`FileSystem.Lock.aquire` ends in `UnexpectedError ENOENT` when the lock is released while it checks it

[gilramir]

Found against: gren 0.6.6, gren-lang/compiler-node 5.0.0, gren-lang/core 7.4.2, gren-lang/node 6.1.3, node 22
Reproduction: https://github.com/gilramir/gren-bug-reports/tree/main/2026-09-16-lock-vanishes-during-metadata

Taking a lock that another process holds only briefly often ends in
UnexpectedError, when it should end with the lock taken or AlreadyLocked.
aquire creates the lock directory; if it exists, it reads its metadata to see
whether the lock is stale. If the other process removes the lock in between,
metadata fails with ENOENT, and the handler that means to retry tests for
the wrong error.

Reproduction

gren.json: a node application with "gren-lang/compiler-node": "5.0.0", "gren-lang/core": "7.4.2", "gren-lang/node": "6.1.3" (indirect "gren-lang/compiler-common": "3.0.0", "gren-lang/url": "6.0.0").

churn.js stands in for another process that holds the lock very briefly:

// Create and remove ./.lock in a tight loop, as another process holding the
// lock very briefly would.
const fs = require("node:fs");
for (;;) {
  try { fs.mkdirSync(".lock"); } catch (e) {}
  try { fs.rmdirSync(".lock"); } catch (e) {}
}

src/Main.gren tries to take the lock 2000 times, without retries, releasing it
whenever it gets it, and counts how each attempt ended:

(source elided here as it's very long)

gren make Main --output=app
node churn.js &
node app

Output (counts vary with timing):

outcome                      count
UnexpectedError ENOENT         953
already locked                 156
lock acquired                  891

Cause

LockMetaCheck { path, attempt, result = Err fsErr } ->
    if FileSystem.errorIsFileExists fsErr then
        -- Lock might have been removed while we asked for metadata
        update fsPerm (Lock { path = fromLockPath path, attempt = attempt + 1 }) model

    else
        UnexpectedError { path = fromLockPath path, error = fsErr }

The comment describes the right recovery, but reading a directory's metadata
never fails with EEXIST, so a lock that has been removed takes the else
branch.

Proposed Fix

The check should be FileSystem.errorIsNoSuchFileOrDirectory

With it, the same program gave 325 already locked and 1675 lock acquired.