Re: [ANNOUNCE] Git v2.32.0-rc3 - t5300 Still Broken on NonStop ia64/x86

[Bryan Turner <bturner@atlassian.com>]

On Wed, Jun 2, 2021 at 1:14 PM Jeff King <peff@peff.net> wrote:
>
> On Wed, Jun 02, 2021 at 08:11:50PM +0000, Eric Wong wrote:
>
> > Jeff King <peff@peff.net> wrote:
> > > And so when he gets this error:
> > >
> > >   fatal: fsync error on '.git/objects/pack/tmp_pack_NkPgqN': Interrupted system call
> > >
> > > presumably we were in fsync() when the signal arrived, and unlike most
> > > other platforms, the call needs to be restarted manually (even though we
> > > set up the signal with SA_RESTART). I'm not sure if this violates POSIX
> > > or not (I couldn't find a definitive answer to the set of interruptible
> > > functions in the standard). But either way, the workaround is probably
> > > something like:
> >
> > "man 3posix fsync" says EINTR is allowed ("manpages-posix-dev"
> > package in Debian non-free).
>
> Ah, thanks. Linux's fsync(3) doesn't mention it, and nor does it appear
> in the discussion of interruptible calls in signals(7). So I was looking
> for a POSIX equivalent of that signals manpage but couldn't find one. :)
>
> > >   #ifdef FSYNC_NEEDS_RESTART
> >
> > The wrapper should apply to all platforms.  NFS (and presumably
> > other network FSes) can be mounted with interrupts enabled.
>
> I don't mind that, as the wrapper is pretty low-cost (and one less
> Makefile knob is nice). If it's widespread, though, I find it curious
> that nobody has run into it before now.

I was dealing with a similar issue[1] recently, albeit not in the Git
codebase but rather with Java. My issue was with epoll_wait, rather
than fsync, which is documented on signal(7) as not restartable even
with SA_RESTART. That led me to this[2] little bit of code inside the
JVM:
#define RESTARTABLE(_cmd, _result) do { \
  do { \
    _result = _cmd; \
  } while((_result == -1) && (errno == EINTR)); \
} while(0)

which they use like this[3]:
RESTARTABLE(epoll_wait(epfd, events, numfds, -1), res);

Not sure what the Git maintainers' view on macros is, but if there
wasn't going to be a Makefile knob perhaps something similar might
make sense as a reusable construct. Of course, it's unclear how often
Git might _need_ such a thing; given this doesn't seem to come up
much, perhaps that's a sign such a macro would end up a waste of
effort. Anyway, just thought I'd share because I was looking at
something similar.

[1] https://github.com/brettwooldridge/NuProcess/issues/124
[2] https://github.com/JetBrains/jdk8u_jdk/blob/94318f9185757cc33d2b8d527d36be26ac6b7582/src/solaris/native/sun/nio/ch/nio_util.h#L33-L37
[3] https://github.com/JetBrains/jdk8u_jdk/blob/94318f9185757cc33d2b8d527d36be26ac6b7582/src/solaris/native/sun/nio/ch/EPoll.c#L92

>
> -Peff