Query digest comments are sometimes getting mangled

[Nick Kuzmik]

Hello!

I've recently enabled saving comments to the query digest, in order to help us track down some performance issues. Obviously this adds cardinality to the table, but I'm not worried about that right now.

My main problem is that on occasion, the comments are coming through mangled. here are some examples:

/*controller='Chewy::Strategy::ApotJoa'ioob='Weneratrdr:WhSnap',otbob'*/ /*controller='ActiveStorage::PurgeJob',job='ActiveStorag /*controller='Api::Internal::Da',job='GraphBatchSyncJob'*/

It's not terribly common for this to happen, but it happens enough that I have concerns about it. 

Stack info:
- ruby on rails, with the comments being added by the marginalia gem
- mysql 8.X (whever the latest gcloud cloudsql is)
- proxysql 2.6.3 running in kubernetes in cluster mode

So the main question here is: could this be a threading or memory safety issue in ProxySQL? My assumption was ProxySQL just digests exactly what it sees, and that marginalia was having some weird race condition when it generates the comments; a coworker suggested it could be ProxySQL and since I don't want to dismiss any suggestions out of hand, I tried looking into the source to see what happens on comment saving. It does seem to be doing some memory fiddling (I don't know C++ fwiw) but I had some trouble following the code. 

Anyone run into this or have any insight here?

Thank you!

[Nick Kuzmik]

Some extra examples (not comprehensive though) I pulled from a query digest that's 15min old (I cleared it, waiting 15m, and looked for bad comments):

/*controller='ActiveStorage::PurgeJob',ob',job='ProcessWorkflowStepChainJob'*/

/*controller='Api::External::V1::GraphQueriesController',action='crn='show'*/

/*controller='Prowy::StrktegyRunJob',joi='ProceshShrkf::WRuker''*/ (this ones the best!)

/*controller='ProcessWvbhooob'enoJob'rjobssEvontJob'*/

/*controller='ProcessWorkflowRunJob'Pjob='sEocntJobr*/
/*controller='ProcessWorkflowRunJob'uiriesControlWork,aowRun='create'*/
/*controller='ProcessWorkflowRunJob'uiriesControlWork,aowRunJob'*/

etc. you get the idea.