Automatically do git/hg add on flake.lock

src/libexpr/flake/flake.cc

@@ -579,22 +579,23 @@ LockedFlake lockFlake(
 
                     newLockFile.write(path);
 
-                    // Rewriting the lockfile changed the top-level
-                    // repo, so we should re-read it.
+                    topRef.input->markChangedFile(
+                        (topRef.subdir == "" ? "" : topRef.subdir + "/") + "flake.lock");
+
+                    /* Rewriting the lockfile changed the top-level
+                       repo, so we should re-read it. FIXME: we could
+                       also just clear the 'rev' field... */
+                    auto prevLockedRef = flake.lockedRef;
                     FlakeCache dummyCache;
                     flake = getFlake(state, topRef, {}, lockFlags.useRegistries, dummyCache);
 
-                    if (flake.lockedRef.input->isImmutable())
+                    /* Make sure that we picked up the change,
+                       i.e. the tree should usually be dirty
+                       now. Corner case: we could have reverted from a
+                       dirty to a clean tree! */
+                    if (flake.lockedRef.input == prevLockedRef.input
+                        && !flake.lockedRef.input->isImmutable())
                         throw Error("'%s' did not change after I updated its 'flake.lock' file; is 'flake.lock' under version control?", flake.originalRef);
-
-                    #if 0
-                    // Hack: Make sure that flake.lock is visible to Git, so it ends up in the Nix store.
-                    runProgram("git", true,
-                        { "-C", *sourcePath, "add",
-                          "--force",
-                          "--intent-to-add",
-                          (topRef.subdir == "" ? "" : topRef.subdir + "/") + "flake.lock" });
-                    #endif
                 }
             } else
                 throw Error("cannot write modified lock file of flake '%s' (use '--no-write-lock-file' to ignore)", topRef);

src/libstore/fetchers/fetchers.hh

@@ -62,6 +62,9 @@ struct Input : std::enable_shared_from_this<Input>
 
     virtual std::optional<Path> getSourcePath() const { return {}; }
 
+    // FIXME: should merge with getSourcePath().
+    virtual void markChangedFile(std::string_view file) const { assert(false); }
+
     virtual void clone(const Path & destDir) const
     {
         throw Error("do not know how to clone input '%s'", to_string());

src/libstore/fetchers/git.cc

@@ -165,6 +165,18 @@ struct GitInput : Input
         return {};
     }
 
+    void markChangedFile(std::string_view file) const override
+    {
+        auto sourcePath = getSourcePath();
+        assert(sourcePath);
+        runProgram("git", true,
+            { "-C", *sourcePath, "add",
+              "--force",
+              "--intent-to-add",
+              std::string(file)
+            });
+    }
+
     std::pair<bool, std::string> getActualUrl() const
     {
         // Don't clone file:// URIs (but otherwise treat them the

src/libstore/fetchers/mercurial.cc

@@ -84,6 +84,17 @@ struct MercurialInput : Input
         return {};
     }
 
+    void markChangedFile(std::string_view file) const override
+    {
+        auto sourcePath = getSourcePath();
+        assert(sourcePath);
+        // FIXME: shut up if file is already tracked.
+        runProgram("hg", true,
+            { "add",
+              *sourcePath + "/" + std::string(file)
+            });
+    }
+
     std::pair<bool, std::string> getActualUrl() const
     {
         bool isLocal = url.scheme == "file";