worker: start worker threads from the built-in snapshot

[codebytere]

new Worker() currently runs the whole internal bootstrap in the new isolate (realm, node, web exposure, the thread and process-state switches), compiling ~80 builtins with the code cache before internal/main/worker_thread starts. Only the main thread deserializes its principal context from the built-in snapshot, and that bootstrap is about half of a worker's cold start.

The bootstrapped principal context in the snapshot is nearly thread-neutral already: is_not_main_thread.js and does_not_own_process_state.js are written as overrides of the main-thread switches, and the only thread-specific data baked into the context were six properties of the worker binding. This lets a worker deserialize the same kNodeMainContextIndex context and EnvSerializeInfo the main thread uses, and applies the two worker-side switches on top.

misc/startup-core.js mode='worker' script='test/fixtures/semicolon.js'                ***    98.45 %  ±0.82%   (21.1 -> 10.6 ms)
misc/startup-core.js mode='worker' script='benchmark/fixtures/empty.mjs'              ***    99.64 %  ±0.60%
misc/startup-core.js mode='worker' script='benchmark/fixtures/require-builtins.js'    ***    47.58 %  ±1.19%   (33.8 -> 22.9 ms)
misc/startup-core.js mode='worker' script='benchmark/fixtures/import-builtins.mjs'    ***    30.03 %  ±0.54%
misc/startup-core.js mode='worker' script='test/fixtures/snapshot/typescript.js'      ***     8.59 %  ±0.46%
misc/startup-core.js mode='process' (all five scripts)                                        ±0.5 %  n.s.

(x64 Linux, 30 runs. An idle worker's RSS also drops from ~12.2 to ~8.1 MiB, heapTotal 8.6 to 5.4 MiB.)

• worker binding: threadId, threadName, isMainThread, isInternalThread, ownsProcessState and resourceLimits become lazy properties of the per-isolate template, computed from the Environment on first read, so no bootstrapped context carries them. Bootstrap itself only takes getEnvMessagePort from the binding.
• CreateEnvironment(): an empty context already means "deserialize from the snapshot". When the caller is a worker (its IsolateData has a Worker), deserialize the main context as before, then run internal/bootstrap/switches/is_not_main_thread and, unless the worker owns process state, does_not_own_process_state after InitializeMainContext(). The isolate error-handler reset stays main-thread only.
• Worker::Run(): take that path only when the snapshot in use is the embedded one (an embedder's own snapshot or a --snapshot-blob one has run application code in its main context, so it is never reused), browser globals aren't disabled (kNoBrowserGlobals changes the bootstrap shape) and --no-worker-snapshot wasn't passed. Otherwise the worker bootstraps from scratch as today, which is also the path for embedders that create environments without a snapshot.
• is_not_main_thread.js: also delete process._debugPause, _startProfilerIdleNotifier and _stopProfilerIdleNotifier, which is_main_thread.js installs.
• --[no-]worker-snapshot per-isolate option, documented, as the escape hatch.

Modules read no options at bootstrap time (getCLIOptionsValues() throws before bootstrapping is done and refreshOptions() runs in pre-execution), so a worker's execArgv can't disagree with anything captured in the context; per-thread runtime state (argv, execArgv, title, env proxy contents, time origin, inspector, message port, stdio) is established after context creation by Worker::Run() and prepareWorkerThreadExecution() as before.

Testing:

• Object.getOwnPropertyNames() plus descriptor kinds of process and globalThis are identical between a from-snapshot and a --no-worker-snapshot worker, as are process listeners, features and versions.
• Same results as main for env: SHARE_ENV and copied env, resourceLimits, piped stdio, nested workers, eval/CJS/ESM/data: entries, exit codes, early terminate(), uncaught errors, structured-clone workerData, argv/execArgv/name, --frozen-intrinsics, --disable-proto=throw, the permission model, and a --build-snapshot user snapshot (workers do not see its globals).
• Full default suite on Release; worker, messageport, broadcastchannel, inspector-worker, async-hooks, process, bootstrap, snapshot and es-module suites on a Debug build (437/437).

An earlier version added a dedicated worker context to the snapshot instead. Any second bootstrapped context in the node_mksnapshot isolate currently fails inside V8's serializer (SerializeBackingStore() on a typed array reached from both contexts, or CHECK(!SerializePendingObject(*code)) in VisitJSDispatchTableEntry()), so reusing the existing context is both the smaller change and the one that works today; i'll file the serializer limitation separately.

---

Disclosure: the code, tests, measurements and this description were written by Claude Code, directed and reviewed by @codebytere.

[nodejs-github-bot]

Review requested:

• @nodejs/config
• @nodejs/startup

[codecov]

Codecov Report

❌ Patch coverage is 91.76471% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.16%. Comparing base (30bff4a) to head (a0d71cf).
⚠️ Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
src/api/environment.cc	61.53%	2 Missing and 3 partials ⚠️
src/node_worker.cc	96.77%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65336      +/-   ##
==========================================
+ Coverage   90.13%   90.16%   +0.03%     
==========================================
  Files         752      752              
  Lines      251568   251649      +81     
  Branches    47270    47290      +20     
==========================================
+ Hits       226759   226909     +150     
+ Misses      16168    16092      -76     
- Partials     8641     8648       +7     

Files with missing lines	Coverage Δ
.../internal/bootstrap/switches/is_not_main_thread.js	100.00% <100.00%> (ø)
src/node_options.cc	79.50% <100.00%> (-0.04%)	⬇️
src/node_options.h	95.37% <100.00%> (+0.02%)	⬆️
src/node_worker.h	91.66% <ø> (ø)
src/node_worker.cc	82.16% <96.77%> (+0.65%)	⬆️
src/api/environment.cc	77.74% <61.53%> (-0.68%)	⬇️

... and 55 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/75913/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/75930/