vfs: integrate with CJS and ESM module loaders by mcollina · Pull Request #63653 · nodejs/node

mcollina · 2026-05-30T05:41:48Z

Makes `require()` and `import` resolve files served by `node:vfs`. Before this PR, mounted VFS files were only visible through `fs.*`; the loaders went straight to the real filesystem.

Mechanism: toggleable wrappers in the loaders. Null fast-path when no VFS is mounted; otherwise the VFS answers stat / read / realpath / package.json.

User-visible side effects:

Each `VirtualFileSystem` instance has a stable `layerId` (per-process monotonic counter).
ESM URLs resolved through a VFS gain `?vfs-layer=N` — visible in `import.meta.url`, matches the HMR cache-busting pattern, lets unmount scope-purge the cascaded loader's cache instead of flushing it globally.
CJS `__filename` is unchanged.

Review guide

Out of scope

SEA + VFS, migrating the C++ `package_configs_` cache, broader permission-model integration.

nodejs-github-bot · 2026-05-30T05:41:54Z

Review requested:

@nodejs/loaders

codecov · 2026-05-30T06:50:02Z

Codecov Report

❌ Patch coverage is 90.71207% with 60 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.32%. Comparing base (39b481b) to head (51b033a).
⚠️ Report is 26 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/vfs/setup.js	85.54%	59 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63653      +/-   ##
==========================================
- Coverage   91.96%   90.32%   -1.64%     
==========================================
  Files         379      732     +353     
  Lines      166638   237271   +70633     
  Branches    25497    44727   +19230     
==========================================
+ Hits       153242   214325   +61083     
- Misses      13099    14656    +1557     
- Partials      297     8290    +7993

Files with missing lines	Coverage Δ
lib/internal/modules/cjs/loader.js	`98.15% <100.00%> (+18.48%)`	⬆️
lib/internal/modules/esm/get_format.js	`94.83% <100.00%> (+21.03%)`	⬆️
lib/internal/modules/esm/load.js	`91.47% <100.00%> (+8.22%)`	⬆️
lib/internal/modules/esm/resolve.js	`99.04% <100.00%> (+11.14%)`	⬆️
lib/internal/modules/helpers.js	`98.75% <100.00%> (+7.70%)`	⬆️
lib/internal/modules/package_json_reader.js	`99.47% <100.00%> (+11.86%)`	⬆️
lib/internal/vfs/setup.js	`86.14% <85.54%> (-0.55%)`	⬇️

... and 472 files with indirect coverage changes

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mcollina · 2026-05-30T10:27:16Z

@joyeecheung take a look, should be easier to review.

@jsumners-nr

Restore the "DO NOT depend on the patchability" warnings in esm/load.js and esm/resolve.js that were dropped along with the fs imports. The warning still applies; it now also points at node:vfs as one of the formal hook mechanisms callers should reach for instead. Addresses review feedback from @jsumners-nr in nodejs#63653

nodejs-github-bot · 2026-06-03T08:04:09Z

CI: https://ci.nodejs.org/job/node-test-pull-request/73901/

nodejs-github-bot · 2026-06-03T11:52:41Z

CI: https://ci.nodejs.org/job/node-test-pull-request/73902/

joyeecheung

A design question recently occurred to me: have we explored the versioning of the mounting?

joyeecheung · 2026-06-03T12:09:57Z

+  const { clearPackageJSONCache } = require('internal/modules/package_json_reader');
+  clearPackageJSONCache();
+  // The ESM cascaded loader's loadCache is intentionally NOT cleared here:
+  // clearing it mid-flight (while another import() is awaiting nested


Isn't it also the case for require() anyway? (You can unmount it at the top level of a CJS module while the graph is loading and things can be broken there, it's always up to the user to ensure they don't unmount from an unsafe place).

An alternative approach would be to add a cache-busting search params to avoid the stale caches, which solves the identity problem. This is also what the test mock module does and how some user-land HMR works, see #61767 (comment)

I wanted to avoid search params because in my experience they are unreliable (in a graph) and I wanted to minimize memory leaks. Not sure if it's totally possible.

