Add GitHub action build reporter

cmd/build.go

@@ -174,6 +174,7 @@ func addBuildFlags(cmd *cobra.Command) {
 	cmd.Flags().StringToString("docker-build-options", nil, "Options passed to all 'docker build' commands")
 	cmd.Flags().String("report", "", "Generate a HTML report after the build has finished. (e.g. --report myreport.html)")
 	cmd.Flags().String("report-segment", os.Getenv("LEEWAY_SEGMENT_KEY"), "Report build events to segment using the segment key (defaults to $LEEWAY_SEGMENT_KEY)")
+	cmd.Flags().Bool("report-github", os.Getenv("GITHUB_OUTPUT") != "", "Report package build success/failure to GitHub Actions using the GITHUB_OUTPUT environment variable")
 }
 
 func getBuildOpts(cmd *cobra.Command) ([]leeway.BuildOption, *leeway.FilesystemCache) {
@@ -255,6 +256,11 @@ func getBuildOpts(cmd *cobra.Command) ([]leeway.BuildOption, *leeway.FilesystemC
 	} else if segmentkey != "" {
 		reporter = append(reporter, leeway.NewSegmentReporter(segmentkey))
 	}
+	if github, err := cmd.Flags().GetBool("report-github"); err != nil {
+		log.Fatal(err)
+	} else if github {
+		reporter = append(reporter, leeway.NewGitHubReporter())
+	}
 
 	dontTest, err := cmd.Flags().GetBool("dont-test")
 	if err != nil {

pkg/leeway/reporter.go

@@ -621,3 +621,35 @@ func (sr *SegmentReporter) track(event string, props segment.Properties) {
 		log.WithField("evt", string(fc)).Debug("reported segment event")
 	}
 }
+
+func NewGitHubReporter() *GitHubActionReporter {
+	return &GitHubActionReporter{}
+}
+
+type GitHubActionReporter struct {
+	NoopReporter
+
+	mu sync.Mutex
+}
+
+func (sr *GitHubActionReporter) PackageBuildFinished(pkg *Package, rep *PackageBuildReport) {
+	sr.mu.Lock()
+	defer sr.mu.Unlock()
+
+	fn, ok := os.LookupEnv("GITHUB_OUTPUT")
+	if !ok || fn == "" {
+		return
+	}
+	f, err := os.OpenFile(fn, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
+	if err != nil {
+		log.WithField("fn", fn).WithError(err).Warn("cannot open GITHUB_OUTPUT file")
+		return
+	}
+	defer f.Close()
+
+	var success bool
+	if rep.Error == nil {
+		success = true
+	}
+	fmt.Fprintf(f, "%s=%v\n", pkg.FilesystemSafeName(), success)
+}