x/sys/unix: Pipe does not work on Darwin

[ianlancetaylor]

The x/sys/unix Pipe implementation is incorrect on Darwin. The x/sys/unix package needs a change along the lines of the change to syscall/syscall_darwin.go in https://golang.org/cl/141639. Unfortunately, it may have to be more complicated if we want to continue supporting Go 1.12 in the x/sys/unix package, as for 1.12 we should probably use the existing version of Pipe.

CC @randall77

[randall77]

Yikes, looks like this never worked. The pipe calling convention is different between the raw syscall and the libc call.
Any idea if there are others like this?

Fortunately I don't think we use x/sys/unix.Pipe anywhere in the stdlib. So no need to vendor in any fix here.

The corresponding x/sys/unix change to https://golang.org/cl/141639 was https://golang.org/cl/154179

It should be easy to keep supporting 1.12. There are separate files for <=1.12 and >1.12 support.

[randall77]

1.12 is actually when we cut over to libc system calls. So only <= 1.11 support would still need support for raw system calls.
But it looks like that support is already gone in x/sys/unix: https://golang.org/cl/250437
So the fix is easy.

[gopherbot]

Change https://golang.org/cl/281432 mentions this issue: unix: fix darwin pipe implementation

[gopherbot]

Change https://golang.org/cl/280622 mentions this issue: unix: don't call testing.T.Fatalf in a background goroutine