feat: Add release-please tag to release PRs so louhi flow will get triggered (#2050)

fixes #2038 

Tested with

```
go run github.com/googleapis/librarian/cmd/librarian release tag-and-release -repo=/Users/ldetmer/workspace2/librarian -pr=ldetmer#1
```


ldetmer@39ab425
contains tag release-please-1

Author: ldetmer

internal/github/github.go

@@ -312,3 +312,16 @@ func (c *Client) FindMergedPullRequestsWithPendingReleaseLabel(ctx context.Conte
 	}
 	return allPRs, nil
 }
+
+// CreateTag creates a lightweight tag in the repository at the given commit SHA.
+// This does NOT create a release, just the tag.
+func (c *Client) CreateTag(ctx context.Context, tagName, commitSHA string) error {
+	slog.Info("Creating tag", "tag", tagName, "commit", commitSHA)
+	ref := "refs/tags/" + tagName
+	tagRef := &github.Reference{
+		Ref:    github.Ptr(ref),
+		Object: &github.GitObject{SHA: github.Ptr(commitSHA), Type: github.Ptr("commit")},
+	}
+	_, _, err := c.Git.CreateRef(ctx, c.repo.Owner, c.repo.Name, tagRef)
+	return err
+}

internal/github/github_test.go

@@ -1081,3 +1081,67 @@ func TestFindMergedPullRequestsWithPendingReleaseLabel(t *testing.T) {
 		})
 	}
 }
+func TestCreateTag(t *testing.T) {
+	t.Parallel()
+	for _, test := range []struct {
+		name      string
+		tagName   string
+		commitSHA string
+		handler   http.HandlerFunc
+		wantErr   bool
+	}{
+		{
+			name:      "Success",
+			tagName:   "v1.2.3",
+			commitSHA: "abcdef123456",
+			handler: func(w http.ResponseWriter, r *http.Request) {
+				if r.Method != http.MethodPost {
+					t.Errorf("unexpected method: got %s, want %s", r.Method, http.MethodPost)
+				}
+				wantPath := "/repos/owner/repo/git/refs"
+				if r.URL.Path != wantPath {
+					t.Errorf("unexpected path: got %s, want %s", r.URL.Path, wantPath)
+				}
+
+				var ref github.Reference
+				if err := json.NewDecoder(r.Body).Decode(&ref); err != nil {
+					t.Fatalf("failed to decode request body: %v", err)
+				}
+				if ref.Ref == nil || *ref.Ref != "refs/tags/v1.2.3" {
+					t.Errorf("unexpected ref: got %v, want %s", ref.Ref, "refs/tags/v1.2.3")
+				}
+				fmt.Fprint(w, `{"ref": "refs/tags/v1.2.3"}`)
+			},
+		},
+		{
+			name:      "API Error",
+			tagName:   "v1.2.3",
+			commitSHA: "abcdef123456",
+			handler:   func(w http.ResponseWriter, r *http.Request) { w.WriteHeader(http.StatusInternalServerError) },
+			wantErr:   true,
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			server := httptest.NewServer(test.handler)
+			defer server.Close()
+
+			repo := &Repository{Owner: "owner", Name: "repo"}
+			client, err := newClientWithHTTP("fake-token", repo, server.Client())
+			if err != nil {
+				t.Fatalf("newClientWithHTTP() error = %v", err)
+			}
+			client.BaseURL, _ = url.Parse(server.URL + "/")
+
+			err = client.CreateTag(context.Background(), test.tagName, test.commitSHA)
+
+			if test.wantErr {
+				if err == nil {
+					t.Errorf("CreateTag() err = nil, expected error")
+				}
+			} else if err != nil {
+				t.Errorf("CreateTag() err = %v, want nil", err)
+			}
+		})
+	}
+}

internal/librarian/librarian.go

@@ -115,6 +115,7 @@ type GitHubClient interface {
 	GetPullRequest(ctx context.Context, number int) (*github.PullRequest, error)
 	CreateRelease(ctx context.Context, tagName, name, body, commitish string) (*github.RepositoryRelease, error)
 	CreateIssueComment(ctx context.Context, number int, comment string) error
+	CreateTag(ctx context.Context, tag, commitish string) error
 }
 
 // ContainerClient is an abstraction over the Docker client.

