fix(librarian): use templates for update-image PR body (#2602)

Fixes #2587

Author: chingor13

internal/librarian/update_image.go

@@ -15,10 +15,13 @@
 package librarian
 
 import (
+	"bytes"
 	"context"
 	"fmt"
+	"html/template"
 	"log/slog"
 	"path/filepath"
+	"strings"
 
 	"github.com/googleapis/librarian/internal/config"
 	"github.com/googleapis/librarian/internal/gitrepo"
@@ -126,18 +129,8 @@ func (r *updateImageRunner) run(ctx context.Context) error {
 	}
 	slog.Info("successful generations", slog.Int("num", len(successfulGenerations)))
 
-	// TODO(#2587): improve PR body content
 	prBodyBuilder := func() (string, error) {
-		body := fmt.Sprintf("feat: update image to %s\n\n", r.image)
-		if len(failedGenerations) > 0 {
-			body += "The following libraries failed to regenerate:\n"
-			for _, lib := range failedGenerations {
-				body += fmt.Sprintf("- %s\n", lib.ID)
-			}
-		} else {
-			body += "All libraries regenerated successfully."
-		}
-		return body, nil
+		return formatUpdateImagePRBody(r.image, failedGenerations)
 	}
 	commitMessage := fmt.Sprintf("feat: update image to %s", r.image)
 	// TODO(#2588): open PR as draft if there are failures
@@ -185,3 +178,33 @@ func (r *updateImageRunner) regenerateSingleLibrary(ctx context.Context, library
 
 	return nil
 }
+
+var updateImageTemplate = template.Must(template.New("updateImage").Parse(`feat: update image to {{.Image}}
+{{ if .FailedLibraries }}
+## Generation failed for
+{{- range .FailedLibraries }}
+- {{ . }}
+{{- end -}}
+{{- end }}
+`))
+
+type updateImagePRBody struct {
+	Image           string
+	FailedLibraries []string
+}
+
+func formatUpdateImagePRBody(image string, failedGenerations []*config.LibraryState) (string, error) {
+	failedLibraries := make([]string, 0, len(failedGenerations))
+	for _, failedGeneration := range failedGenerations {
+		failedLibraries = append(failedLibraries, failedGeneration.ID)
+	}
+	data := &updateImagePRBody{
+		Image:           image,
+		FailedLibraries: failedLibraries,
+	}
+	var out bytes.Buffer
+	if err := updateImageTemplate.Execute(&out, data); err != nil {
+		return "", fmt.Errorf("error executing template %w", err)
+	}
+	return strings.TrimSpace(out.String()), nil
+}

internal/librarian/update_image_test.go

@@ -760,3 +760,57 @@ func TestUpdateImageRunnerRun(t *testing.T) {
 		})
 	}
 }
+
+func TestFormatUpdateImagePRBody(t *testing.T) {
+	t.Parallel()
+	for _, test := range []struct {
+		name              string
+		image             string
+		failedGenerations []*config.LibraryState
+		want              string
+		wantErr           bool
+		wantErrMsg        string
+	}{
+		{
+			name:  "success",
+			image: "some-image",
+			want:  "feat: update image to some-image",
+		},
+		{
+			name:  "multiple errors",
+			image: "some-image",
+			failedGenerations: []*config.LibraryState{
+				{
+					ID: "library-id-1",
+				},
+				{
+					ID: "library-id-2",
+				},
+			},
+			want: `feat: update image to some-image
+
+## Generation failed for
+- library-id-1
+- library-id-2`,
+		},
+	} {
+		t.Run(test.name, func(t *testing.T) {
+			t.Parallel()
+			got, err := formatUpdateImagePRBody(test.image, test.failedGenerations)
+
+			if test.wantErr {
+				if err == nil {
+					t.Fatalf("formatUpdateImagePRBody() error = %v, wantErr %v", err, test.wantErr)
+				}
+
+				if !strings.Contains(err.Error(), test.wantErrMsg) {
+					t.Fatalf("want error message: %s, got: %s", test.wantErrMsg, err.Error())
+				}
+				return
+			}
+			if diff := cmp.Diff(got, test.want); diff != "" {
+				t.Errorf("%s: formatUpdateImagePRBody() mismatch (-want +got):%s", test.name, diff)
+			}
+		})
+	}
+}