autocomplete: Terminate the already-running search when there is a new one

Before this fix, when there was a search in progress in `_startSearch`,
and then for some reason, `_startSearch` was called again, like through
`refreshStaleUserResults`, the new search would do its work and update
`_results` field, but also the current search would be retried and this
in its turn, would also update `_results` in the same way as before,
doing the same thing for the second time.

This fix terminates the current in-progress search and leaves the room
for the new search to do its work.

Author: sm-sayedi

lib/model/autocomplete.dart

@@ -235,14 +235,11 @@ class MentionAutocompleteView extends ChangeNotifier {
   _startSearch(MentionAutocompleteQuery query) async {
     List<MentionAutocompleteResult>? newResults;
 
-    while (true) {
-      try {
-        newResults = await _computeResults(query);
-        break;
-      } on ConcurrentModificationError {
-        // Retry
-        // TODO backoff?
-      }
+    try {
+      newResults = await _computeResults(query);
+    } on ConcurrentModificationError {
+      // Terminate this search as there is a new search going on
+      return;
     }
 
     if (newResults == null) {

test/model/autocomplete_test.dart

@@ -274,28 +274,31 @@ void main() {
     }
   });
 
-  test('MentionAutocompleteView mutating store.users while in progress causes retry', () async {
+  test('MentionAutocompleteView mutating store.users while in progress causes '
+    'the results to be recomputed only one time ', () async {
     const narrow = CombinedFeedNarrow();
     final store = eg.store();
     for (int i = 0; i < 1500; i++) {
       await store.addUser(eg.user(userId: i, email: 'user$i@example.com', fullName: 'User $i'));
     }
     final view = MentionAutocompleteView.init(store: store, narrow: narrow);
 
-    bool done = false;
-    view.addListener(() { done = true; });
+    int notifiedCount = 0;
+    view.addListener(() { notifiedCount++; });
     view.query = MentionAutocompleteQuery('User 10000');
 
     await Future(() {});
-    check(done).isFalse();
+    check(notifiedCount).equals(0);
     await store.addUser(eg.user(userId: 10000, email: '[email protected]', fullName: 'User 10000'));
     await Future(() {});
-    check(done).isFalse();
+    check(notifiedCount).equals(0);
     await Future(() {});
-    check(done).isTrue();
+    check(notifiedCount).equals(1);
     check(view.results).single
       .isA<UserMentionAutocompleteResult>()
       .userId.equals(10000);
+    await Future(() {});
+    check(notifiedCount).equals(1);
     // new result sticks; no "zombie" result from `store.users` pre-mutation
     for (int i = 0; i < 10; i++) { // for good measure
       await Future(() {});