Doesn't this now leak all the time whenever new ESM are loaded through the VFS? Unless the identifiers are reused by subsequent mounting (i.e. graph 1, 2, 3 overlaps), the modules stay stale in the cache. With a search param in the identifier I think it will be possible walk through the caches and purge everything identified by an umounted VFS version in the search params during unmounting. Not that it eliminates the leaks (#63186 will continue to be an issue), but at least it reduces the leaks when the graphs don't overlap?

mcollina · 2026-06-03T12:40:18Z

A design question recently occurred to me: have we explored the versioning of the mounting?

what do you mean? You mean multiple vfs layers on top of each other?

joyeecheung · 2026-06-03T13:02:48Z

what do you mean? You mean multiple vfs layers on top of each other?

For the stacks to have some kind of version number/ID to identify the current status?

BTW I just noticed that there's no mention of unmount() and mount() in the VFS docs..

mcollina · 2026-06-03T20:10:47Z

BTW I just noticed that there's no mention of unmount() and mount() in the VFS docs..

I did purge them when doing the splitting; I forgot to bring them back. I'll add them to this PR.

mcollina · 2026-06-03T20:11:45Z

For the stacks to have some kind of version number/ID to identify the current status?

No but we totally should.

Each VirtualFileSystem now exposes a per-process monotonically increasing `layerId`, assigned at construction. The id is stable across mount/unmount cycles for the lifetime of the instance and surfaces in: - debug() output for register / deregister so the layer stack is visible when NODE_DEBUG=vfs is enabled; - the overlap ERR_INVALID_STATE message, which now names the layer ids of the conflicting mounts. The id is the building block for tagging cache entries with the owning VFS, which a follow-up will use to replace the global loader-cache flush in deregisterVFS with a scoped purge. Refs: nodejs#63653 Signed-off-by: Matteo Collina <hello@matteocollina.com>

Replace the global loader-cache flush in deregisterVFS with a scope-purge that only drops entries owned by the unmounting VFS. Per-layer ownership is determined two ways: - For CJS-style filename-keyed caches (Module._cache, Module._pathCache, the CJS stat cache, the helpers.js realpath cache, and the package.json caches) entries are filtered with `vfs.shouldHandle(filename)`. __filename stays a clean absolute path so user code that does `path.dirname(__filename)` or similar is unaffected. - For the ESM cascaded loader's loadCache, entries are tagged at resolve time: when finalizeResolution() detects the resolved path is VFS-owned (via the new loaderGetLayerForPath hook), it appends `?vfs-layer=<id>` to the URL. The tag surfaces in `import.meta.url`, matching the cache-busting pattern used by HMR tooling. On deregister, entries whose URL carries the tag for the unmounting layer are deleted. Multi-mount setups no longer pay the cross-VFS cache-warmup penalty when a single VFS unmounts, and ESM modules loaded from a VFS become reachable for purge instead of leaking forever in the cascaded loader. New helpers exposed for VFS: - cjs/loader.js: clearStatCacheForVFS - helpers.js: purgeRealpathCacheForVFS, loaderGetLayerForPath - package_json_reader.js: purgePackageJSONCacheForVFS Adds test-vfs-scoped-cache-purge covering both the multi-mount isolation and the import.meta.url tag visibility. Refs: nodejs#63653 Signed-off-by: Matteo Collina <hello@matteocollina.com>

mcollina · 2026-06-05T06:47:09Z

@joyeecheung PTAL

Each VirtualFileSystem now exposes a per-process monotonically increasing `layerId`, assigned at construction. The id is stable across mount/unmount cycles for the lifetime of the instance and surfaces in: - debug() output for register / deregister so the layer stack is visible when NODE_DEBUG=vfs is enabled; - the overlap ERR_INVALID_STATE message, which now names the layer ids of the conflicting mounts. The id is the building block for tagging cache entries with the owning VFS, which a follow-up will use to replace the global loader-cache flush in deregisterVFS with a scoped purge. Refs: nodejs#63653 Signed-off-by: Matteo Collina <hello@matteocollina.com>

