[robhogan]

Without Watchman, Metro watches with FallbackWatcher on Linux and Windows. Its startup walk lstats every entry, files included, just to learn its type:

metro/packages/metro-file-map/src/watchers/FallbackWatcher.js

Lines 505 to 515 in 56efab3

	const visit = async (entry: string) => {
	let stats;
	let names;
	try {
	stats = await fsPromises.lstat(entry);
	if (!stats.isDirectory()) {
	if (stats.isSymbolicLink()) {
	symlinkCallback(entry, stats);
	} else if (stats.isFile()) {
	fileCallback(entry, stats);
	}

This walks by the types readdir reports instead. A directory that appears while watching still needs stats for its events, so that walk now stats only the entries it reports. The walk also now descends into a watch root that's a symlink, as the crawler does.

Benchmark

This takes 40% off watch-mode startup on Linux (1,617ms -> 964ms) and 20% off on Windows. macOS uses NativeWatcher, so it's unaffected.

	main	#2104	This PR	vs main	vs #2104
Linux: Watch-mode start, warm cache	1,896ms	1,617ms	964ms	-49.2% (-49.7% to -48.7%)	-40.4% (-40.7% to -39.7%)
Windows: Watch-mode start, warm cache	2,554ms	2,429ms	1,939ms	-24.1% (-24.7% to -23.7%)	-20.2% (-20.9% to -19.8%)
Linux: Watch-mode start, cold cache	1,882ms	1,599ms	972ms	-48.4% (-48.9% to -48.1%)	-39.2% (-39.9% to -39.0%)
Windows: Watch-mode start, cold cache	2,547ms	2,419ms	1,911ms	-25.0% (-25.3% to -24.6%)	-21.0% (-21.3% to -20.5%)

Changelog

 - **[Performance]**: metro-file-map: Start the file watcher without stat-ing every file (Linux and Windows, without Watchman)

Test plan

New unit test that startup doesn't lstat anything it finds. Benchmark as in #2104.