Subject: Re: Git 2.54.0-rc1, subtests of t5310, t5326, t5327

From: Junio C Hamano <[email-masked]>
Date: Wed, 08 Apr 2026 15:35:25 -0700

Junio C Hamano <[email-masked]> writes:

> Junio C Hamano <[email-masked]> writes:
>
>> To be quite honest, I am not sure if it is even worth using writev()
>> if we need a loop that protects against shrot writes, so unless I am
>> grossly mistaken (e.g., perhaps there is some guarantee that there
>> won't be any short writes for writev() that sends data smaller than
>> 64k that I missed in the docs), the best course of action might be
>> to revert the change to use writev() and use the two write(2)s as
>> before, *if* we actually observe that the current code is broken by
>> short writes.
>
> Ah, sorry, I should have double checked the actual code.  We already
> use a looping writev_in_full() that wraps writev(), so there is
> nothing extra that we still need to do to prepare for short writes.
>
> Unfortunately, comparing write_in_full() vs writev_in_full(), there
> is nothing that corresponds to xwrite() that can be used to hide the
> short writes and chomps an originally larger I/O into smaller
> pieces.

Oops, the beauty of having xwrite() is *not* that it hides short
writes (it doesn't), but it can be used to pretend that short writes
happened on platforms with unreasonably small I/O limit by setting
MAX_IO_SIZE to unusually low.  But the point that ...

> Unlike write() that we may receive a single linear large
> sequence of bytes, which we can choose to chomp into artificially
> smaller pieces and write them out (up to 8MB by default), writev()
> API lets the caller to prepare chunks of memory and I do not think
> there is a good way for the writev_in_full() at the lower layer to
> chomp these into smaller pieces, and even if we could, that would
> defeat the whole reason why we rewrote the original code that used
> write_in_full() into using writev(), i.e., to avoid extra allocation
> (and extra system calls---but if your I/O layer is limited to very
> small writes, no matter how we chop it, you will have to issue extra
> system calls to flush all of the data out).
>
> So, I dunno.

... I doubt that there exists a good way to have xwritev() that
wraps around writev() and pretend that a short write happened,
instead of issuing a large I/O, still stands.