x/sys/unix: sendmsg does not permit sending multiple buffers

[eapache]

(I realize syscall has moved to its own repo as of 1.4, but that repo doesn't have a bug tracker so I assume this is still the right place to file such things, please correct me if I'm wrong.)

The unix Sendmsg and SendmsgN system calls permit passing in multiple buffers through the use of an io-vector structure, however the Go wrappers for these functions take only a single []byte and hard-code the length of the io-vector to 1. It would be very useful for performance in some cases (such as IBM/sarama#238) if Sendmsg could be made to take a [][]byte and construct the equivalent io-vector.

[minux]

We can't change the existing functions, but how about introducing this?

func SendmsgMultiple(fd int, p [][]byte, oob []byte, to Sockaddr, flags int) (n int, err error)

Any suggestions for a better name?

[ianlancetaylor]

GIven the existence of net.Buffers, perhaps we can do something along the lines of SendmsgBuffers. Not that we should import "net", but we could add a Buffers type to x/sys/unix. I'm not quite sure that makes sense, though.

[dpifke]

Superseded by #52885?

[ianlancetaylor]

Yes, thanks.