I found and tested a fix for the mysql thread safety problem I have been troubleshooting.
Commenting out line 154 of dbd_mysql.c succeeds in fixing the problem when libdbi-mysql is build against recent libmysqlclient libraries. This is the were mysql_server_end() is called after each dbi instance if finished. In a multi-threaded application, multiple dbi instances can come and go in various threads. With recent libmysqlclient library implementations, mysql_server_end() should only be called ONCE, at the end of the entire program/process, not multiple times when a thread is finished with a dbi intsnace. libmysqlclient's mysql_server_end() function performs some final libmysqlclient memory clean-up. So if it isn't called, and the application ends, it's not really a problem - the OS will clean up the memory allocation. A few other thread safety notes that might be helpful to others. To be thread safe: 1. Each program thread must use it's own libdbi instance (as documented in libdbi). 2. The dbi_conn_connect function must be protected by a mutex, since it results in an underlying libmysqlclient call that is not thread safe, using memory objects allocated once by the libmysqlclient library when it was first initialized. This is exactly the memory that was causing trouble (freed) when mysql_server_end() is called by libdbi_shutdown_r in onw thread when the libdbi library is still in use by other threads prior to program termination. So... is libdbi and the database drivers still maintained by someone? How can I get this fix included so it will be present when folks install the driver for mysql on their various systems? Thanks, Ethan... On Fri, 2019-11-15 at 09:35 -0700, Ethan Funk wrote: > I think I found the problem in the mysql dbd driver code.: > > dbd_finalize() calls mysql_server_end(), which is deprecated in the > current mysqlclient21 library, but results in a call to a replacement > function which is intended to be called only once, at the end of a > process. With the call being made inside dbd_finalize(), the > mysql_server_end() gets called each time my application code finished > with a libdbi instance. Reading between the lines in the MySQL > documentation, it looks think the mysql_server_end() call to it's new > counter part frees memory which is allocated by a pthread_once() > function triggered by mysql_library_init(), which itself is called by > mysql_init() via the libdbi driver on each connection attempt. So my > code's first libdbi connection results in the memory being allocated, > with subsequent calls in other threads being ignored. But > dbd_finalize() gets called at the end of EVERY libdbi instance closure, > so the memory if freed over and over again. I don't see a mechanism > for a once-per-process driver shutdown callback in libdbi, so I want to > try commenting out mysql_servefr_end() in dbd_finalize(). This will > leak memory which is never freed at the end of the process, but so > what. It is only allocated once per process, so the operating system > process shutdown will free the memory. > > Unfortunately, I can't seem to build my modified code for testing with > the libdi_dbd project code. "./configure" is giving me the following > error: > configure: error: Invalid libdbi directory - include files not found. > > This is after configure has already indicated that yes, it has located > the dbi header files, which I installed with the libdbi-dev package > with the Ubuntu apt command. > > How might I be able to fix this error, and proceed with testing my code > fix? > > Thanks, > Ethan... > > On Wed, 2019-11-13 at 14:19 -0700, Ethan Funk wrote: > > I am having a problem with libdbd_mysql on linux (Ubuntu 19.10) > > reagding thread fasety. > > There used to be two versions of libmysqlclient, one was thread safe, > > one was not. However, > > now there seems to be only one client library, which I understood was > > thread safe: libmysqlclient.so.21. > > > > Any insight into the dump below would be greatly appreciated! > > > > Ethan... > > > > Here is a valgrind dump of a read-already-freed error that occurs in > > what I will call thread #3 > > when it attempts to connect to MySQL. This thread has it's own libdbi > > instance, yet mysql_real_connect > > is trying to read memory allocated by the first thread (thread #1) that > > first called mysql_real_connect, > > after that same memory was freed by another thread (thread #2) that > > called mysql_server_end as it was > > finishing. Each thread has it's own libdbi instance, so why is > > libmysqlclient sharring memory between > > threads? It's looking like the libmysqlclient isn't thread safe. > > > > ==2828== Invalid read of size 1 > > ==2828== at 0x4A3A042: strlen (vg_replace_strmem.c:461) > > ==2828== by 0x141A46C1: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x141A5468: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x141A54A8: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x141A373E: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x4FE041E: __pthread_once_slow (pthread_once.c:116) > > ==2828== by 0x141A320D: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x141A41B2: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x141A429E: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x1415472B: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x1415492A: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x141577A8: mysql_real_connect (in /usr/lib/x86_64- > > linux-gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x12F47435: dbd_connect (in /usr/lib/x86_64-linux- > > gnu/dbd/libdbdmysql.so) > > ==2828== by 0x4DC37AE: dbi_conn_connect (in /usr/lib/x86_64-linux- > > gnu/libdbi.so.1.1.0) > > ==2828== by 0x134BFA: dbSetupConnection (database.c:211) > > ==2828== by 0x1357F6: DeleteLogEntry (database.c:440) > > ==2828== by 0x131ECE: taskCallProcInThread (tasks.c:73) > > ==2828== by 0x4FD7668: start_thread (pthread_create.c:479) > > ==2828== by 0x567B322: clone (clone.S:95) > > > > Memory was freed in a thread (lets call it thread #2) that ran and > > completed. It created it's own > > dbi instance, did it's thing, and then closed it's MySQL connection and > > shutdown the instance. > > So why is it freeing memory deep in the mysql driver that it did not > > allocate? > > > > ==2828== Address 0x1343e698 is 424 bytes inside a block of size 4,088 > > free'd > > ==2828== at 0x4A37F90: free (vg_replace_malloc.c:540) > > ==2828== by 0x141A9B8A: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x141A8756: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x14148982: mysql_server_end (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x12F4684C: dbd_finalize (in /usr/lib/x86_64-linux- > > gnu/dbd/libdbdmysql.so) > > ==2828== by 0x4DC2E4D: dbi_shutdown_r (in /usr/lib/x86_64-linux- > > gnu/libdbi.so.1.1.0) > > ==2828== by 0x134440: dbi_instance_free (database.c:56) > > ==2828== by 0x4FD6600: __nptl_deallocate_tsd.part.0 > > (pthread_create.c:301) > > ==2828== by 0x4FD7689: __nptl_deallocate_tsd (pthread_create.c:256) > > ==2828== by 0x4FD7689: start_thread (pthread_create.c:490) > > ==2828== by 0x567B322: clone (clone.S:95) > > > > This is in the first thread in the program to make use of libdbi/MySQL, > > and it is still > > running/connected to MySQL using a unique libdbi intance. > > > > ==2828== Block was alloc'd at > > ==2828== at 0x4A36E96: malloc (vg_replace_malloc.c:309) > > ==2828== by 0x141A9A30: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x141A9AEB: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x141A4E1C: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x1419CFA0: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x1419D95E: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x1416FD29: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x141A2A66: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x141A3771: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x4FE041E: __pthread_once_slow (pthread_once.c:116) > > ==2828== by 0x141A320D: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x141A41B2: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x141A429E: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x1415472B: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x1415492A: ??? (in /usr/lib/x86_64-linux- > > gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x141577A8: mysql_real_connect (in /usr/lib/x86_64- > > linux-gnu/libmysqlclient.so.21.1.17) > > ==2828== by 0x12F47435: dbd_connect (in /usr/lib/x86_64-linux- > > gnu/dbd/libdbdmysql.so) > > ==2828== by 0x4DC37AE: dbi_conn_connect (in /usr/lib/x86_64-linux- > > gnu/libdbi.so.1.1.0) > > ==2828== by 0x134BFA: dbSetupConnection (database.c:211) > > ==2828== by 0x13D2F4: GetItemMetaData (database.c:2960) > > ==2828==
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