From: Tom Lane <tgl@sss.pgh.pa.us>
To: exclusion@gmail.com
Cc: pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #16685: The ecpg thread/descriptor test fails sometimes on Windows
Date: Sat, 24 Oct 2020 12:54:23 -0400
Message-ID: <380144.1603558463@sss.pgh.pa.us> (raw)
In-Reply-To: <367782.1603554604@sss.pgh.pa.us>
References: <16685-d6cd241872c101d3@postgresql.org>
<367782.1603554604@sss.pgh.pa.us>
I wrote:
> I'll bet that the correct fix is
> static void
> set_descriptors(struct descriptor *value)
> {
> + pthread_once(&descriptor_once, descriptor_key_init);
> pthread_setspecific(descriptor_key, value);
> }
No, looking closer, there are only two calls of set_descriptors()
and each of them is preceded by a call to get_descriptors().
So it should not be possible to get here without descriptor_key
having been initialized.
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.)
regards, tom lane
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To: pgsql-bugs@postgresql.org
Cc: tgl@sss.pgh.pa.us, exclusion@gmail.com, pgsql-bugs@lists.postgresql.org
Subject: Re: BUG #16685: The ecpg thread/descriptor test fails sometimes on Windows
In-Reply-To: <380144.1603558463@sss.pgh.pa.us>
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