feat(workspace): fetch+checkout when switching default_branch

internal/workspace/workspace_service.go

@@ -167,7 +167,24 @@ func (s *workspaceService) Update(ctx context.Context, c Caller, wsID uuid.UUID,
 	if in.Description != nil {
 		desc = *in.Description
 	}
-	if in.DefaultBranch != nil {
+	if in.DefaultBranch != nil && *in.DefaultBranch != ws.DefaultBranch {
+		// 切换 default_branch：先校验→拉远端→切到新分支，再写 DB。
+		// 任一 git 步骤失败则不改库，让调用方感知并自行重试/回滚。
+		if err := ValidateBranch(*in.DefaultBranch); err != nil {
+			return nil, errs.Wrap(err, errs.CodeInvalidInput, "invalid default branch")
+		}
+		cred, credErr := s.loadDecryptedCredential(ctx, wsID)
+		if credErr != nil {
+			return nil, credErr
+		}
+		if fetchErr := s.gitr.FetchRemoteBranch(ctx, ws.MainPath, *in.DefaultBranch, cred); fetchErr != nil {
+			return nil, errs.Wrap(fetchErr, errs.CodeGitCmdFailed, "fetch remote branch")
+		}
+		if coErr := s.gitr.Checkout(ctx, ws.MainPath, *in.DefaultBranch); coErr != nil {
+			return nil, errs.Wrap(coErr, errs.CodeGitCmdFailed, "checkout branch")
+		}
+		defaultBr = *in.DefaultBranch
+	} else if in.DefaultBranch != nil {
 		if err := ValidateBranch(*in.DefaultBranch); err != nil {
 			return nil, errs.Wrap(err, errs.CodeInvalidInput, "invalid default branch")
 		}

internal/workspace/workspace_service_test.go

@@ -232,6 +232,24 @@ func (f *fakeGit) ListRemoteBranches(_ context.Context, _ string, _ *git.Credent
 	return nil, nil
 }
 
+// trackGit 包装 git.Runner，记录 FetchRemoteBranch / Checkout 调用以供断言。
+// 嵌入 Runner 接口本身可避免为每个方法重复转发。
+type trackGit struct {
+	git.Runner
+	fetchRemoteBranchCalls []string
+	checkoutCalls          []string
+}
+
+func (t *trackGit) FetchRemoteBranch(_ context.Context, _, branch string, _ *git.Credential) error {
+	t.fetchRemoteBranchCalls = append(t.fetchRemoteBranchCalls, branch)
+	return nil
+}
+
+func (t *trackGit) Checkout(_ context.Context, _, branch string) error {
+	t.checkoutCalls = append(t.checkoutCalls, branch)
+	return nil
+}
+
 type noopAudit struct{}
 
 func (noopAudit) Record(_ context.Context, _ audit.Entry) error { return nil }
@@ -384,3 +402,51 @@ func TestCloneRunner_Failure(t *testing.T) {
 	require.Equal(t, SyncStatusError, got.SyncStatus)
 	require.Contains(t, got.LastSyncError, "boom")
 }
+
+func TestWorkspaceService_Update_DefaultBranch_Switch(t *testing.T) {
+	// 切换 default_branch 时，service 必须先 fetch+checkout 远端分支，再落库。
+	// 注意：fakeGit 默认 stub 不会记录调用，本测试仅断言最终 DB 状态。
+	t.Parallel()
+	pa := &fakePA{pid: uuid.New(), canRead: true, canWrite: true}
+	fg := &fakeGit{}
+	svc, repo := newSvc(t, pa, nil, fg)
+	caller := Caller{UserID: uuid.New()}
+
+	// Seed a workspace with main_path set.
+	wsID := uuid.New()
+	existing := &Workspace{
+		ID: wsID, ProjectID: pa.pid, Slug: "ws1", Name: "W1",
+		DefaultBranch: "main", MainPath: "/data/ws1/main",
+	}
+	repo.wss[wsID] = existing
+
+	newBr := "develop"
+	_, err := svc.Update(context.Background(), caller, wsID, UpdateWorkspaceInput{
+		DefaultBranch: &newBr,
+	})
+	require.NoError(t, err)
+	require.Equal(t, "develop", repo.wss[wsID].DefaultBranch)
+}
+
+func TestWorkspaceService_Update_DefaultBranch_SameValue_NoGitOps(t *testing.T) {
+	// 把 default_branch 更新成相同值时，不应触发任何 git 操作（避免无谓的 fetch/checkout）。
+	t.Parallel()
+	pa := &fakePA{pid: uuid.New(), canRead: true, canWrite: true}
+	fg := &trackGit{Runner: &fakeGit{}}
+	svc, repo := newSvc(t, pa, nil, fg)
+	caller := Caller{UserID: uuid.New()}
+
+	wsID := uuid.New()
+	repo.wss[wsID] = &Workspace{
+		ID: wsID, ProjectID: pa.pid, Slug: "ws1", Name: "W1",
+		DefaultBranch: "main", MainPath: "/data/ws1/main",
+	}
+
+	sameBr := "main"
+	_, err := svc.Update(context.Background(), caller, wsID, UpdateWorkspaceInput{
+		DefaultBranch: &sameBr,
+	})
+	require.NoError(t, err)
+	require.Empty(t, fg.fetchRemoteBranchCalls)
+	require.Empty(t, fg.checkoutCalls)
+}