Subject: Re: BUG #16685: The ecpg thread/descriptor test fails sometimes on
 Windows
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-bugs@lists.postgresql.org
References: <16685-d6cd241872c101d3@postgresql.org>
 <367782.1603554604@sss.pgh.pa.us> <380144.1603558463@sss.pgh.pa.us>
From: Alexander Lakhin <exclusion@gmail.com>
Message-ID: <dedbf623-ae82-5635-a639-944395697799@gmail.com>
Date: Sat, 24 Oct 2020 21:00:00 +0300
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24.10.2020 19:54, Tom Lane wrote:
> I'm forced to the conclusion that there's something wrong with
> ECPG's emulation of pthread_once ...
>
> ... and now that I look at it, it seems just as obvious what
> is wrong there:
>
> void
> win32_pthread_once(volatile pthread_once_t *once, void (*fn) (void))
> {
> 	if (!*once)
> 	{
> 		pthread_mutex_lock(&win32_pthread_once_lock);
> 		if (!*once)
> 		{
> 			*once = true;
> 			fn();
> 		}
> 		pthread_mutex_unlock(&win32_pthread_once_lock);
> 	}
> }
>
> We should not set *once until AFTER we execute fn().
> Otherwise, other threads passing through pthread_once()
> will mistakenly fall through, expecting the initialization
> to be done already.
>
> (So in this view, adding a sleep just before fn() would
> make the failure more reproducible.)
Yes, adding "pg_usleep(1000L);" just before fn() leads to 15 of 100
tests failed (without the delay more than 100 iterations could pass
successfully).
And the reverse test construction:
            fn();
            pg_usleep(1000L);
            *once = true;
Makes all the tests (I ran 30x100 iterations) pass just fine.
Thank you for looking into this!

Best regards,
Alexander