Respond to CR.

// FREEBIE

SignalServiceKit/src/Messages/OWSMessageSender.m

@@ -954,11 +954,29 @@ NSString *const OWSMessageSenderRateLimitedException = @"RateLimitedException";
 
     NSString *localNumber = [TSAccountManager localNumber];
     if ([localNumber isEqualToString:recipient.uniqueId]) {
+        OWSAssert([message isKindOfClass:[OWSOutgoingSyncMessage class]]);
+        // Messages send to the "local number" should be sync messages.
+        //
+        // We can skip sending sync messages if we know that we have no linked
+        // devices. However, we need to be sure to handle the case where the
+        // linked device list has just changed.
+        //
+        // The linked device list is reflected in two separate pieces of state:
+        //
+        // * OWSDevice's state is updated when you link or unlink a device.
+        // * SignalRecipient's state is updated by 409 "Mismatched devices"
+        //   responses from the service.
+        //
+        // If _both_ of these pieces of state agree that there are no linked
+        // devices, then can safely skip sending sync message.
+
+        // 1. Check OWSDevice's state.
         __block BOOL hasSecondaryDevices;
         [self.dbConnection readWithBlock:^(YapDatabaseReadTransaction *transaction) {
             hasSecondaryDevices = [OWSDevice hasSecondaryDevicesWithTransaction:transaction];
         }];
 
+        // 2. Check SignalRecipient's state.
         BOOL hasDeviceMessages = deviceMessages.count > 0;
 
         if (!hasSecondaryDevices && !hasDeviceMessages) {
@@ -975,7 +993,7 @@ NSString *const OWSMessageSenderRateLimitedException = @"RateLimitedException";
         } else if (hasSecondaryDevices) {
             // We may have just linked a new secondary device which is not yet reflected in
             // the SignalRecipient that corresponds to ourself.  Proceed.  Client should learn
-            // of new secondary devices when this message send fails.
+            // of new secondary devices via 409 "Mismatched devices" response.
             DDLogWarn(@"%@ sync message has no device messages but account has secondary devices.", self.logTag);
         } else if (hasDeviceMessages) {
             OWSFail(@"%@ sync message has device messages for unknown secondary devices.", self.logTag);