`fcntl` module provides no access to F_GETLK operation

Wrapper/access for fcntl(fd, F_GETLK, …)

The fcntl module currently exposes the F_GETLK constant itself, but unfortunately because the types in struct flock are OS and CFLAGS dependent (off_t, pid_t), the operation is hard to work-around with struct.unpack.

Pitch

Exposing F_GETLK is particularly useful to determine if a process that created a lock file or pid file is actually still alive, vs. whether the lock/pid file was left uncleaned from a previous exit/crash/etc. The l_pid field is the most useful, but if wrapping the call anyway it might make more sense to just expose all values.

I've already implemented this, a PR is following shortly.

Linked PRs

• gh-112372
• gh-96695

Start with the fcntl module's existing F_GETLK constant and review the platform-dependent struct flock types described in the issue. Check linked PRs gh-112372 and gh-96695 before starting, since the requested implementation is already reported as complete. Done means exposing F_GETLK access without requiring callers to guess OS- and CFLAGS-dependent layouts.