fix(git): refuse remote switches that would silently detach HEAD

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

@@ -4939,6 +4939,58 @@ it.layer(layerTest)("GitVcsDriver core integration", (it) => {
         }),
     );
 
+    it.effect.each(["untracked", "other-remote"] as const)(
+      "refuses a remote switch that collides with a local branch (%s)",
+      (tracking) =>
+        Effect.gen(function* () {
+          const cwd = yield* makeTmpDir();
+          const { initialBranch } = yield* initRepoWithCommit(cwd);
+          const remote = yield* makeTmpDir("git-remote-");
+          yield* git(remote, ["init", "--bare"]);
+          yield* git(cwd, ["remote", "add", "upstream", remote]);
+          yield* git(cwd, ["push", "upstream", "HEAD:refs/heads/feature"]);
+          yield* git(cwd, ["fetch", "upstream"]);
+          yield* git(cwd, ["checkout", "--no-track", "-b", "feature"]);
+          yield* git(cwd, ["commit", "--allow-empty", "-m", "Local work"]);
+          if (tracking === "other-remote") {
+            yield* git(cwd, ["remote", "add", "origin", remote]);
+            yield* git(cwd, ["fetch", "origin"]);
+            yield* git(cwd, ["branch", "--set-upstream-to=origin/feature", "feature"]);
+          }
+          const featureHead = yield* git(cwd, ["rev-parse", "feature"]);
+          const upstream = yield* git(cwd, [
+            "for-each-ref",
+            "--format=%(upstream)",
+            "refs/heads/feature",
+          ]);
+          yield* git(cwd, ["checkout", initialBranch]);
+          const head = yield* git(cwd, ["rev-parse", "HEAD"]);
+          yield* writeTextFile(cwd, "README.md", "Uncommitted work\n");
+          const driver = yield* GitVcsDriver.GitVcsDriver;
+
+          const result = yield* driver
+            .switchRef({ cwd, refName: "upstream/feature" })
+            .pipe(Effect.result);
+          assert.isTrue(Result.isFailure(result));
+          assert.equal(yield* git(cwd, ["branch", "--show-current"]), initialBranch);
+          assert.equal(yield* git(cwd, ["rev-parse", "HEAD"]), head);
+          assert.equal(yield* git(cwd, ["rev-parse", "feature"]), featureHead);
+          assert.equal(
+            yield* git(cwd, ["for-each-ref", "--format=%(upstream)", "refs/heads/feature"]),
+            upstream,
+          );
+          assert.equal(
+            yield* (yield* FileSystem.FileSystem).readFileString(
+              (yield* Path.Path).join(cwd, "README.md"),
+            ),
+            "Uncommitted work\n",
+          );
+          // Selecting the existing local branch remains an explicit, attached checkout.
+          assert.equal((yield* driver.switchRef({ cwd, refName: "feature" })).refName, "feature");
+          assert.equal(yield* git(cwd, ["symbolic-ref", "HEAD"]), "refs/heads/feature");
+        }),
+    );
+
     it.effect("rejects a missing branch without restoring a matching dirty file", () =>
       Effect.gen(function* () {
         const cwd = yield* makeTmpDir();

apps/server/src/vcs/GitVcsDriverCore.ts

@@ -5309,15 +5309,13 @@ export const makeGitVcsDriverCore = Effect.fn("makeGitVcsDriverCore")(function*
 
       const checkoutArgs = localInputExists
         ? ["checkout", input.refName]
-        : remoteExists && !localTrackingBranch && localTrackedBranchTargetExists
-          ? ["checkout", input.refName]
-          : newTrackingBranch
-            ? ["checkout", newTrackingBranch]
-            : remoteExists && !localTrackingBranch
-              ? ["checkout", "--track", input.refName]
-              : remoteExists && localTrackingBranch
-                ? ["checkout", localTrackingBranch]
-                : ["checkout", input.refName];
+        : newTrackingBranch
+          ? ["checkout", newTrackingBranch]
+          : remoteExists && !localTrackingBranch
+            ? ["checkout", "--track", input.refName]
+            : remoteExists && localTrackingBranch
+              ? ["checkout", localTrackingBranch]
+              : ["checkout", input.refName];
 
       yield* refuseOptionLikeRefName("GitVcsDriver.switchRef", input.cwd, checkoutArgs.at(-1));