metro-file-map: Identify TreeFS directory nodes by shape, not realm

[robhogan]

TreeFS uses an internal instanceof Map check in its internal isDirectory function, to identify whether a given node represented a directory (Map) or file (Array/tuple).

The only problem with instanceof Map is under Jest, in tests where we're exercising the file map cache. In Jest, tests are executed in v8 contexts where Map gets a new prototype, such that a file map cache v8-serialised in one test is deserialised but can't be traversed in another test, because instanceof Map checks fail on the foreign maps.

I discovered this while writing some unrelated tests that failed unexpectedly. We already have at least three tests in main that pass by accident (isDirectory returns false despite the node being a Map, just a foreign one) and effectively weren't testing anything. (Luckily, they all pass anyway)

An easy fix is to invert the implementation of isDirectory so that it becomes "not null and not a file", using Array.isArray, which is portable across realms.

As it turns out, this is also marginally faster.

Changelog: [Internal]

Test plan

Microbenchmark

isDirectory is hot so just to confirm this doesn't regress:

const m = new Map([['a', 1]]);
const f = [0, 1, 2, 3, 0];
const nodes = [m, f, m, f, m, m, f, m];
const fn =
  process.argv[2] === 'instanceof'
    ? n => n instanceof Map
    : n => n != null && !Array.isArray(n);
let best = Infinity;
for (let r = 0; r < 5; r++) {
  let c = 0;
  const t = process.hrtime.bigint();
  for (let i = 0; i < 2e8; i++) c += fn(nodes[i & 7]) ? 1 : 0;
  best = Math.min(best, Number(process.hrtime.bigint() - t) / 1e6);
}
console.log(process.argv[2], (best | 0) + ' ms');

Check	Run 1	Run 2	Per call
node instanceof Map	216 ms	216 ms	1.08 ns
node != null && !Array.isArray(node)	180 ms	179 ms	0.90 ns

Array.isArray is marginally faster with a mix of inputs - implemented with a slot read rather than a prototype walk.

[meta-codesync]

@vzaidman has imported this pull request. If you are a Meta employee, you can view this in D120355345.