Show the merge prompt before fetching so Merge has no blank pause

internal/agents/mergein.go

@@ -117,12 +117,7 @@ func (m *Manager) projectMerge(projectID string) *projectMerge {
 }
 
 func (m *Manager) startMerge(ctx context.Context, pm *projectMerge, task api.Task, project api.Project, user api.User) error {
-	err := git.FetchFastForward(ctx, project.RepoPath, project.DefaultBranch)
-	if err != nil {
-		return err
-	}
-
-	err = m.store.SetMergingBy(ctx, task.Id, user.Login)
+	err := m.store.SetMergingBy(ctx, task.Id, user.Login)
 	if errors.Is(err, store.ErrNotFound) {
 		return fmt.Errorf("%w: task %s is being discarded", ErrConflict, task.Id)
 	}
@@ -131,13 +126,83 @@ func (m *Manager) startMerge(ctx context.Context, pm *projectMerge, task api.Tas
 	}
 	pm.holder = task.Id
 
-	prompt := mergeInPrompt(project.DefaultBranch) + fmt.Sprintf(" When this turn ends, byre squashes this branch onto %s and pushes it; if you stop, call stop_merge first so it doesn't. ", project.DefaultBranch) + summaryRequest
-	_, err = m.sendMergeIn(ctx, task, user, prompt)
+	sess, err := m.mergeSession(ctx, task, user)
 	if err != nil {
 		m.takeMerge(task.Id)
 		pm.holder = ""
+		return err
 	}
-	return err
+
+	prompt := mergeInPrompt(project.DefaultBranch) + fmt.Sprintf(" When this turn ends, byre squashes this branch onto %s and pushes it; if you stop, call stop_merge first so it doesn't. ", project.DefaultBranch) + summaryRequest
+	go m.launchMerge(sess, task, project, prompt)
+	return nil
+}
+
+func (m *Manager) mergeSession(ctx context.Context, task api.Task, user api.User) (api.AgentSession, error) {
+	sessions, err := m.store.ListSessions(ctx, task.Id)
+	if err != nil {
+		return api.AgentSession{}, err
+	}
+
+	sess := api.AgentSession{}
+	for _, s := range sessions {
+		if s.State != api.AgentSessionStateCompleted {
+			sess = s
+		}
+	}
+	if sess.Id == "" {
+		sess, err = m.newSession(ctx, task, user, "")
+		if err != nil {
+			return api.AgentSession{}, err
+		}
+	}
+
+	err = transition(ctx, m.store, sess.Id, api.AgentSessionStateIdle, api.AgentSessionStateRunning, "byre")
+	return sess, err
+}
+
+func (m *Manager) launchMerge(sess api.AgentSession, task api.Task, project api.Project, prompt string) {
+	ctx := context.Background()
+	err := m.deliverMerge(ctx, sess, task, project, prompt)
+	if err == nil {
+		return
+	}
+	slog.Error("start merge turn", "task", task.Id, "err", err)
+	m.failStart(ctx, sess.Id, "byre", err)
+	m.abandonMerge(task.Id, project.Id)
+}
+
+func (m *Manager) deliverMerge(ctx context.Context, sess api.AgentSession, task api.Task, project api.Project, prompt string) error {
+	l, err := m.ensureLive(ctx, sess, task)
+	if err != nil {
+		return err
+	}
+	err = m.syncModel(ctx, l, sess.Model)
+	if err != nil {
+		return err
+	}
+	_, err = m.prepareMessage(ctx, l, "byre", prompt, nil)
+	if err != nil {
+		return err
+	}
+	m.mu.Lock()
+	l.fetching = true
+	m.mu.Unlock()
+
+	err = git.FetchFastForward(ctx, project.RepoPath, project.DefaultBranch)
+
+	m.mu.Lock()
+	l.fetching = false
+	interrupted := l.interrupted
+	m.mu.Unlock()
+
+	if err != nil {
+		return err
+	}
+	if interrupted {
+		return errors.New("interrupted before the merge started")
+	}
+	return l.sess.send(ctx, prompt, nil)
 }
 
 func (m *Manager) recoverMerges(ctx context.Context) error {