Re: lib/54661: pthread_self() returns invalid value in some circumstances

[Joerg Sonnenberger <joerg%bec.de@localhost>]

On Wed, Oct 30, 2019 at 10:44:39AM -0400, Tom Lane wrote:
> Joerg Sonnenberger <joerg%bec.de@localhost> writes:
> > The following reply was made to PR lib/54661; it has been noted by GNATS.
> >  On Wed, Oct 30, 2019 at 06:10:00AM +0000, tgl%sss.pgh.pa.us@localhost wrote:
> >>> pthread_self() sometimes returns an invalid pointer ((void *) -1),
> >>> as illustrated in How-To-Repeat.
>  
> >  Why do you think it's an invalid pointer? It's an opaque value.
> >  Comparing it against any fixed value is a bug in the caller.
> 
> I tend to agree that Python shouldn't be checking for -1, but even
> if it removed that check, this behavior would still be problematic,
> because pthread_self() is returning a value that won't work correctly
> in later thread operations.  For example, it will not compare equal
> to the result of a later pthread_self() if somebody successfully
> loads/initializes libpthread.so in between.
> 
> I've now looked at the relevant libpthread code, and what seems to be
> happening is this: libpython.so is invoking libpthread's version of
> pthread_self(), but that is just redirecting to the stub version in
> libc, evidently because pthread__init hasn't been called.  So the
> question is why not.  I see bug fixes in libpthread's CVS history
> that claim to be fixing cases where libpthread is dlopen'd, but
> somehow that's not working here.  Maybe the case where libpthread
> is a dependency of a dlopen'd library isn't covered?
> 
> I also kind of wonder why the redirect-to-the-stubs behavior is
> there at all.  Why not just forcibly do pthread__init, if it's not
> been done already, when one of libpthread's functions is called?

We didn't allow dynamic loading of libpthread at all for a long time. I
still believe allowing it was a mistake, but *shrug*. We certainly do
not support calling pthread_create if libpthread was not initially
loaded. As much pthread_self() can only ever return the main thread.

Joerg