[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 (eg node_modules without retainAllFiles) are handed over too, for the watcher to walk unless it ignores them as well.

This is internal to the built-in node crawler. A crawlerFactory crawler, a recrawl, a root added with addRoot and the node crawl after a failed Watchman crawl 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 1,887ms to 492ms (-74%) on Linux, and from 2,577ms to 525ms (-80%) on Windows. On macOS, which uses NativeWatcher, PRs 2-5 stay within 5% of #2104, which is noise: #2106 changes nothing there and measures +4.6%.

	main	#2107	This PR	vs main	vs #2107
Linux: Watch-mode start, warm cache	1,887ms	695ms	492ms	-73.9% (-74.2% to -73.7%)	-29.2% (-29.9% to -28.6%)
Windows: Watch-mode start, warm cache	2,577ms	727ms	525ms	-79.6% (-79.7% to -79.5%)	-27.7% (-28.1% to -27.4%)
Linux: Watch-mode start, cold cache	1,885ms	652ms	462ms	-75.5% (-75.8% to -75.0%)	-29.2% (-30.0% to -27.7%)
Windows: Watch-mode start, cold cache	2,562ms	701ms	493ms	-80.8% (-80.8% to -80.6%)	-29.6% (-29.9% to -29.3%)
Linux: Heap after a watch-mode start	31.7MB	28.1MB	28.1MB	-11.3% (-11.3% to -11.3%)	-0.1% (-0.1% to -0.1%)
Windows: Heap after a watch-mode start	31.7MB	28.2MB	28.2MB	-11.3% (-11.3% to -11.3%)	-0.0% (-0.0% to -0.0%)

Changelog

 - **[Performance]**: metro-file-map: Set up file watching during the crawl instead of in a separate walk before 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.