Reschedule presubmit targets when base ref is changed

app_dart/lib/src/request_handlers/github/webhook_subscription.dart

@@ -280,7 +280,7 @@ class GithubWebhookSubscription extends SubscriptionHandler {
     final RepositorySlug slug = pr.base!.repo!.slug();
     final bool isTipOfTree = pr.base!.ref == Config.defaultBranch(slug);
     final GitHub gitHubClient = await config.createGitHubClient(pullRequest: pr);
-    await _validateRefs(gitHubClient, pr);
+    await _validateRefs(gitHubClient, pr, pullRequestEvent);
     if (kNeedsTests.contains(slug) && isTipOfTree) {
       switch (slug.name) {
         case 'flutter':
@@ -490,6 +490,7 @@ class GithubWebhookSubscription extends SubscriptionHandler {
   Future<void> _validateRefs(
     GitHub gitHubClient,
     PullRequest pr,
+    PullRequestEvent pullRequestEvent,
   ) async {
     final RepositorySlug slug = pr.base!.repo!.slug();
     String body;
@@ -538,6 +539,8 @@ class GithubWebhookSubscription extends SubscriptionHandler {
           base: Config.defaultBranch(slug),
         );
         await gitHubClient.issues.createComment(slug, pr.number!, body);
+        // Not that this should cancel all existing running tasks.
+        await _scheduleIfMergeable(pullRequestEvent);
       }
     }
   }

app_dart/test/request_handlers/github/webhook_subscription_test.dart

@@ -4,6 +4,7 @@
 
 import 'package:cocoon_service/src/model/appengine/commit.dart';
 import 'package:cocoon_service/src/model/luci/buildbucket.dart';
+import 'package:cocoon_service/src/model/luci/push_message.dart' as pm;
 import 'package:cocoon_service/src/request_handlers/github/webhook_subscription.dart';
 import 'package:cocoon_service/src/service/cache_service.dart';
 import 'package:cocoon_service/src/service/config.dart';
@@ -272,13 +273,15 @@ void main() {
     test('Acts on opened against master when default is main', () async {
       const int issueNumber = 123;
 
-      tester.message = generateGithubWebhookMessage(
+      final pm.PushMessage pushMessage = generateGithubWebhookMessage(
         action: 'opened',
         number: issueNumber,
         baseRef: 'master',
         slug: Config.engineSlug,
       );
 
+      tester.message = pushMessage;
+
       when(pullRequestsService.listFiles(Config.engineSlug, issueNumber)).thenAnswer(
         (_) => Stream<PullRequestFile>.value(
           PullRequestFile()..filename = 'packages/flutter/blah.dart',
@@ -308,6 +311,11 @@ void main() {
           argThat(contains('master -> main')),
         ),
       ).called(1);
+
+      // Count two calls for the initial triggering and after the update of the
+      // base branch reference.
+      expect(scheduler.triggerPresubmitTargetsCallCount, 2);
+      scheduler.resetTriggerPresubmitTargetsCallCount();
     });
 
     // We already schedule checks when a draft is opened, don't need to re-test

app_dart/test/src/service/fake_scheduler.dart

@@ -61,13 +61,9 @@ class FakeScheduler extends Scheduler {
 
   int cancelPreSubmitTargetsCallCnt = 0;
 
-  int get cancelPreSubmitTargetsCallCount {
-    return cancelPreSubmitTargetsCallCnt;
-  }
+  int get cancelPreSubmitTargetsCallCount => cancelPreSubmitTargetsCallCnt;
 
-  void resetCancelPreSubmitTargetsCallCount() {
-    cancelPreSubmitTargetsCallCnt = 0;
-  }
+  void resetCancelPreSubmitTargetsCallCount() => cancelPreSubmitTargetsCallCnt = 0;
 
   @override
   Future<void> cancelPreSubmitTargets({
@@ -78,6 +74,22 @@ class FakeScheduler extends Scheduler {
     cancelPreSubmitTargetsCallCnt++;
   }
 
+  int triggerPresubmitTargetsCnt = 0;
+
+  int get triggerPresubmitTargetsCallCount => triggerPresubmitTargetsCnt;
+
+  void resetTriggerPresubmitTargetsCallCount() => triggerPresubmitTargetsCnt = 0;
+
+  @override
+  Future<void> triggerPresubmitTargets({
+    required PullRequest pullRequest,
+    String reason = 'Newer commit available',
+    List<String>? builderTriggerList,
+  }) async {
+    await super.triggerPresubmitTargets(pullRequest: pullRequest);
+    triggerPresubmitTargetsCnt++;
+  }
+
   int addPullRequestCallCnt = 0;
 
   int get addPullRequestCallCount {