fix(server): background status fetch no longer breaks a concurrent git pull

apps/server/src/vcs/GitVcsDriverCore.test.ts

@@ -4097,6 +4097,37 @@ it.layer(layerTest)("GitVcsDriver core integration", (it) => {
       }),
     );
 
+    it.effect("leaves FETCH_HEAD to the user's own pulls in background upstream fetches", () =>
+      Effect.gen(function* () {
+        const cwd = yield* makeTmpDir();
+        const remote = yield* makeTmpDir("git-vcs-driver-remote-");
+        const updater = yield* makeTmpDir("git-vcs-driver-updater-");
+        const { initialBranch } = yield* initRepoWithCommit(cwd);
+        yield* git(remote, ["init", "--bare"]);
+        yield* git(cwd, ["remote", "add", "origin", remote]);
+        yield* git(cwd, ["push", "-u", "origin", initialBranch]);
+
+        yield* git(updater, ["clone", remote, "."]);
+        yield* git(updater, ["config", "user.email", "test@test.com"]);
+        yield* git(updater, ["config", "user.name", "Test"]);
+        yield* writeTextFile(updater, "remote.txt", "remote\n");
+        yield* git(updater, ["add", "remote.txt"]);
+        yield* git(updater, ["commit", "-m", "remote commit"]);
+        yield* git(updater, ["push", "origin", initialBranch]);
+        const fetchHeadPath = (yield* Path.Path).join(cwd, ".git", "FETCH_HEAD");
+        assert.equal(NodeFS.existsSync(fetchHeadPath), false);
+
+        const status = yield* (yield* GitVcsDriver.GitVcsDriver).statusDetailsRemote(cwd);
+
+        assert.equal(status.behindCount, 1);
+        assert.equal(
+          NodeFS.existsSync(fetchHeadPath),
+          false,
+          "background status fetch wrote .git/FETCH_HEAD, which an overlapping `git pull` merges from",
+        );
+      }),
+    );
+
     it.effect("uses origin HEAD for default-branch detection with a non-origin upstream", () =>
       Effect.gen(function* () {
         const cwd = yield* makeTmpDir();

apps/server/src/vcs/GitVcsDriverCore.ts

@@ -1363,6 +1363,8 @@ export const makeGitVcsDriverCore = Effect.fn("makeGitVcsDriverCore")(function*
       "--quiet",
       "--no-tags",
       "--no-auto-gc",
+      // FETCH_HEAD is written without a lock; overlapping a user's `git pull` makes that pull fail.
+      "--no-write-fetch-head",
       "--",
       remoteName,
     ];