| Subject: | DBD::MySQL does seem to be thread-safe |
Current DBD::MySQL documentation reads:
The multithreading capabilities of DBD::mysql depend completely on the
underlying C libraries. The modules are working with handle data only, no
global variables are accessed or (to the best of my knowledge) thread
unsafe functions are called. Thus DBD::mysql is believed to be completely
thread safe, if the C libraries are thread safe and you don't share
handles among threads.
However, MySQL documentation has stronger requirements <https://dev.mysql.com/doc/refman/5.1/en/c-api-threaded-clients.html>:
When you call mysql_init(), MySQL creates a thread-specific variable for
the thread that is used by the debug library (among other things). If
you call a MySQL function before the thread has called mysql_init(), the
thread does not have the necessary thread-specific variables in place
and you are likely to end up with a core dump sooner or later. To avoid
problems, you must do the following:
(1) Call mysql_library_init() before any other MySQL functions. It is
not thread-safe, so call it before threads are created, or protect the
call with a mutex.
(2) Arrange for mysql_thread_init() to be called early in the thread
handler before calling any MySQL function. If you call mysql_init(), it
will call mysql_thread_init() for you.
(3) In the thread, call mysql_thread_end() before calling
pthread_exit(). This frees the memory used by MySQL thread-specific
variables.
The preceding notes regarding mysql_init() also apply to
mysql_connect(), which calls mysql_init().
Current DBD::MySQL source does not call mysql_library_init() at all. Also it does not call mysql_thread_end() before exiting a thread. (Actually there is no Perl thread-specific code like START_MY_CXT.)
Finally, some MySQL distributions provide thread-safe MySQL client in standalone libmysqlclient_r.so library and DBD::MySQL still prefers the non-threaded libmysqlclient.so version (Makefile.PL should use mysql_config -libs_r).
It would be great to make the DBD::MySQL thread safe by following the MySQL requirements or drop the misleading text from the documentation.