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Discovered through a compile warning that says:

static bool isc::db::MySqlHolder::<lambda()>::_FUN()’ will never be NULL

for this line:

bool MySqlHolder::atexit_ = []{atexit([]{mysql_library_end();});return true;};

What it's saying is that the address of the lambda is always different than zero. Which gets casted to a true when assigned to atexit_. That's not the concerning part. To the best of my understanding, the lambda never gets called here. And so atexit() is never called. And so mysql_library_end() never gets called. Quote from the MySQL docs:

To avoid memory leaks after the application is done using the library (for example, after closing the connection to the server), be sure to call mysql_library_end() explicitly. This enables memory managment to be performed to clean up and free resources used by the library.

So this could be a socket leak? If I remember, unclosed sockets outlive the process.

Compiler is g++11, but I remember getting it in previous versions also. Flags are -Wall -Wextra, but they might not be required.