On June 3, 2021 4:22 PM, Bryan Turner wrote:
>On Wed, Jun 2, 2021 at 1:14 PM Jeff King <peff@xxxxxxxx> wrote:
>>
>> On Wed, Jun 02, 2021 at 08:11:50PM +0000, Eric Wong wrote:
>>
>> > Jeff King <peff@xxxxxxxx> 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
I can only suggest, from other OpenSource projects I'm on, that make extensive use of macros, that they are very difficult to debug and sometimes harder to integrate with platform-specific compatibility layers. I would rather have something explicit in compat.c that gdb could make sense of during a debug session if necessary. Trying to debug macros is harsh, rather like debugging -O2 without -g.
I used to be a big fan of macros, but grew out of it 😉. Just my take on it.
-Randall
