
 
 std::ostream & operator << (std::ostream & str, const FlakeRef & flakeRef);
diff --git a/tests/functional/flakes/meson.build b/tests/functional/flakes/meson.build
index 368c43876..213c388a6 100644
--- a/tests/functional/flakes/meson.build
+++ b/tests/functional/flakes/meson.build
@@ -32,6 +32,7 @@ suites += {
     'symlink-paths.sh',
     'debugger.sh',
     'source-paths.sh',
+    'old-lockfiles.sh',
   ],
   'workdir': meson.current_source_dir(),
 }
diff --git a/tests/functional/flakes/old-lockfiles.sh b/tests/functional/flakes/old-lockfiles.sh
new file mode 100644
index 000000000..fd36abdcc
--- /dev/null
+++ b/tests/functional/flakes/old-lockfiles.sh
@@ -0,0 +1,60 @@
+#!/usr/bin/env bash
+
+source ./common.sh
+
+requireGit
+
+repo="$TEST_ROOT/repo"
+
+createGitRepo "$repo"
+
+cat > "$repo/flake.nix" <<EOF
+{
+  inputs = {
+    dependency.url = "git+file:///no-such-path?dir=subdir";
+  };
+  outputs = { dependency, self }: {
+    hi = dependency.an_output;
+  };
+}
+EOF
+
+cat > "$repo/flake.lock" <<EOF
+{
+  "nodes": {
+    "dependency": {
+      "locked": {
+        "dir": "subdir",
+        "lastModified": 1746721011,
+        "narHash": "sha256-9aIDvIdyHAfQyvT5SwPgYxUUhf1GwQVAWq+qa5LcEQE=",
+        "ref": "refs/heads/master",
+        "rev": "432058dbfc82b0369bc9cce440e4af2aece52b54",
+        "revCount": 1,
+        "type": "git",
+        "url": "file:///no-such-path?dir=subdir"
+      },
+      "original": {
+        "dir": "subdir",
+        "type": "git",
+        "url": "file:///no-such-path?dir=subdir"
+      }
+    },
+    "root": {
+      "inputs": {
+        "dependency": "dependency"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}
+EOF
+
+git -C "$repo" add flake.nix flake.lock
+git -C "$repo" commit -a -m foo
+
+cp "$repo/flake.lock" "$repo/flake.lock.old"
+
+nix flake lock "$repo"
+
+cmp "$repo/flake.lock" "$repo/flake.lock.old"