From: Junio C Hamano Date: Wed, 08 Apr 2026 22:35:25 GMT Subject: Re: Git 2.54.0-rc1, subtests of t5310, t5326, t5327 Message-ID: In-Reply-To: Junio C Hamano writes: > Junio C Hamano 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.