Check stack traces (and variables), per thread: thread apply all bt [full]
Print variables, up to complex one:
thread 1
print do_command::thd->query_string.string.str
Set new values for variables (global and per thread, even those formally read-only in MySQL while it’s running):
set max_connections=5000
set opt_log_slave_updates=1
Call functions (that may do complex changes):
call rpl_filter->add_do_db(strdup("hehehe"))
Set breakpoints and watchpoints
Work interactively or use gdb as a command line utility (-batch)
Use macros, Python scripting, more…
All these may not work, fail, hang, crash, produce obscure errors…
You have to read and understand the source code
