[robhogan]

TreeFS.getSerializableSnapshot() deep-copies the whole tree - a Map per directory and an array per file - before every cache write:

metro/packages/metro-file-map/src/lib/TreeFS.js

Lines 1476 to 1486 in 1461d81

	#cloneTree(root: DirectoryNode): DirectoryNode {
	const clone: DirectoryNode = new Map();
	for (const [name, node] of root) {
	if (isDirectory(node)) {
	clone.set(name, this.#cloneTree(node));
	} else {
	clone.set(name, [...node]);
	}
	}
	return clone;
	}

DiskCacheManager serialises the snapshot as soon as it asks for it, so nothing can change it in between and the copy is thrown away:

metro/packages/metro-file-map/src/cache/DiskCacheManager.js

Lines 119 to 121 in 1461d81

	const data = getSnapshot();
	this.#hasUnwrittenChanges = false;
	const serialized = serialize(data);

This returns the tree itself. That saves a copy of the file map on every cold start, and on every save of the cache while watching.

A custom cache manager (unstable_fileMapCacheManagerFactory) that holds on to the snapshot to serialise later would now see later changes, so CacheManager.write's docs now say to serialise what getSnapshot returns straight away, or copy it. Expo's fork (@expo/metro-file-map) already skips the deep copy, for the same reason.

Benchmark

This takes 28-51% off writing the cache (Windows: 19.4ms -> 9.5ms), or 2-5% off a cold start on Linux and Windows. macOS's cold starts are too noisy to show it. The same copy was made on every save of the cache while watching, which this benchmark doesn't exercise.

	main	#2108	This PR	vs main	vs #2108
macOS: Cache write in a cold start	7.8ms	6.6ms	4.3ms	-44.4% (-45.1% to -43.6%)	-34.2% (-35.4% to -33.7%)
Linux: Cache write in a cold start	30ms	28ms	20ms	-31.9% (-39.7% to -22.0%)	-27.9% (-32.0% to -25.1%)
Windows: Cache write in a cold start	20ms	19.4ms	9.5ms	-53.5% (-54.9% to -36.4%)	-51.0% (-53.2% to -32.1%)
macOS: Cold start	129ms	96ms	90ms	-30.2% (-36.0% to -22.6%)	-5.7% (-14.6% to +8.9%)
Linux: Cold start	648ms	324ms	319ms	-50.8% (-51.7% to -50.1%)	-1.8% (-5.0% to -0.5%)
Windows: Cold start	363ms	179ms	170ms	-53.2% (-53.8% to -52.5%)	-5.1% (-6.8% to -2.6%)
macOS: Watch-mode start, cold cache	134ms	98ms	96ms	-28.6% (-29.5% to -27.0%)	-2.7% (-4.5% to +0.2%)
Linux: Watch-mode start, cold cache	1,892ms	470ms	451ms	-76.1% (-76.3% to -75.8%)	-3.9% (-4.9% to -0.9%)
Windows: Watch-mode start, cold cache	2,556ms	496ms	490ms	-80.8% (-80.9% to -80.8%)	-1.1% (-1.6% to -0.7%)

Changelog

 - **[Performance]**: metro-file-map: Write the cache without copying the file map first

Test plan

New unit test that DiskCacheManager serialises the snapshot before anything else can run. Benchmark as in #2104, with 24 rounds on macOS.