MySQL Bugs: #32857: Application linked dynamically to MyODBC crashes

Bug #32857	Application linked dynamically to MyODBC crashes
Submitted:	29 Nov 2007 21:43	Modified:	31 Dec 2007 14:50
Reporter:	Bogdan Degtyariov
Status:	Closed
Category:	Connector/ODBC	Severity:	S3 (Non-critical)
Version:	3.51.22	OS:	Linux
Assigned to:	Bogdan Degtyariov	Target Version:
Tags:	linking, thread

Description:
A multithreaded (even STL) application that loads MyODBC directly (MyODBC is linked
dynamically) without UnixODBC crashes when calling SQLAllocStmt(). This bug never occurs
with UnixODBC because the driver manager creates additional synchronization points. 

How to repeat:
The test case shall be ready soon.

Sorry, the application is single-threaded, so it is not a synchronization problem.

This IS the problem with threads synchronization. Occurs when MyODBC is loaded/linked
directly to application or through unixODBC providing the Threading parameter in
odbcinst.ini is lower than 3 (3 is the default value if the parameter is omitted. That is
why ODBC programs did not crash at that point if linked against unixODBC and the parameter
not set explicitly).

Unfortunately, there is no test case that could reproduce the bug on 100%. Everything we
have is a C program that can reproduce the crash if running in infinite loop for some long
time (up to 2-3 minutes).

Crashes have been fixed (see the patch below) by adding locks in proper places.

Patch without test case. It was not included because the problem occured randomly providing the loop is running infinitely ~3min

Attachment: patch32857.diff (application/octet-stream, text), 1.29 KiB.

Bogdan, approved

A note has been added to the 3.51.23 changelog: 

When using unixODBC or directly linked applications where the thread level is set to less
than 3 (within odbcinst.ini), a thread synchronization issue would lead to an application
crash. This was because SQLAllocStmt() and SQLFreeStmt() did not synchronize access to the
list of statements associated with a connection.