xattrs: use unix flocks when writing multiple xattrs

[butonic]

checking the flock implementation options...

• x/sys/unix ... current state of this PR ... ties us to unix
• https://github.com/gofrs/flock ... uses a dedicated file ... I still like this ... but it seems unmaintaind
• wait for official lockfile: Proposal: make the internal lockedfile package public
• use https://github.com/rogpeppe/go-internal/blob/master/lockedfile/lockedfile.go until then? because it seems maintained.

[dragotin]

I think this can only be the start.

A bit of copy & paste from the flock() manpage (linux system call):

• flock() does not lock files over NFS.
• flock() places advisory locks only; given suitable permissions on a file, a process is free to ignore the use of flock() and perform I/O on the file.
• flock() is blocking. I would have implemented a lock base function that uses the unblocking variant of flock, but waits for a few milliseconds and tries again.
• The read function All() (at least) needs to place a shared lock for read. That must be considered here as well.
• Any reason why you did not choose a flock golang library that is multi platform?

[butonic]

https://github.com/gofrs/flock was the first implementation we looked at, but stumbled over the fact that it is intendet to write a lock file, not folders. Furthermore, we discovered the proposal to expose the existing locking implementation in the internal golang/go/src/cmd/go/internal/lockedfile package. But all threads lead to file locks, so I implemented a unix based lock.

[butonic]

we will use https://github.com/rogpeppe/go-internal/blob/master/lockedfile/lockedfile.go https://github.com/gofrs/flock to write a {node}.flock file when setting multiple extended attributes, OR when creating a application level log ({node}.lock).

it comes with nice TryLockCtx methods which wrap around the functionality we want.

[dragotin]

Awesome stuff!