If $CFG->enable_read_only_sessions_debug was not enabled, debugging could be
enabled part-way through a request when restart_with_write_lock was
called. This meant that a diff between the initial and final session
would be made during write_close(), although the intial session state
was never captured. This generated false positives in the logs, and it
thought any variable set in the session was a change from the original
value.
This ensures that debugging is enabled before the debug flag is allowed
to change, preventing false positives.
Mark Johnson
217154895c