在事务中更新不同的数据库（Laravel）

For our web application, written in Laravel, we use transactions to update the database. We have separated our data cross different database (for ease, let's say "app" and "user"). During an application update, an event is fired to update some user statistics in the user database. However, this application update may be called as part of a transaction on the application database, so the code structure looks something as follows.

DB::connection('app')->beginTransaction();
// ...
DB::connection('user')->doStuff();
// ...
DB::connection('app')->commit();

It appears that any attempt to start a transaction on the user connection (since a single query already creates an implicit transaction) while in a game transaction does not work, causing a deadlock (see InnoDB status output). I also used innotop to get some more information, but while it showed the lock wait, it did not show by which query it was blocked. There was a lock present on the user table, but I could not find it's origin. Relevant output is shown below:

The easy solution would be to pull out the user operation from the transaction, but since the actual code is slightly more complicated (the doStuff actually happens somewhere in a nested method called during the transaction and is called from different places), this is far from trivial. I would very much like the doStuff to be part of the transaction, but I do not see how the transaction can span multiple databases.

What is the reason that this situation causes a deadlock and is it possible to run the doStuff on the user database within this transaction, or do we have to find an entirely different solution like queueing the events for execution afterwards?

I have found a way to solve this issue using a workaround. My hypothesis was that it was trying to use the app connection to update the user database, but after forcing it to use the user connection, it still locked. Then I switched it around and used the app connection to update the user database. Since we have joins between the databases, we have database users with proper access, so that was no issue. This actually solved the problem.

That is, the final code turned out to be something like

DB::connection('app')->beginTransaction();
// ...
DB::connection('app')->table('user.users')->doStuff(); // Pay attention to this line!
// ...
DB::connection('app')->commit();