clean code and add comments

js/models/messages.js

@@ -1209,6 +1209,10 @@
       return errors[0][0];
     },
 
+    /**
+     * This function is called by inbox_view.js when a message was successfully sent for one device.
+     * So it might be called several times for the same message
+     */
     async handleMessageSentSuccess(sentMessage) {
       let sentTo = this.get('sent_to') || [];
 
@@ -1220,13 +1224,22 @@
         sentMessage.group &&
         sentMessage.group instanceof libsession.Types.OpenGroup;
 
-      // Handle the sync logic here
-      if (
+      // We trigger a sync message only when the message is not to one of our devices, AND
+      // the message is not for an open group (there is no sync for opengroups, each device pulls all messages), AND
+      // if we did not sync or trigger a sync message for this specific message already
+      const shouldTriggerSyncMessage =
         !isOurDevice &&
         !isOpenGroupMessage &&
         !this.get('synced') &&
-        !this.get('sentSync')
-      ) {
+        !this.get('sentSync');
+
+      // A message is synced if we triggered a sync message (sentSync)
+      // and the current message was sent to our device (so a sync message)
+      const shouldMarkMessageAsSynced =
+        isOurDevice && !isOpenGroupMessage && this.get('sentSync');
+
+      // Handle the sync logic here
+      if (shouldTriggerSyncMessage) {
         const contentDecoded = textsecure.protobuf.Content.decode(
           sentMessage.plainTextBuffer
         );
@@ -1234,7 +1247,7 @@
         if (dataMessage) {
           this.sendSyncMessage(dataMessage);
         }
-      } else if (isOurDevice && this.get('sentSync')) {
+      } else if (shouldMarkMessageAsSynced) {
         this.set({ synced: true });
       }
       if (!isOpenGroupMessage) {

ts/test/test-utils/utils/message.ts

@@ -111,4 +111,8 @@ export class MockConversation {
 
     return this.isPrimary ? this.id : generateFakePubKey().key;
   }
+
+  public get(obj: string) {
+    return (this.attributes as any)[obj];
+  }
 }