properly handle deleting sha from query parameters (#5009)

Fix: flutter/flutter#184372

Author: ievdokdm

.vscode/launch.json

@@ -4,6 +4,18 @@
     // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
     "version": "0.2.0",
     "configurations": [
+        {
+            "name": "dashboard",
+            "cwd": "dashboard",
+            "request": "launch",
+            "flutterMode": "debug",
+            "deviceId": "chrome",
+            "type": "dart",
+            "args": [
+                "--web-port",
+                "8080",
+            ]
+        },
         {
             "name": "dashboard (fake auth)",
             "cwd": "dashboard",
@@ -12,7 +24,9 @@
             "deviceId": "chrome",
             "type": "dart",
             "args": [
-                "--dart-define=FAKE_AUTH=true"
+                "--dart-define=FAKE_AUTH=true",
+                "--web-port",
+                "8080",
             ]
         },
     ]

dashboard/lib/state/presubmit.dart

@@ -336,6 +336,10 @@ class PresubmitState extends ChangeNotifier {
       _lastFetchedSha = null;
       _jobs = null;
       _selectedJob = null;
+      // Reset to force re-fetch of available SHAs and pick the latest one
+      if (sha == null) {
+        _lastFetchedPr = null;
+      }
     }
 
     if (changed) {

dashboard/test/state/presubmit_test.dart

@@ -367,4 +367,41 @@ void main() {
       ),
     ).called(1);
   });
+  test(
+    'syncUpdate with null sha resets _lastFetchedPr to force re-fetch when pr is the same',
+    () async {
+      const summaries = [
+        PresubmitGuardSummary(
+          headSha: 'latest',
+          creationTime: 123,
+          guardStatus: GuardStatus.succeeded,
+        ),
+      ];
+      when(
+        mockCocoonService.fetchPresubmitGuardSummaries(
+          pr: '123',
+          repo: 'flutter',
+        ),
+      ).thenAnswer(
+        (_) async =>
+            const CocoonResponse<List<PresubmitGuardSummary>>.data(summaries),
+      );
+
+      presubmitState.update(repo: 'flutter', pr: '123', sha: 'sha1');
+      expect(presubmitState.sha, 'sha1');
+
+      await Future<void>.delayed(Duration.zero);
+      clearInteractions(mockCocoonService);
+
+      presubmitState.update(pr: '123', sha: null);
+
+      verify(
+        mockCocoonService.fetchPresubmitGuardSummaries(
+          pr: '123',
+          repo: 'flutter',
+        ),
+      ).called(1);
+      expect(presubmitState.sha, 'latest');
+    },
+  );
 }