In `markConclusion` do not throw exception if remainingJobs is zero (#5113)

In `markConclusion` do not throw exception if remainingJobs is zero but keep it zero

fix: https://github.com/flutter/flutter/issues/189989

app_dart/lib/src/service/firestore/unified_check_run.dart

diff --git a/app_dart/lib/src/service/firestore/unified_check_run.dart b/app_dart/lib/src/service/firestore/unified_check_run.dart
index 81bafcf..dcb0d89 100644
--- a/app_dart/lib/src/service/firestore/unified_check_run.dart
+++ b/app_dart/lib/src/service/firestore/unified_check_run.dart
@@ -615,11 +615,15 @@
           // So if the test existed and either remaining or failed_count is changed;
           // the response is valid.
           if (state.status.isComplete) {
-            // Guard against going negative and log enough info so we can debug.
-            if (remaining == 0) {
-              throw '$logCrumb: field "${PresubmitGuard.fieldRemainingJobs}" is already zero for $transaction / ${presubmitGuardDocument.fields}';
+            // If remaining is 0 we should not decrement it and we should log
+            // this fact.
+            if (remaining > 0) {
+              remaining -= 1;
+            } else {
+              log.error(
+                '$logCrumb: field "${PresubmitGuard.fieldRemainingJobs}" is already zero for $transaction / ${presubmitGuardDocument.fields}',
+              );
             }
-            remaining -= 1;
             valid = true;
           }
 

app_dart/test/service/firestore/unified_check_run_test.dart

diff --git a/app_dart/test/service/firestore/unified_check_run_test.dart b/app_dart/test/service/firestore/unified_check_run_test.dart
index c9353b5..dfb3d79 100644
--- a/app_dart/test/service/firestore/unified_check_run_test.dart
+++ b/app_dart/test/service/firestore/unified_check_run_test.dart
@@ -326,6 +326,51 @@
         expect(checkDoc.buildNumber, 456);
         expect(checkDoc.buildId, Int64.MAX_VALUE);
       });
+      test('keeps remaining jobs at 0 if it is already 0', () async {
+        final guardDoc = await firestoreService.getDocument(
+          'projects/flutter-dashboard/databases/cocoon/documents/presubmit_guards/${guardId.documentId}',
+        );
+        final guard = PresubmitGuard.fromDocument(guardDoc);
+        guard.remainingJobs = 0;
+        await firestoreService.writeViaTransaction(
+          documentsToWrites([guard], exists: true),
+        );
+
+        final state = const PresubmitJobState(
+          jobName: 'linux',
+          status: TaskStatus.succeeded,
+          attemptNumber: 1,
+          startTime: 2000,
+          endTime: 3000,
+        );
+
+        final result = await UnifiedCheckRun.markConclusion(
+          firestoreService: firestoreService,
+          guardId: guardId,
+          state: state,
+        );
+
+        expect(result.result, PresubmitGuardConclusionResult.ok);
+        expect(result.remaining, 0);
+
+        final updatedGuardDoc = await firestoreService.getDocument(
+          'projects/flutter-dashboard/databases/cocoon/documents/presubmit_guards/${guardId.documentId}',
+        );
+        final updatedGuard = PresubmitGuard.fromDocument(updatedGuardDoc);
+        expect(updatedGuard.remainingJobs, 0);
+        expect(updatedGuard.jobs['linux'], TaskStatus.succeeded);
+
+        final checkDoc = await PresubmitJob.fromFirestore(
+          firestoreService,
+          PresubmitJobId(
+            slug: slug,
+            checkRunId: 123,
+            jobName: 'linux',
+            attemptNumber: 1,
+          ),
+        );
+        expect(checkDoc.status, TaskStatus.succeeded);
+      });
     });
     group('reInitializeFailedChecks', () {
       late PresubmitGuardId fusionGuardId;