Fix presubmit retry of missing builder (#3708)

Currently the presubmit LUCI pubsub handler has a couple of un-ACK'd messages because the PR has a ci.yaml that references a builder that was recently removed. The current code to determine whether to retry the job is throwing an uncaught exception when validating the config, causing the handler to return a 500.

This catches that error, and forces the job not to retry, so that the message can be processed. Not retrying is the correct behavior, since it would fail again anway.

app_dart/lib/src/request_handlers/presubmit_luci_subscription_v2.dart

@@ -143,10 +143,16 @@ class PresubmitLuciSubscriptionV2 extends SubscriptionHandlerV2 {
       sha: userData['commit_sha'] as String,
     );
     late CiYaml ciYaml;
-    if (commit.branch == Config.defaultBranch(commit.slug)) {
-      ciYaml = await scheduler.getCiYaml(commit, validate: true);
-    } else {
-      ciYaml = await scheduler.getCiYaml(commit);
+    try {
+      if (commit.branch == Config.defaultBranch(commit.slug)) {
+        ciYaml = await scheduler.getCiYaml(commit, validate: true);
+      } else {
+        ciYaml = await scheduler.getCiYaml(commit);
+      }
+    } on FormatException {
+      // If ci.yaml no longer passes validation (for example, because a builder
+      // has been removed), ensure no retries.
+      return 0;
     }
 
     // Do not block on the target not found.

app_dart/test/request_handlers/presubmit_luci_subscription_v2_test.dart

@@ -300,4 +300,58 @@ void main() {
       ),
     ).called(1);
   });
+
+  test('Build not rescheduled if ci.yaml fails validation.', () async {
+    scheduler.failCiYamlValidation = true;
+    when(
+      mockGithubChecksService.updateCheckStatus(
+        build: anyNamed('build'),
+        userDataMap: anyNamed('userDataMap'),
+        luciBuildService: anyNamed('luciBuildService'),
+        slug: anyNamed('slug'),
+        rescheduled: false,
+      ),
+    ).thenAnswer((_) async => true);
+    when(mockGithubChecksService.taskFailed(any)).thenAnswer((_) => true);
+    when(mockGithubChecksService.currentAttempt(any)).thenAnswer((_) => 1);
+
+    final Map<String, dynamic> userDataMap = {
+      'repo_owner': 'flutter',
+      'commit_branch': Config.defaultBranch(Config.flutterSlug),
+      'commit_sha': 'abc',
+      'repo_name': 'flutter',
+    };
+
+    tester.message = createPushMessageV2(
+      Int64(1),
+      status: bbv2.Status.SUCCESS,
+      builder: 'Linux C',
+      userData: userDataMap,
+    );
+
+    final bbv2.BuildsV2PubSub buildsV2PubSub = createBuild(
+      Int64(1),
+      status: bbv2.Status.SUCCESS,
+      builder: 'Linux C',
+    );
+
+    await tester.post(handler);
+    verifyNever(
+      mockLuciBuildService.rescheduleBuild(
+        build: buildsV2PubSub.build,
+        builderName: 'Linux C',
+        userDataMap: userDataMap,
+        rescheduleAttempt: 1,
+      ),
+    );
+    verify(
+      mockGithubChecksService.updateCheckStatus(
+        build: anyNamed('build'),
+        userDataMap: anyNamed('userDataMap'),
+        luciBuildService: anyNamed('luciBuildService'),
+        slug: anyNamed('slug'),
+        rescheduled: false,
+      ),
+    ).called(1);
+  });
 }

app_dart/test/src/service/fake_scheduler_v2.dart

@@ -45,14 +45,22 @@ class FakeSchedulerV2 extends SchedulerV2 {
   /// [CiYaml] value to be injected on [getCiYaml].
   CiYaml? ciYaml;
 
+  /// If true, getCiYaml will throw a [FormatException] when validation is
+  /// enforced, simulating failing validation.
+  bool failCiYamlValidation = false;
+
   @override
   Future<CiYaml> getCiYaml(
     Commit commit, {
     CiYaml? totCiYaml,
     RetryOptions? retryOptions,
     bool validate = false,
-  }) async =>
-      ciYaml ?? _defaultConfig;
+  }) async {
+    if (validate && failCiYamlValidation) {
+      throw const FormatException('Failed validation!');
+    }
+    return ciYaml ?? _defaultConfig;
+  }
 
   @override
   Future<Commit> generateTotCommit({required String branch, required RepositorySlug slug}) async {