feat(dev-infra): support user-failures when computing branches for target label (#38223)

The merge tool provides a way for configurations to determine the branches
for a label lazily. This is supported because it allows labels to respect
the currently selected base branch through the Github UI. e.g. if `target: label`
is applied on a PR and the PR is based on the patch branch, then the change
could only go into the selected target branch, while if it would be based on
`master`, the change would be cherry-picked to `master` too. This allows for
convenient back-porting of changes if they did not apply cleanly to the primary
development branch (`master`).

We want to expand this function so that it is possible to report failures if an
invalid target label is appplied (e.g. `target: major` not allowed in
some situations), or if the Github base branch is not valid for the given target
label (e.g. if `target: lts` is used, but it's not based on a LTS branch).

PR Close #38223

dev-infra/pr/merge/config.ts

@@ -31,6 +31,9 @@ export interface TargetLabel {
    * List of branches a pull request with this target label should be merged into.
    * Can also be wrapped in a function that accepts the target branch specified in the
    * Github Web UI. This is useful for supporting labels like `target: development-branch`.
+   *
+   * @throws {InvalidTargetLabelError} Invalid label has been applied to pull request.
+   * @throws {InvalidTargetBranchError} Invalid Github target branch has been selected.
    */
   branches: TargetLabelBranchResult|((githubTargetBranch: string) => TargetLabelBranchResult);
 }

dev-infra/pr/merge/pull-request.ts

@@ -12,7 +12,7 @@ import {GitClient} from '../../utils/git';
 
 import {PullRequestFailure} from './failures';
 import {matchesPattern} from './string-pattern';
-import {getBranchesFromTargetLabel, getTargetLabelFromPullRequest} from './target-label';
+import {getBranchesFromTargetLabel, getTargetLabelFromPullRequest, InvalidTargetBranchError, InvalidTargetLabelError} from './target-label';
 import {PullRequestMergeTask} from './task';
 
 /** Interface that describes a pull request. */
@@ -83,6 +83,20 @@ export async function loadAndValidatePullRequest(
       labels.some(name => matchesPattern(name, config.commitMessageFixupLabel));
   const hasCaretakerNote = !!config.caretakerNoteLabel &&
       labels.some(name => matchesPattern(name, config.caretakerNoteLabel!));
+  let targetBranches: string[];
+
+  // If branches are determined for a given target label, capture errors that are
+  // thrown as part of branch computation. This is expected because a merge configuration
+  // can lazily compute branches for a target label and throw. e.g. if an invalid target
+  // label is applied, we want to exit the script gracefully with an error message.
+  try {
+    targetBranches = await getBranchesFromTargetLabel(targetLabel, githubTargetBranch);
+  } catch (error) {
+    if (error instanceof InvalidTargetBranchError || error instanceof InvalidTargetLabelError) {
+      return new PullRequestFailure(error.failureMessage);
+    }
+    throw error;
+  }
 
   return {
     url: prData.html_url,
@@ -92,8 +106,8 @@ export async function loadAndValidatePullRequest(
     githubTargetBranch,
     needsCommitMessageFixup,
     hasCaretakerNote,
+    targetBranches,
     title: prData.title,
-    targetBranches: getBranchesFromTargetLabel(targetLabel, githubTargetBranch),
     commitCount: prData.commits,
   };
 }

dev-infra/pr/merge/target-label.ts

@@ -9,6 +9,22 @@
 import {MergeConfig, TargetLabel} from './config';
 import {matchesPattern} from './string-pattern';
 
+/**
+ * Unique error that can be thrown in the merge configuration if an
+ * invalid branch is targeted.
+ */
+export class InvalidTargetBranchError {
+  constructor(public failureMessage: string) {}
+}
+
+/**
+ * Unique error that can be thrown in the merge configuration if an
+ * invalid label has been applied to a pull request.
+ */
+export class InvalidTargetLabelError {
+  constructor(public failureMessage: string) {}
+}
+
 /** Gets the target label from the specified pull request labels. */
 export function getTargetLabelFromPullRequest(config: MergeConfig, labels: string[]): TargetLabel|
     null {
@@ -21,8 +37,14 @@ export function getTargetLabelFromPullRequest(config: MergeConfig, labels: strin
   return null;
 }
 
-/** Gets the branches from the specified target label. */
-export function getBranchesFromTargetLabel(
-    label: TargetLabel, githubTargetBranch: string): string[] {
-  return typeof label.branches === 'function' ? label.branches(githubTargetBranch) : label.branches;
+/**
+ * Gets the branches from the specified target label.
+ *
+ * @throws {InvalidTargetLabelError} Invalid label has been applied to pull request.
+ * @throws {InvalidTargetBranchError} Invalid Github target branch has been selected.
+ */
+export async function getBranchesFromTargetLabel(
+    label: TargetLabel, githubTargetBranch: string): Promise<string[]> {
+  return typeof label.branches === 'function' ? await label.branches(githubTargetBranch) :
+                                                await label.branches;
 }