Re: narwhal and PGDLLIMPORT

On Mon, Oct 20, 2014 at 10:24:47PM +0200, Andres Freund wrote:
> On 2014-10-20 01:03:31 -0400, Noah Misch wrote:
> > On Wed, Oct 15, 2014 at 12:53:03AM -0400, Noah Misch wrote:
> > I happened to try the same contrib/dblink test suite on PostgreSQL built with
> > modern MinGW-w64 (i686-4.9.1-release-win32-dwarf-rt_v3-rev1).  That, too, gave
> > a crash-like symptom starting with commit 846e91e.  Specifically, a backend
> > that LOADed any module linked to libpq (libpqwalreceiver, dblink,
> > postgres_fdw) would suffer this after calling exit(0):
> >
> > ===
> > 3056 2014-10-20 00:40:15.163 GMT LOG:  disconnection: session time: 0:00:00.515 user=cyg_server database=template1
host=127.0.0.1port=3936 
> >
> > This application has requested the Runtime to terminate it in an unusual way.
> > Please contact the application's support team for more information.
> >
> > This application has requested the Runtime to terminate it in an unusual way.
> > Please contact the application's support team for more information.
> > 9300 2014-10-20 00:40:15.163 GMT LOG:  server process (PID 3056) exited with exit code 3
> > ===
> >
> > The mechanism turned out to be disjoint from the mechanism behind the
> > ancient-compiler crash.  Based on the functions called from exit(), my best
> > guess is that exit() encountered recursion and used something like an abort()
> > to escape.
>
> Hm.
>
> >  (I can send the gdb transcript if anyone is curious to see the
> > gory details.)
>
> That would be interesting.

Attached.  ("rep 100 s" calls a macro equivalent to issuing "s" 100 times.)

> > The proximate cause was commit 846e91e allowing modules to use
> > shared libgcc.  A 32-bit libpq acquires 64-bit integer division from libgcc.
> > Passing -static-libgcc to the link restores the libgcc situation as it stood
> > before commit 846e91e.  The main beneficiary of shared libgcc is C++/Java
> > exception handling, so PostgreSQL doesn't care.  No doubt there's some deeper
> > bug in libgcc or in PostgreSQL; loading a module that links with shared libgcc
> > should not disrupt exit().  I'm content with this workaround.
>
> I'm unconvinced by this reasoning. Popular postgres extensions like
> postgis do use C++. It's imo not hard to imagine situations where
> switching to a statically linked libgcc statically could cause problems.

True; I was wrong to say that PostgreSQL doesn't care.  MinGW builds of every
released PostgreSQL version use static libgcc.  That changed as an unintended
consequence of a patch designed to remove our reliance on dlltool.  Given the
lack of complaints about our historic use of static libgcc, I think it's fair
to revert to 9.3's use thereof.  Supporting shared libgcc would be better
still, should someone make that effort.