One way to pass information in a way that does not require an extra roundtrip like SHOW WARNINGS would require is to use the system variable session tracker. It can be used to pass arbitrary information to the client in the final OK packet sent after authentication. It's a little bit outside of the intended scope of the tracker but given the extremely useful ability to pass arbitrary text data to clients it's (IMO) a pretty nice idea.

This could also be used to deliver out-of-band messages to the client that would not require the use of the existing warning mechanism and could display warnings without needing user interaction. It could even be used to implement a MotD type global messages that would be displayed to connected clients when the administrator sets a global variable to some value.

In 11.3 we'll likely have TLS everywhere, enabled by default. There will be only a minority of users who'd use these plugins over a non-secure connection, as it would require them to disable ssl explicitly in my.cnf.

markus makela, I'd rather suggest a separate session tracker, like session warning tracker, not sysvar tracker. and it needs a limit of how many warnings to send

Yes, a separate session tracker would be ideal. Perhaps something like session_track_warnings=5 to limit it to 5 warnings.

markus makela, we're diverging. the original issue was about a client-side warning, not about how to send warnings from the server. server side warnings tracker needs a new issue