Hold a task's merge while another task in the project is merging

internal/agents/manager_test.go

@@ -917,6 +917,75 @@ func TestMergeFetchesMovedRemote(t *testing.T) {
 	}
 }
 
+func TestMergeHeldWhileAnotherMerges(t *testing.T) {
+	t.Parallel()
+	e := newEnv(t)
+	m := e.manager()
+	second := e.addTask()
+
+	for i, task := range []api.Task{e.task, second} {
+		_, err := m.Create(e.ctx, task, e.user, fmt.Sprintf("Create a file task%d.txt containing exactly the word task.", i), testModel)
+		if err != nil {
+			t.Fatalf("create session: %v", err)
+		}
+		e.waitIdle()
+	}
+
+	first, err := m.Merge(e.ctx, e.task, e.project, e.user)
+	if err != nil {
+		t.Fatalf("merge first: %v", err)
+	}
+	if first.MergeQueuedAt != nil {
+		t.Fatalf("first merge held: %+v", first)
+	}
+	held, err := m.Merge(e.ctx, second, e.project, e.user)
+	if err != nil {
+		t.Fatalf("merge second: %v", err)
+	}
+	if held.MergeQueuedAt == nil {
+		t.Fatalf("second merge not held: %+v", held)
+	}
+	_, err = m.Merge(e.ctx, held, e.project, e.user)
+	if !errors.Is(err, agents.ErrConflict) {
+		t.Fatalf("merging a held task again: %v, want conflict", err)
+	}
+
+	events := e.waitFor("second task merged", func(ev api.Event) bool {
+		return ev.Kind == "task.merged" && *ev.TaskId == second.Id
+	})
+	firstMerged, unqueued := -1, -1
+	for i, ev := range events {
+		switch {
+		case ev.Kind == "task.merged" && *ev.TaskId == e.task.Id:
+			firstMerged = i
+		case ev.Kind == "task.merge_unqueued" && *ev.TaskId == second.Id:
+			unqueued = i
+		case ev.Kind == "inbox.created" && ev.Payload["kind"] == string(api.SessionError):
+			t.Fatalf("merge filed an error: %v", ev.Payload)
+		}
+	}
+	if firstMerged < 0 || unqueued < firstMerged {
+		t.Fatalf("held merge started at event %d, first merged at %d", unqueued, firstMerged)
+	}
+
+	tasks := map[string]api.Task{}
+	for _, id := range []string{e.task.Id, second.Id} {
+		tasks[id], err = e.store.GetTask(e.ctx, id)
+		if err != nil {
+			t.Fatalf("get task: %v", err)
+		}
+		if tasks[id].Status != api.TaskStatusMerged || tasks[id].MergeCommit == nil || tasks[id].MergeQueuedAt != nil {
+			t.Fatalf("task after merge %+v", tasks[id])
+		}
+	}
+	if head := gitOut(t, e.remote, "rev-parse", "main"); head != *tasks[second.Id].MergeCommit {
+		t.Fatalf("remote main at %s, want second merge commit %s", head, *tasks[second.Id].MergeCommit)
+	}
+	if parent := gitOut(t, e.remote, "rev-parse", "main^"); parent != *tasks[e.task.Id].MergeCommit {
+		t.Fatalf("second merge commit parent %s, want first merge commit %s", parent, *tasks[e.task.Id].MergeCommit)
+	}
+}
+
 func TestCreateTaskTool(t *testing.T) {
 	t.Parallel()
 	e := newEnv(t)