x/sys/unix: Add wrapper for readv/writev

[Merovius]

I'd like to propose exposing readv/writev (and maybe preadv/pwritev and maybeer preadv2/pwritev2) in x/sys/unix, as {Read,Write}v(fd int, iov [][]byte) (int, error).

My usecase is writing to a file in O_APPEND mode, which offers atomicity-guarantees per write. Issuing multiple writes loses those guarantees, so I have to resort to a) allocating a temporary buffer and concatenating the individual []bytes, or b) use unix.Syscall. The latter is fine, but a wrapper would allow to avoid unsafe.

I don't personally care about the p… and …v2 variants, but they could be added at the same time. I'm not sure how portable they are, though, beyond Linux.

I can send a CL if this is accepted.

[Merovius]

(See also #21676 which discusses, among other things, exposing an interface to writev for *os.File)

[ianlancetaylor]

In general we permit adding any existing system call to x/net/unix. Using [][]byte as the second argument seems right to me. So this is fine.

[tklauser]

I assume you meant x/sys/unix, not x/net/unix?

It's fine to add the non-portable preadv/pwritev/preadv2/pwritev2 variants for Linux only. We already have several wrappers for Linux-only syscalls.

[Merovius]

@tklauser yes I do, thanks, corrected my initial post as well :) I'll send a CL later today.

[gopherbot]

Change https://golang.org/cl/212157 mentions this issue: unix: add Readv/Writev family of syscalls for linux

[Merovius]

I only have Linux machines, so AIUI I can't generate the constants and everything for other platforms.
Also, I mostly copied the interaction with the race-detector from Read/Write - I have no idea if that was needed or correct in this case.

[gopherbot]

Change https://golang.org/cl/224238 mentions this issue: unix: Add readv/writev for illumos