Re: [PATCH v2] midx: use buffered I/O to talk to pack-objects

[Jeff King <peff@peff.net>]

On Wed, Aug 12, 2020 at 01:28:22PM -0700, Junio C Hamano wrote:

> René Scharfe <l.s.r@web.de> writes:
> 
> >  		nth_midxed_object_oid(&oid, m, i);
> > -		xwrite(cmd.in, oid_to_hex(&oid), the_hash_algo->hexsz);
> > -		xwrite(cmd.in, "\n", 1);
> > +		fprintf(cmd_in, "%s\n", oid_to_hex(&oid));
> 
> I do think it is silly to send an object name and terminating LF in
> two different system calls per object.
> 
> The original uses xwrite() so that it does not have to worry about
> having to restart interrupted system calls and such.  Do we need to
> do that ourselves now or does the stdio layer take care of it for
> us?

I think we're OK in this instance because we are not expecting writes to
happen at any given moment. We might do a partial write() for any given
fprintf(), but we're OK as long as by the fflush() at the end everything
is written.

For more general cases where we do care about ordering (e.g., if we were
interleaving writes and reads with something like cat-file), we'd need
to fflush() after each write. And I'd expect fflush() to retry across
partial write() or EINTR anyway as a quality-of-implementation thing.
Which doesn't make it true, but glibc empirically seems to behave that
way, and I think we'd be better off adding a transparent wrapper to
fflush() on any system that doesn't.

-Peff