vfs: enforce mutual exclusion in MemFS.Lock

[jbowens]

Previously MemFS's Lock method did not implement mutual exclusion under the assumption that databases are given separate processes which do not share memory. This commit adapts Lock to enforce mutual exclusion, preventing accidentally using same FS concurrently from more than one Pebble instance. It's believed that cockroachdb/cockroach#110645 is the result of Cockroach acceptance tests opening a MemFS store twice, with one Pebble instance deleting files from beneath another Pebble instance.

[cockroach-teamcity]

This change is 

[RaduBerinde]

Reviewable status: 0 of 3 files reviewed, 2 unresolved discussions (waiting on @itsbilal and @jbowens)

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vfs/mem_fs.go line 468 at r1 (raw file):

	// close the same MemFS-backed database multiple times. We want mutual
	// exclusion in this case too. See cockroachdb/cockroach#110645.
	_, loaded := y.lockedFiles.Swap(fullname, true)

[nit] the value could be nil here (I initially wondered if the true value has some significance)

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vfs/mem_fs.go line 474 at r1 (raw file):

		return nil, syscall.EAGAIN
	}
	// Otherwise, we successfully acquired the lock. Locks are visible in the

"locks are visible in the file" - did you mean "filesystem"?

[jbowens]

TFTR!

Reviewable status: 0 of 3 files reviewed, all discussions resolved (waiting on @itsbilal)

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vfs/mem_fs.go line 468 at r1 (raw file):

Previously, RaduBerinde wrote…

[nit] the value could be nil here (I initially wondered if the true value has some significance)

Done.

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vfs/mem_fs.go line 474 at r1 (raw file):

Previously, RaduBerinde wrote…

"locks are visible in the file" - did you mean "filesystem"?

Done.