message: Avoid double-sends after send-message request succeeds

This implements the waitPeriodExpired -> waiting state transition.

GitHub discussion:
  zulip#1472 (comment)

Author: PIG208

lib/model/message.dart

@@ -530,12 +530,14 @@ const kSendMessageOfferRestoreWaitPeriod = Duration(seconds: 10);  // TODO(#1441
 /// [MessageStore.sendMessage] call and before its eventual deletion.
 ///
 /// ```
-///              Got an [ApiRequestException].
-///          ┌──────┬──────────┬─────────────► failed
-/// (create) │      │          │                 │
-///    └► hidden   waiting   waitPeriodExpired ──┴──────────────► (delete)
-///          │      ▲   │      ▲                   User restores
-///          └──────┘   └──────┘                   the draft.
+///                             Got an [ApiRequestException].
+///          ┌──────┬────────────────────────────┬──────────► failed
+///          │      │                            │              │
+///          │      │      [sendMessage]         │              │
+/// (create) │      │      request succeeds.     │              │
+///    └► hidden   waiting ◄─────────────── waitPeriodExpired ──┴─────► (delete)
+///          │      ▲   │                     ▲               User restores
+///          └──────┘   └─────────────────────┘               the draft.
 ///         Debounce     [sendMessage] request
 ///         timed out.   not finished when
 ///                      wait period timed out.
@@ -559,7 +561,8 @@ enum OutboxMessageState {
   /// outbox message is shown to the user.
   ///
   /// This state can be reached after staying in [hidden] for
-  /// [kLocalEchoDebounceDuration].
+  /// [kLocalEchoDebounceDuration], or when the request succeeds after the
+  /// outbox message reaches [OutboxMessageState.waitPeriodExpired].
   waiting,
 
   /// The [sendMessage] HTTP request did not finish in time and the user is
@@ -717,7 +720,8 @@ mixin _OutboxMessageStore on PerAccountStoreBase {
     final isStateTransitionValid = switch (newState) {
       OutboxMessageState.hidden => false,
       OutboxMessageState.waiting =>
-        oldState == OutboxMessageState.hidden,
+        oldState == OutboxMessageState.hidden
+        || oldState == OutboxMessageState.waitPeriodExpired,
       OutboxMessageState.waitPeriodExpired =>
         oldState == OutboxMessageState.waiting,
       OutboxMessageState.failed =>
@@ -803,6 +807,14 @@ mixin _OutboxMessageStore on PerAccountStoreBase {
     // Cancel the timer that would have had us start presuming that the
     // send might have failed.
     _outboxMessageWaitPeriodTimers.remove(localMessageId)?.cancel();
+    if (_outboxMessages[localMessageId]!.state
+          == OutboxMessageState.waitPeriodExpired) {
+      // The user was offered to restore the message since the request did not
+      // complete for a while.  Since the request was successful, we expect the
+      // message event to arrive eventually.  Stop inviting the the user to
+      // retry, to avoid double-sends.
+      _updateOutboxMessage(localMessageId, newState: OutboxMessageState.waiting);
+    }
   }
 
   TopicName _processTopicLikeServer(TopicName topic, {

test/model/message_test.dart

@@ -202,6 +202,34 @@ void main() {
       checkState().equals(OutboxMessageState.waiting);
     }));
 
+    test('waitPeriodExpired -> waiting and never return to waitPeriodExpired', () => awaitFakeAsync((async) async {
+      await prepare(stream: stream);
+      await prepareMessages([eg.streamMessage(stream: stream)]);
+      // Set up a [sendMessage] request that succeeds after enough delay,
+      // for the outbox message to reach the waitPeriodExpired state.
+      // TODO extract helper to add prepare an outbox message with a delayed
+      //   successful [sendMessage] request if we have more tests like this
+      connection.prepare(json: SendMessageResult(id: 1).toJson(),
+        delay: kSendMessageOfferRestoreWaitPeriod + Duration(seconds: 1));
+      final future = store.sendMessage(
+        destination: streamDestination, content: 'content');
+      async.elapse(kSendMessageOfferRestoreWaitPeriod);
+      checkState().equals(OutboxMessageState.waitPeriodExpired);
+
+      // Wait till the [sendMessage] request succeeds.
+      async.elapse(Duration(seconds: 1));
+      await future;
+      checkState().equals(OutboxMessageState.waiting);
+
+      // Wait till we reach at least [kSendMessageOfferRestoreWaitPeriod] after
+      // returning to the waiting state.
+      async.elapse(kSendMessageOfferRestoreWaitPeriod);
+      async.flushTimers();
+      // The outbox message should stay in the waiting state;
+      // it should not transition to waitPeriodExpired.
+      checkState().equals(OutboxMessageState.waiting);
+    }));
+
     group('… -> failed', () {
       test('hidden -> failed', () => awaitFakeAsync((async) async {
         await prepareOutboxMessageToFailAfterDelay(Duration.zero);