fix(internal/librarian): generate should fail with error if all libraries failed (#1876)

Fixes #1787

Author: zhumin8

internal/librarian/generate.go

@@ -141,13 +141,18 @@ func (r *generateRunner) run(ctx context.Context) error {
 		}
 		prBody += fmt.Sprintf("feat: generated %s\n", libraryID)
 	} else {
+		failedGenerations := 0
 		for _, library := range r.state.Libraries {
 			if err := r.generateSingleLibrary(ctx, library.ID, outputDir); err != nil {
 				// TODO(https://github.com/googleapis/librarian/issues/983): record failure and report in PR body when applicable
 				slog.Error("failed to generate library", "id", library.ID, "err", err)
 				prBody += fmt.Sprintf("%s failed to generate\n", library.ID)
+				failedGenerations++
 			}
 		}
+		if failedGenerations > 0 && failedGenerations == len(r.state.Libraries) {
+			return fmt.Errorf("all %d libraries failed to generate", failedGenerations)
+		}
 	}
 
 	if err := saveLibrarianState(r.repo.GetDir(), r.state); err != nil {

internal/librarian/generate_test.go

@@ -829,6 +829,32 @@ func TestGenerateScenarios(t *testing.T) {
 			wantGenerateCalls: 2,
 			wantBuildCalls:    1,
 		},
+		{
+			name: "generate all, all fail should report error",
+			repo: newTestGitRepo(t),
+			state: &config.LibrarianState{
+				Image: "gcr.io/test/image:v1.2.3",
+				Libraries: []*config.LibraryState{
+					{
+						ID:   "lib1",
+						APIs: []*config.API{{Path: "some/api1"}},
+						SourceRoots: []string{
+							"src/a",
+						},
+					},
+				},
+			},
+			container: &mockContainerClient{
+				failGenerateForID: "lib1",
+				generateErrForID:  errors.New("generate error"),
+			},
+			ghClient:          &mockGitHubClient{},
+			build:             true,
+			wantErr:           true,
+			wantErrMsg:        "all 1 libraries failed to generate",
+			wantGenerateCalls: 1,
+			wantBuildCalls:    0,
+		},
 		{
 			name: "generate skips libraries with no APIs",
 			repo: newTestGitRepo(t),