Fix backfiller when fetching ToT when running on a non-default branch. (#4711)

Closes flutter/flutter#169086.

This was actually a pretty bad bug, it's lucky we caught this (thx experimental branch!)

Author: matanlurey

app_dart/lib/src/request_handlers/scheduler/batch_backfiller.dart

@@ -12,7 +12,6 @@ import 'package:meta/meta.dart';
 
 import '../../../cocoon_service.dart';
 import '../../model/ci_yaml/ci_yaml.dart';
-import '../../model/commit_ref.dart';
 import '../../model/firestore/task.dart' as fs;
 import '../../request_handling/exceptions.dart';
 import '../../service/firestore/commit_and_tasks.dart';
@@ -63,6 +62,7 @@ final class BatchBackfiller extends RequestHandler {
   //
   // See https://github.com/flutter/flutter/issues/168738.
   Future<void> _backfillExperimentalBranch(RepositorySlug slug) async {
+    log.debug('Running experimental branch backfiller for "$slug"');
     final fsGrid = await _firestore.queryRecentCommitsAndTasks(
       slug,
       commitLimit: config.flags.backfillerCommitLimit,
@@ -116,21 +116,20 @@ final class BatchBackfiller extends RequestHandler {
       );
 
       // Download the ToT .ci.yaml targets.
-      final ciYaml = await _ciYamlFetcher.getCiYamlByCommit(
-        CommitRef(
-          slug: slug,
-          sha: fsGrid.first.commit.sha,
-          branch: Config.defaultBranch(slug),
-        ),
-        postsubmit: true,
-      );
+      final totCiYaml = await _ciYamlFetcher.getTipOfTreeCiYaml(slug: slug);
 
       final totTargets = [
-        ...ciYaml.postsubmitTargets(),
-        if (ciYaml.isFusion)
-          ...ciYaml.postsubmitTargets(type: CiType.fusionEngine),
+        ...totCiYaml.postsubmitTargets(),
+        if (totCiYaml.isFusion)
+          ...totCiYaml.postsubmitTargets(type: CiType.fusionEngine),
       ];
-      log.debug('Fetched ${totTargets.length} tip-of-tree targets');
+      if (totTargets.isEmpty) {
+        log.warn(
+          'Did not fetch any tip-of-tree targets. Backfill will do nothing!',
+        );
+      } else {
+        log.debug('Fetched ${totTargets.length} tip-of-tree targets');
+      }
 
       grid = BackfillGrid.from([
         for (final CommitAndTasks(:commit, :tasks) in fsGrid)

app_dart/lib/src/service/scheduler/ci_yaml_fetcher.dart

@@ -69,13 +69,7 @@ interface class CiYamlFetcher {
     }
 
     final isFusion = commit.slug == Config.flutterSlug;
-    final totCommit = await _fetchTipOfTreeCommit(slug: commit.slug);
-    final totYaml = await _getCiYaml(
-      commit: totCommit,
-      validate: validate,
-      isFusionCommit: isFusion,
-      useGitOnBorgFallback: true,
-    );
+    final totYaml = await getTipOfTreeCiYaml(slug: commit.slug);
     return _getCiYaml(
       commit: commit,
       validate: validate,
@@ -85,6 +79,16 @@ interface class CiYamlFetcher {
     );
   }
 
+  /// Returns the latest `ci.yaml`(s) for the default branch of [slug].
+  Future<CiYamlSet> getTipOfTreeCiYaml({required RepositorySlug slug}) async {
+    return await _getCiYaml(
+      commit: await _fetchTipOfTreeCommit(slug: slug),
+      validate: false,
+      isFusionCommit: slug == Config.flutterSlug,
+      useGitOnBorgFallback: true,
+    );
+  }
+
   /// Creates and returns, using a cache, the `.ci.yaml` file for a commit.
   Future<CiYamlSet> _getCiYaml({
     required CommitRef commit,

app_dart/test/src/service/fake_ci_yaml_fetcher.dart

@@ -11,13 +11,22 @@ import 'package:github/src/common/model/repos.dart';
 import 'package:yaml/yaml.dart';
 
 final class FakeCiYamlFetcher implements CiYamlFetcher {
-  FakeCiYamlFetcher({this.ciYaml, this.failCiYamlValidation = false});
+  FakeCiYamlFetcher({
+    this.ciYaml,
+    this.totCiYaml,
+    this.failCiYamlValidation = false,
+  });
 
   /// The value that should be returned as a canned response for [getCiYamlByCommit].
   ///
   /// If omitted (`null`) defaults to a configuration with a single target.
   CiYamlSet? ciYaml;
 
+  /// The value that should be returned as a canned response for [getTipOfTreeCiYaml].
+  ///
+  /// If omitted (`null`), defaults to the same response as [ciYaml].
+  CiYamlSet? totCiYaml;
+
   /// Sets [ciYaml] by loading a YAML document.
   ///
   /// Optionally may also specify [engine] for a fusion [CiYamlSet].
@@ -59,6 +68,19 @@ final class FakeCiYamlFetcher implements CiYamlFetcher {
     );
   }
 
+  @override
+  Future<CiYamlSet> getTipOfTreeCiYaml({required RepositorySlug slug}) async {
+    final ci =
+        totCiYaml ??
+        ciYaml ??
+        _createDefault(slug: slug, commitBranch: Config.defaultBranch(slug));
+    return CiYamlSet(
+      slug: slug,
+      branch: Config.defaultBranch(slug),
+      yamls: ci.configs.map((k, v) => MapEntry(k, v.config)),
+    );
+  }
+
   static CiYamlSet _createDefault({
     required RepositorySlug slug,
     required String commitBranch,