fix: Call send_sync_msg() only from the SMTP loop (#5780)

`Context::send_sync_msg()` mustn't be called from multiple tasks in parallel to avoid sending the
same sync items twice because sync items are removed from the db only after successful
sending. Let's guarantee this by calling `send_sync_msg()` only from the SMTP loop. Before
`send_sync_msg()` could be called in parallel from the SMTP loop and another task doing
e.g. `chat::sync()` which led to `test_multidevice_sync_chat` being flaky because of events
triggered by duplicated sync messages.

src/sync.rs

@@ -120,6 +120,7 @@ impl Context {
     /// Adds most recent qr-code tokens for a given chat to the list of items to be synced.
     /// If device synchronization is disabled,
     /// no tokens exist or the chat is unpromoted, the function does nothing.
+    /// The caller should perform `SchedulerState::interrupt_smtp()` on its own to trigger sending.
     pub(crate) async fn sync_qr_code_tokens(&self, chat_id: Option<ChatId>) -> Result<()> {
         if !self.should_send_sync_msgs().await? {
             return Ok(());
@@ -154,6 +155,7 @@ impl Context {
 
     /// Adds deleted qr-code token to the list of items to be synced
     /// so that the token also gets deleted on the other devices.
+    /// This interrupts SMTP on its own.
     pub(crate) async fn sync_qr_code_token_deletion(
         &self,
         invitenumber: String,
@@ -164,10 +166,16 @@ impl Context {
             auth,
             grpid: None,
         }))
-        .await
+        .await?;
+        self.scheduler.interrupt_smtp().await;
+        Ok(())
     }
 
     /// Sends out a self-sent message with items to be synchronized, if any.
+    ///
+    /// Mustn't be called from multiple tasks in parallel to avoid sending the same sync items twice
+    /// because sync items are removed from the db only after successful sending. We guarantee this
+    /// by calling `send_sync_msg()` only from the SMTP loop.
     pub async fn send_sync_msg(&self) -> Result<Option<MsgId>> {
         if let Some((json, ids)) = self.build_sync_json().await? {
             let chat_id =
@@ -608,7 +616,8 @@ mod tests {
         alice
             .set_config(Config::Displayname, Some("Alice Human"))
             .await?;
-        alice.pop_sent_msg().await; // Sync message
+        alice.send_sync_msg().await?;
+        alice.pop_sent_msg().await;
         let msg = bob.recv_msg(&alice.pop_sent_msg().await).await;
         assert_eq!(msg.text, "hi");