bot: delete backport branches when a backport is merged (#14068)

The backport workflow runs when any PR is merged to any banch.
Prior to this change, the workflow was a no-op if the target branch
was a release branch.

Update the workflow so that when it runs on merged backports
it deletes the auto-generated backport branch.

Also fixes #12412

.github/workflows/robot/internal/bot/backport.go

@@ -50,6 +50,17 @@ func (b *Bot) Backport(ctx context.Context) error {
 		return trace.Wrap(err)
 	}
 
+	// If this workflow is running on a release branch, then it means
+	// a backport was merged and there's no need to open additional
+	// backport PRs. We can safely delete the remote branch though,
+	// because we know this is an internal contributor's merged PR.
+	if isReleaseBranch(b.c.Environment.UnsafeBase) &&
+		isBotBackportBranch(b.c.Environment.UnsafeHead) &&
+		!pull.Fork {
+		log.Printf("backport merged to %v, deleting branch %v", b.c.Environment.UnsafeBase, b.c.Environment.UnsafeHead)
+		return trace.Wrap(git("push", "origin", "--delete", b.c.Environment.UnsafeHead))
+	}
+
 	// Extract backport branches names from labels attached to the Pull
 	// Request. If no backports were requested, return right away.
 	branches := findBranches(pull.UnsafeLabels)

.github/workflows/robot/internal/bot/bot.go

@@ -151,3 +151,11 @@ func isAutoGeneratedFile(name string) bool {
 		strings.HasSuffix(name, "_pb.d.ts") ||
 		strings.Contains(name, "webassets/")
 }
+
+func isReleaseBranch(branch string) bool {
+	return strings.HasPrefix(branch, "branch/")
+}
+
+func isBotBackportBranch(branch string) bool {
+	return strings.HasPrefix(branch, "bot/backport")
+}

.github/workflows/robot/internal/bot/label.go

@@ -61,7 +61,7 @@ func (b *Bot) labels(ctx context.Context, files []github.PullRequestFile) ([]str
 	var labels []string
 
 	// The branch name is unsafe, but here we are simply adding a label.
-	if strings.HasPrefix(b.c.Environment.UnsafeHead, "branch/") {
+	if isReleaseBranch(b.c.Environment.UnsafeBase) {
 		log.Println("Label: Found backport branch.")
 		labels = append(labels, "backport")
 	}