syscall: add Flock implementation for Illumos

[bcmills]

On most Unix and Unix-like platforms, the Go syscall package provides the Flock function (and the associated constants LOCK_EX, LOCK_SH, LOCK_UN).

That functionality is not provided for Solaris, because (as far as I am aware) Oracle Solaris still does not provide the corresponding system call (see also #24684). However, it appears that the Illumos kernel has supported flock since 2015 (https://www.illumos.org/issues/3252).

We should update the syscall package to provide Flock on illumos. Then we can switch cmd/go/internal/lockedfile/internal/filelock over to use it, and hopefully avoid apparent kernel bugs that affect the current Fcntl implementation used on AIX and Solaris (#32817).

CC @jclulow @gdamore @tklauser

[jclulow]

It was my impression that the syscall package was effectively frozen at this stage. Is that just from an exposed API perspective? That is, is this OK because there's already Flock in there for other platforms?

[bcmills]

My personal view is that it's ok because:

1. there is already a Flock implementation (with the same semantics) on other platforms, and
2. we would use Flock within the standard library.

(Either of those on its own might not justify extending the syscall package for that configuration, but given that Flock already exists it's simpler not to require extra build tags in order to use it from some other package.)

[bradfitz]

Yeah, it's okay to modify syscall for the reasons Bryan mentioned. (API already existing + for internal use)

[gopherbot]

Change https://golang.org/cl/222277 mentions this issue: cmd/go/internal/modload: use a global lockfile to avoid spurious EDEADLK on AIX and Solaris

[gopherbot]

Change https://golang.org/cl/255377 mentions this issue: unix: add Flock on illumos

[gopherbot]

Change https://golang.org/cl/255258 mentions this issue: syscall, cmd/go/internal/lockedfile/internal/filelock: add and use Flock on illumos