[robhogan]

Without Watchman, Watcher starts its backends before the crawl, so that nothing changed during the crawl is missed:

metro/packages/metro-file-map/src/Watcher.js

Lines 118 to 127 in 56efab3

	// Non-Watchman backends have no clock to subscribe `since`, so they must
	// already be watching before the crawl begins, and their events buffered,
	// for changes made while crawling not to be missed. Watchman backends are
	// started by watch(), which subscribes `since` the crawl's clock to cover
	// the same window. If the Watchman crawl falls back to the node crawler,
	// there is no clock, so #crawl() starts non-Watchman backends before the
	// node crawl instead.
	if (!options.useWatchman) {
	await this.#startBackendsBeforeCrawl();
	}

For FallbackWatcher that means walking the tree once to watch it and again to crawl it. This lets the crawl do both: the crawler hands each directory to the watcher just before listing it, the order the watcher's own walk uses. Directories the crawl skips as ignored are handed over too, for the watcher to walk unless it ignores them as well (eg node_modules without retainAllFiles).

This is internal to the built-in node crawler. A crawlerFactory crawler, a recrawl and a root added with addRoot keep the watcher's own walk.

Benchmark

This takes another 28-30% off watch-mode startup on Linux and Windows. With the rest of this stack, watch-mode startup without Watchman goes from 1,896ms to 510ms (-73%) on Linux, and from 2,554ms to 522ms (-80%) on Windows. On macOS, which uses NativeWatcher, PRs 2-5 measure within noise of #2104.

	main	#2107	This PR	vs main	vs #2107
Linux: Watch-mode start, warm cache	1,896ms	707ms	510ms	-73.1% (-73.4% to -72.8%)	-27.9% (-28.6% to -27.2%)
Windows: Watch-mode start, warm cache	2,554ms	721ms	522ms	-79.6% (-79.7% to -79.4%)	-27.7% (-28.0% to -27.2%)
Linux: Watch-mode start, cold cache	1,882ms	699ms	491ms	-73.9% (-74.5% to -73.5%)	-29.7% (-31.1% to -28.5%)
Windows: Watch-mode start, cold cache	2,547ms	709ms	507ms	-80.1% (-80.2% to -80.0%)	-28.5% (-28.9% to -27.9%)
Linux: Heap after a watch-mode start	31.8MB	28.3MB	28.3MB	-11.1% (-11.1% to -11.1%)	-0.1% (-0.1% to -0.1%)
Windows: Heap after a watch-mode start	31.9MB	28.4MB	28.4MB	-11.1% (-11.1% to -11.1%)	-0.1% (-0.1% to -0.1%)
Linux: RSS after a watch-mode start	197.6MB	175.2MB	174.3MB	-11.8% (-12.3% to -11.3%)	-0.5% (-0.9% to +0.0%)
Windows: RSS after a watch-mode start	206.1MB	182.9MB	183.9MB	-10.7% (-11.3% to -10.3%)	+0.6% (+0.4% to +0.8%)

Changelog

 - **[Performance]**: metro-file-map: Set up file watching during the crawl instead of after it, on Linux and Windows without Watchman

Test plan

New unit tests, and a watch-integration-test.js case for a directory the crawl skips but the watcher doesn't. Benchmark as in #2104.