Re: Problem in SQLFreeHandle (Statement)

On 06/17/2014 11:06 PM, Jade Koskela wrote:
> I couldn't reproduce the behavior using the test program, or a similar one.
> I recompiled the driver to enable extra logging.
> Here is the grep on ENTER_CONN_CS, the second address is the address of the
> mutex (pthread recursive).
> This was compiled on OSX with Clang, and then also with GCC with the same
> result. Behavior doesn't reproduce on Windows.
>
>
> [140735257568016][[SQLFreeHandle]]
>
> [140735257568016]*** ENTER_CONN_CS [140210608634616] ***
>
> [140735257568016]*** ENTER_CONN_CS [140210608634616] ***
>
> [140735257568016]*** LEAVE_CONN_CS [140210608634616] ***
>
> [140735257568016]*** LEAVE_CONN_CS [140210608634616] ***
>
> [140735257568016]*** LEAVE_CONN_CS [140210608634616] ***
>
> [140735257568016][SQLGetConnectAttrW]
>
> [140735257568016]*** ENTER_CONN_CS [140210608634616] ***
>
> [140735257568016]*** LEAVE_CONN_CS [140210608634616] ***
>
> [140735257568016][SQLGetConnectAttrW]
>
> [140735257568016]*** ENTER_CONN_CS [140210608634616] ***
>
> [140735257568016]*** LEAVE_CONN_CS [140210608634616] ***
>
> [140735257568016][SQLGetConnectAttrW]
>
> [140735257568016]*** ENTER_CONN_CS [140210608634616] ***
>
> [140735257568016]*** LEAVE_CONN_CS [140210608634616] ***
>
> [4466282496][[SQLAllocHandle]]
>
> [4466282496]*** ENTER_CONN_CS [140210608634616] *** <- new thread
> grabbed mutex, count=1
>
> [4466282496]**** PGAPI_AllocStmt: hdbc = 0x7f8553857000, stmt =
> 0x7f8552d17100
>
> [4466282496]*** LEAVE_CONN_CS [140210608634616] *** <- count=0
>
> [4466282496]**** PGAPI_Prepare: STMT_ALLOCATED, copy
>
> [4466282496]*** ENTER_CONN_CS [140210608634616] *** <- count=1
>
> [4466282496]*** ENTER_CONN_CS [140210608634616] *** <- count=2
>
> [140735257568016]*** LEAVE_CONN_CS [140210608634616] *** <- this thread
> doesn't own, count=2

A-ha. The problem is that after issuing the query cancel, the
SQLCancel() function calls DiscardStatementSvp(), to discard any
savepoints associated with the statement. SetStatementSvp() and
DiscardStatementSvp() maintain a counter of how many times the CONN_CS
has been acquired, and DiscardStatementSvp() tries to completely release
it by releasing it as many times as the counter says it's been locked.
However, unlocking a pthread mutex from a different thread than the one
that locked it is undefined behavior. Apparently it happens to work on
other platforms by accident, but OS X is somehow more picky about it.

We never actually check the return value of pthread_mutex_lock() /
pthread_mutex_unlock(), so it's possible that they are reporting errors
even on other platforms, and we just don't pay attention to it. That
ought to be fixed too, at least in debug builds. Also for debugging
purposes, I think we should add a thread ID field next to the
lock_CC_for_rb field to remember which thread is holding the locks, as a
sanity check.

Attached are two patches. The first one is a narrow fix, which should
fix the bug. It simply removes the DiscardStatementSvp() call. AFAICS
it's never needed: if the statement was busy executing in a different
thread (this is the case you hit), we send a query cancel request to the
server, and it's not cool to do anything else with the statement. And in
the codepaths where the statement was not busy executing, PGAPI_Cancel()
already called DiscardStatementSvp(), with the appropriate lock held.

The second patch refactors PGAPI_Cancel() and adds comments, to make it
more readable. It's not absolutely required, but I found PGAPI_Cancel()
very difficult to debug as it is.

Can you test these patches, to confirm that it fixes the bug you're seeing?

- Heikki