[Bug stdio/15142] Missing locking in _IO_cleanup

["fweimer at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=15142

--- Comment #9 from Florian Weimer <fweimer at redhat dot com> ---
(In reply to Dmitry Vyukov from comment #8)
> > For streams which are blocked in writing, POSIX does not really give us a way to make forward progress because we have to flush the unwritten data before exiting.
> 
> Is it really the case for this program?

No, this program does not have any unflushed data to be written, hence my
comment about a more complex locking protocol avoiding the issue.

Exit flushing is special and not specified as equivalent to fflush (NULL), so
maybe it's sufficient to put read-only streams on a separate list, and flush
only writable streams on exit. But it's not clear to me if it's worth making
changes here if that only fixes this LLVM test case, and the real-world issues
are with applications exiting with pending unwritten data.

> If a write does not happen before exit (which is the case in any such
> blocking), then program cannot potentially know the write has even started
> before fflush/exit, so it cannot possibly expect the write side-effects to
> be flushed.
> 
> What am I missing?

There are cases where we must block according to POSIX. Lack of blocking is
observable by another process.

> > We could perhaps add another flag to fopen/fdopen that indicates that the stream should not participate in fflush (NULL) or exit flushing.
> 
> Should we worry about all of the existing programs that will start hanging?

Andreas Schwab wrote this:

“
This has been part of SUSE/openSUSE for several years, and I have not
seen any complaints so far.  It's more likely that you get a crash
during the unlocked access to the streams.
”

<https://inbox.sourceware.org/libc-alpha/mvmr0pptpmm.fsf@suse.de/>

This reduced my worries considerably.

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