Release workers and watchers after failed Bundler initialisation

[robhogan]

Follow-up to #1854, which made Bundler initialisation errors reject ready().

This fixes cases where a failed transformer could lead Metro to hang in a zombie state.

Teardown still didn't account for a failed initialisation, so Metro could leak worker processes and file watchers when, for example, the transformer fails to load:

• Transformer created its WorkerFarm - which spawns jest-worker child processes when maxWorkers > 1 - before computing the transform cache key. getTransformCacheKey is exactly where a missing transformer or Babel preset throws ([Metro 0.84.x] Silent catch in Bundler._initializedPromise causes misleading Cannot read properties of undefined (reading 'transformFile') instead of the real error #1808), and when it did, the constructor threw away the only reference to the farm, so nothing could kill it. Computing the cache key first means any constructor failure happens before workers start.
• Bundler.end() awaited ready() before ending anything, so after a failed initialisation it rejected without calling DependencyGraph.end(), leaving the file map's watcher, health check interval and file processor running. It now waits for initialisation to settle, ends the transformer if one was constructed, and always ends the dependency graph.
• DependencyGraph.end() had the same shape with respect to a failed fileMap.build(), so it now ends the file map regardless.

Initialisation errors are still surfaced through ready() and the reporter - end() now resolves either way, since teardown should complete even if startup didn't.

(Before #1854, end() failed at the same point by dereferencing an undefined _transformer, so none of this is a regression.)

Changelog:

 - **[Fix]**: Release transform workers and file watchers when Metro fails to initialise

Test plan

End to end - a project whose transformerPath points at a module that throws on load, the same way a missing Babel preset does:

const error = new Error("Cannot find module 'babel-preset-missing'");
error.code = 'MODULE_NOT_FOUND';
throw error;

with maxWorkers: 2, resolver.useWatchman: false and a no-op cache store (the transform cache key is only computed with a cache enabled). A script runs Metro from source, either through Metro.runBuild() or through Metro.runMetro(config, {watch: true}) followed by server.ready() and server.end(). It then waits 1s, counts its own child processes, lists process.getActiveResourcesInfo(), and forces an exit if it's still alive 10s later.

	main (1f83bfc)	Transformer change only	This diff
runBuild()	Rejects. 2 worker processes left running, hangs until forced exit	Rejects. Nothing left running, exits	Rejects. Nothing left running, exits
runMetro(), end()	end() rejects. 2 worker processes and the file map's change interval left running, hangs until forced exit	end() rejects. Change interval left running, hangs until forced exit	end() resolves. Nothing left running, exits

So the Transformer reordering is enough for one-shot builds, and the end() changes are needed as well for a watching server.

Unit tests - new cases in Bundler-test, Transformer-test and DependencyGraph-test. The three covering the fixes each fail without the source changes and pass with them.

yarn jest packages/metro
yarn flow check
yarn lint

[Copilot]

Copilot review overview

🟡 Changes recommended

Ensure dependency-graph cleanup runs even when transformer teardown rejects.

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Open (1)

• Ensure dependency graph cleanup after transformer teardown failures · New

What changed in this PR

Fixes Metro resource leaks when Bundler or DependencyGraph initialization fails.

Changes:

• Defers transformer worker creation until cache-key computation succeeds.
• Makes teardown resilient to failed initialization.
• Adds regression tests for worker and file-map cleanup.

The Bundler teardown should still ensure DependencyGraph.end() runs if transformer cleanup rejects.

File	Description
packages/​metro/​src/​node-haste/​DependencyGraph.js	Cleans up the file map after failed builds.
packages/​metro/​src/​node-haste/​__tests__/​DependencyGraph-test.js	Tests file-map cleanup.
packages/​metro/​src/​DeltaBundler/​Transformer.js	Delays worker creation until initialization succeeds.
packages/​metro/​src/​DeltaBundler/​__tests__/​Transformer-test.js	Tests worker suppression on cache-key failure.
packages/​metro/​src/​Bundler.js	Handles teardown after failed initialization.
packages/​metro/​src/​__tests__/​Bundler-test.js	Tests Bundler cleanup behavior.

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[Copilot]

Copilot review overview

🟢 Approval recommended

No unresolved issues were identified, and all reviewed changes are covered by regression tests.

Review effort: Lite
Findings: None

Resolved since last review (1)

• Ensure dependency graph cleanup after transformer teardown failures