assert only sending sync messages when appropriate

// FREEBIE

src/Messages/OWSMessageSender.m

@@ -594,6 +594,19 @@ NSString *const OWSMessageSenderRateLimitedException = @"RateLimitedException";
 {
     TSThread *thread = message.thread;
 
+#ifdef DEBUG
+    // Assert that we're only sending sync messages when necessary
+    if ([message isKindOfClass:[OWSOutgoingSyncMessage class]]) {
+        __block BOOL hasSecondaryDevices = NO;
+        [self.storageManager.dbConnection readWithBlock:^(YapDatabaseReadTransaction *_Nonnull transaction) {
+            hasSecondaryDevices = [OWSDevice hasSecondaryDevicesWithTransaction:transaction];
+        }];
+        if (!hasSecondaryDevices) {
+            OWSFail(@"Sending sync message when we have no devices to sync to.");
+        }
+    }
+#endif
+
     dispatch_async([OWSDispatch sendingQueue], ^{
         if ([thread isKindOfClass:[TSGroupThread class]]) {
             TSGroupThread *gThread = (TSGroupThread *)thread;
@@ -1089,10 +1102,16 @@ NSString *const OWSMessageSenderRateLimitedException = @"RateLimitedException";
 - (void)handleMessageSentLocally:(TSOutgoingMessage *)message
 {
     if (message.shouldSyncTranscript) {
-        // TODO: I suspect we shouldn't optimistically set hasSyncedTranscript.
-        //       We could set this in a success handler for [sendSyncTranscriptForMessage:].
-        [message updateWithHasSyncedTranscript:YES];
-        [self sendSyncTranscriptForMessage:message];
+        __block BOOL hasSecondaryDevices = NO;
+        [self.dbConnection readWithBlock:^(YapDatabaseReadTransaction *_Nonnull transaction) {
+            hasSecondaryDevices = [OWSDevice hasSecondaryDevicesWithTransaction:transaction];
+        }];
+        if (hasSecondaryDevices) {
+            // TODO: I suspect we shouldn't optimistically set hasSyncedTranscript.
+            //       We could set this in a success handler for [sendSyncTranscriptForMessage:].
+            [message updateWithHasSyncedTranscript:YES];
+            [self sendSyncTranscriptForMessage:message];
+        }
     }
 
     [OWSDisappearingMessagesJob setExpirationForMessage:message];