fix(auto_submit): Pass requestSha to GitHub merge API (#5107)

Defense in depth - prevent autosubmit from trying to land a pushed (but not approved) pr. 

This is not a real issue since we have branch protection rules, but it's good to have.

Author: jtmcdole

auto_submit/lib/service/validation_service.dart

@@ -82,7 +82,20 @@ ${pullRequest.title!.replaceFirst('Revert "Revert', 'Reland')}
     if (pullRequest.isMergeQueueEnabled) {
       return _enqueuePullRequest(slug, pullRequest);
     } else {
-      return _mergePullRequest(number, commitMessage, slug);
+      if (pullRequest.head?.sha == null) {
+        return (
+          result: false,
+          message:
+              'Failed to merge ${slug.fullName}/#${pullRequest.number}: invalid head',
+          method: SubmitMethod.merge,
+        );
+      }
+      return _mergePullRequest(
+        number,
+        commitMessage,
+        slug,
+        requestSha: pullRequest.head!.sha!,
+      );
     }
   }
 
@@ -122,8 +135,9 @@ ${pullRequest.title!.replaceFirst('Revert "Revert', 'Reland')}
   Future<MergeResult> _mergePullRequest(
     int number,
     String commitMessage,
-    github.RepositorySlug slug,
-  ) async {
+    github.RepositorySlug slug, {
+    required String requestSha,
+  }) async {
     try {
       github.PullRequestMerge? result;
 
@@ -134,6 +148,7 @@ ${pullRequest.title!.replaceFirst('Revert "Revert', 'Reland')}
           slug: slug,
           number: number,
           mergeMethod: github.MergeMethod.squash,
+          requestSha: requestSha,
         );
       }, retryIf: (Exception e) => e is RetryableException);
 

auto_submit/test/service/pull_request_validation_service_test.dart

@@ -288,6 +288,33 @@ void main() {
       );
 
       expect(result.message, contains('Reland "My first PR!"'));
+      expect(githubService.mergePrShaMap[0], pullRequest.head?.sha);
+    });
+
+    test('Fails for invalid sha', () async {
+      final pullRequest = generatePullRequest(
+        prNumber: 1001,
+        repoName: slug.name,
+        title: 'Revert "Revert "My first PR!"',
+        mergeable: true,
+      );
+      pullRequest.head!.sha = null;
+      githubService.pullRequestData = pullRequest;
+      githubService.mergeRequestMock = PullRequestMerge(
+        merged: true,
+        sha: pullRequest.mergeCommitSha,
+      );
+
+      final result = await validationService.submitPullRequest(
+        config: config,
+        pullRequest: pullRequest,
+      );
+
+      expect(
+        result.message,
+        contains('Failed to merge flutter/cocoon/#1001: invalid head'),
+      );
+      expect(githubService.mergePrShaMap[1001], isNull);
     });
 
     test(

auto_submit/test/src/service/fake_github_service.dart

@@ -341,6 +341,9 @@ class FakeGithubService implements GithubService {
 
   bool throwExceptionOnMerge = false;
 
+  /// Recorded commit shas made during [mergePullRequest].
+  Map<int, String?> mergePrShaMap = <int, String?>{};
+
   /// If useMergeRequestMockList is true then we will return elements from that
   /// list until it is empty.
   ///
@@ -358,6 +361,7 @@ class FakeGithubService implements GithubService {
       throw Exception('Exception occurred during merging of pull request.');
     }
     verifyPullRequestMergeCallMap[number] = slug;
+    mergePrShaMap[number] = requestSha;
     if (useMergeRequestMockList) {
       return pullRequestMergeMockList.removeAt(0);
     } else {