internal/librarian/mocks_test.go

@@ -40,13 +40,15 @@ type mockGitHubClient struct {
 	searchPullRequestsCalls int
 	getPullRequestCalls     int
 	createReleaseCalls      int
+	createTagCalls          int
 	createPullRequestErr    error
 	addLabelsToIssuesErr    error
 	getLabelsErr            error
 	replaceLabelsErr        error
 	searchPullRequestsErr   error
 	getPullRequestErr       error
 	createReleaseErr        error
+	createTagErr            error
 	createdPR               *github.PullRequestMetadata
 	labels                  []string
 	pullRequests            []*github.PullRequest
@@ -97,6 +99,11 @@ func (m *mockGitHubClient) CreateRelease(ctx context.Context, tagName, releaseNa
 	return m.createdRelease, m.createReleaseErr
 }
 
+func (m *mockGitHubClient) CreateTag(ctx context.Context, tagName, commitish string) error {
+	m.createTagCalls++
+	return m.createTagErr
+}
+
 // mockContainerClient is a mock implementation of the ContainerClient interface for testing.
 type mockContainerClient struct {
 	ContainerClient

internal/librarian/tag_and_release.go

@@ -174,9 +174,17 @@ func (r *tagAndReleaseRunner) processPullRequest(ctx context.Context, p *github.
 		slog.Warn("no release details found in pull request body, skipping")
 		return nil
 	}
+
+	// Add a tag to the release commit to trigger louhi flow: "release-please-{pr number}"
+	//TODO: remove this logic as part of https://github.com/googleapis/librarian/issues/2044
+	commitSha := p.GetMergeCommitSHA()
+	tagName := fmt.Sprintf("release-please-%d", p.GetNumber())
+	if err := r.ghClient.CreateTag(ctx, tagName, commitSha); err != nil {
+		return fmt.Errorf("failed to create tag %s: %w", tagName, err)
+	}
+
 	for _, release := range releases {
 		slog.Info("creating release", "library", release.Library, "version", release.Version)
-		commitish := p.GetMergeCommitSHA()
 
 		lib := r.state.LibraryByID(release.Library)
 		if lib == nil {
@@ -186,7 +194,7 @@ func (r *tagAndReleaseRunner) processPullRequest(ctx context.Context, p *github.
 		// Create the release.
 		tagName := formatTag(lib, release.Version)
 		releaseName := fmt.Sprintf("%s %s", release.Library, release.Version)
-		if _, err := r.ghClient.CreateRelease(ctx, tagName, releaseName, release.Body, commitish); err != nil {
+		if _, err := r.ghClient.CreateRelease(ctx, tagName, releaseName, release.Body, commitSha); err != nil {
 			return fmt.Errorf("failed to create release: %w", err)
 		}
 

internal/librarian/tag_and_release_test.go

@@ -374,6 +374,7 @@ func TestProcessPullRequest(t *testing.T) {
 		wantErrMsg             string
 		wantCreateReleaseCalls int
 		wantReplaceLabelsCalls int
+		wantCreateTagCalls     int
 	}{
 		{
 			name:                   "happy path",
@@ -382,6 +383,7 @@ func TestProcessPullRequest(t *testing.T) {
 			state:                  state,
 			wantCreateReleaseCalls: 1,
 			wantReplaceLabelsCalls: 1,
+			wantCreateTagCalls:     1,
 		},
 		{
 			name:     "no release details",
@@ -405,6 +407,7 @@ func TestProcessPullRequest(t *testing.T) {
 			state:                  state,
 			wantErrMsg:             "failed to create release",
 			wantCreateReleaseCalls: 1,
+			wantCreateTagCalls:     1,
 		},
 		{
 			name: "replace labels fails",
@@ -416,6 +419,17 @@ func TestProcessPullRequest(t *testing.T) {
 			wantErrMsg:             "failed to replace labels",
 			wantCreateReleaseCalls: 1,
 			wantReplaceLabelsCalls: 1,
+			wantCreateTagCalls:     1,
+		},
+		{
+			name: "create tag fails",
+			pr:   prWithRelease,
+			ghClient: &mockGitHubClient{
+				createTagErr: errors.New("create tag error"),
+			},
+			state:              state,
+			wantErrMsg:         "failed to create tag",
+			wantCreateTagCalls: 1,
 		},
 	} {
 		t.Run(test.name, func(t *testing.T) {