syscall: add Mmap for Solaris and Illumos?

[bcmills]

Solaris supports an mmap syscall.

x/sys/unix.Mmap is defined for GOOS=solaris, and runtime.mmap appears to use the system call as well.

And yet, even though Mmap exists on Solaris and syscall.Mmap exists on most Go Unix ports, for some reason syscall.Mmap does not appear to be defined for GOOS=solaris or GOOS=illumos.

We're looking at possibly using syscall.Mmap in cmd/go in https://go.dev/cl/403975 (CC @matloob). Would it make sense to add syscall.Mmap, syscall.Munmap, and related constants for Solaris and Illumos for consistency with other platforms?

(CC @ianlancetaylor @tklauser @golang/solaris @golang/illumos)

[ianlancetaylor]

Although the syscall package is frozen, I think that this kind of addition is OK since it matches other targets.

[tklauser]

I too think this is OK.

There have been similar additions in the past when a particular function was present in syscall for some, but not all platforms. See e.g. https://go.dev/cl/403394, https://go.dev/cl/391434, https://go.dev/cl/390714 for some recent examples where this was the case for solaris and illumos.

[gopherbot]

Change https://go.dev/cl/413374 mentions this issue: syscall: add Mmap and Munmap on solaris

[gopherbot]

Change https://go.dev/cl/413375 mentions this issue: cmd/go/internal/mmap: use syscall.Mmap on solaris