Use F_OFD_SETLK instead of exfile_api()

[alandekok]

Message

Linux and apparently OSX support F_OFD_SETLK, which is what you would really expect from F_SETLK.

https://lwn.net/Articles/586904/

https://www.gnu.org/software/libc/manual/html_node/Open-File-Description-Locks.html

https://man7.org/linux/man-pages/man2/fcntl.2.html

https://github.com/apple/darwin-xnu/blob/main/bsd/sys/fcntl.h

It's not clear if FreeBSD has them.

This would let us get rid of most of exfile.c(), and replace it with just open / lock / close.

The issue with normal F_SETLK is that they are process specific. So two threads can't lock the same file. With F_OFD_SETLK, different threads should be able to lock the same file without an issue.

[arr2036]

The linux man page states:

F_OFD_SETLK (struct flock *)
              Acquire an open file description lock (when l_type is
              F_RDLCK or F_WRLCK) or release an open file description
              lock (when l_type is F_UNLCK) on the bytes specified by
              the l_whence, l_start, and l_len fields of lock.  If a
              conflicting lock is held by another process, this call
              returns -1 and sets [errno](https://man7.org/linux/man-pages/man3/errno.3.html) to EAGAIN.

[arr2036]

The key bit being "held by another process".

[alandekok]

From the GNU page:

Open file description locks are useful in the same sorts of situations as process-associated locks. They can also be used to synchronize file access between threads within the same process by having each thread perform its own open of the file, to obtain its own open file description.

[arr2036]

Would be very useful if they work :)

[arr2036]

https://www.austingroupbugs.net/view.php?id=768 - Potentially made it into FreeBSD in 2022?