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From: Heikki Linnakangas on 8 Apr 2010 10:59
On Windows, syslogger uses two threads. The main thread loops and polls
if any SIGHUPs have been received or if the log file needs to be
rotated. Another thread, "pipe thread", does ReadFile() on the pipe that
other processes send their log messages to. ReadFile() blocks, and
whenever new data arrives, it is processed in the pipe thread.

Both threads use palloc()/pfree(), which are not thread-safe :-(.

It's hard to trigger a crash because the main thread mostly just sleeps,
and the pipe thread only uses palloc()/pfree() when it receives chunked
messages, larger than 512 bytes. Browsing the CVS history, this was made
visibly broken by the patch that introduced the message chunking. Before
that the pipe thread just read from the pipe and wrote to the log file,
which was safe. It has always used ereport() to report read errors,
though, which can do palloc(), but there shouldn't normally be any read
errors.

I chatted with Magnus about this, and he suggested using a Windows
critical section to make sure that only one of the threads is active at
a time. That seems suitable for back-porting, but I'd like to get rid of
this threading in CVS head, it seems too error-prone.

The reason it uses threads like this on Windows is explained in the
comments:
> /*
> * Worker thread to transfer data from the pipe to the current logfile.
> *
> * We need this because on Windows, WaitForSingleObject does not work on
> * unnamed pipes: it always reports "signaled", so the blocking ReadFile won't
> * allow for SIGHUP; and select is for sockets only.
> */

But Magnus pointed out that our pgpipe() implementation on Windows
actually creates a pair of sockets instead of pipes, for exactly that
reason, so that you can use select() on the returned file descriptor.
For some reason syslogger explicitly doesn't use pgpipe() on Windows,
though, but calls CreatePipe(). I don't see any explanation why.

I'm going to see what happens if I remove all the #ifdef WIN32 blocks in
syslogger, and let it use pgpipe() and select() instead of the extra thread.

Thoughts?

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com

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