Replace the global loader-cache flush in deregisterVFS with a scope-purge that only drops entries owned by the unmounting VFS. Per-layer ownership is determined two ways: - For CJS-style filename-keyed caches (Module._cache, Module._pathCache, the CJS stat cache, the helpers.js realpath cache, and the package.json caches) entries are filtered with `vfs.shouldHandle(filename)`. __filename stays a clean absolute path so user code that does `path.dirname(__filename)` or similar is unaffected. - For the ESM cascaded loader's loadCache, entries are tagged at resolve time: when finalizeResolution() detects the resolved path is VFS-owned (via the new loaderGetLayerForPath hook), it appends `?vfs-layer=<id>` to the URL. The tag surfaces in `import.meta.url`, matching the cache-busting pattern used by HMR tooling. On deregister, entries whose URL carries the tag for the unmounting layer are deleted. Multi-mount setups no longer pay the cross-VFS cache-warmup penalty when a single VFS unmounts, and ESM modules loaded from a VFS become reachable for purge instead of leaking forever in the cascaded loader. New helpers exposed for VFS: - cjs/loader.js: clearStatCacheForVFS - helpers.js: purgeRealpathCacheForVFS, loaderGetLayerForPath - package_json_reader.js: purgePackageJSONCacheForVFS Adds test-vfs-scoped-cache-purge covering both the multi-mount isolation and the import.meta.url tag visibility. Refs: nodejs#63653 Signed-off-by: Matteo Collina <hello@matteocollina.com>

jasnell · 2026-06-15T13:59:04Z

+let _loaderRealpath = null;
+let _loaderLegacyMainResolve = null;
+let _loaderGetFormatOfExtensionlessFile = null;
+let _loaderGetLayerForPath = null;


What's with the _ prefixes?

jasnell · 2026-06-16T09:22:07Z

+  const moduleCache = cjsLoader.Module._cache;
+  for (const filename of ObjectKeys(moduleCache)) {
+    if (isOwnedByVFS(vfs, filename)) {
+      delete moduleCache[filename];


Can this be made to use a Map instead. Using the delete is not ideal.

jasnell · 2026-06-16T09:23:44Z

+  const cjsLoader = require('internal/modules/cjs/loader');
+  const moduleCache = cjsLoader.Module._cache;
+  for (const filename of ObjectKeys(moduleCache)) {
+    if (isOwnedByVFS(vfs, filename)) {


Nit: isOwnedByVFS includes a try/catch that swallows possible errors. If that is throwing for whatever reason independent of the specific filename, then it's possible that every filename iteration here could trigger the catch case in the worst case, leading to a new wasted error and stack capture on every iteration.

joyeecheung

The code is doing a suspicious amount of path.resolve() everywhere (which would resolve the paths to process.cwd(), not against VFS, and the result would be observable in e.g. error messages), and the same filename can go through that multiple times repeatedly in various places, which seems rather wasteful..

nodejs-github-bot · 2026-06-16T10:10:10Z

CI: https://ci.nodejs.org/job/node-test-pull-request/74176/

ShogunPanda

LGTM!

mcollina · 2026-06-19T16:08:18Z

@joyeecheung PTAL

nodejs-github-bot · 2026-06-19T16:20:03Z

CI: https://ci.nodejs.org/job/node-test-pull-request/74271/

Co-authored-by: James M Snell <jasnell@gmail.com> Signed-off-by: Matteo Collina <hello@matteocollina.com>

nodejs-github-bot added lib / src Issues and PRs related to general changes in the lib or src directory. needs-ci PRs that need a full CI run. labels May 30, 2026

mcollina requested review from ShogunPanda, jasnell and joyeecheung May 30, 2026 10:26

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build: migrate from Workers to Docker jsdelivr/globalping-dash#204

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module: add --experimental-strip-private-modules #63869

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vfs: integrate with CJS and ESM module loaders

572c4bc

Co-authored-by: James M Snell <jasnell@gmail.com> Signed-off-by: Matteo Collina <hello@matteocollina.com>

mcollina force-pushed the vfs-module-loader-integration branch from 10412dc to 572c4bc Compare June 20, 2026 10:01

#	File	Role
1	`lib/internal/modules/helpers.js`	The hook surface: 9 `loader*` wrappers + 2 setters.
2	`lib/internal/modules/cjs/loader.js`	`stat()` + TS read routed through wrappers.
3	`lib/internal/modules/esm/resolve.js`	Routed; appends `?vfs-layer=N` after `finalizeResolution`.
4	`lib/internal/modules/esm/load.js`	`getSourceSync` reads via the wrapper.
5	`lib/internal/modules/esm/get_format.js`	Extensionless format detection routed.
6	`lib/internal/modules/package_json_reader.js`	4 C++ binding calls routed; adds scoped purge.
7	`lib/internal/vfs/file_system.js`	Exposes `vfs.layerId`.
8	`lib/internal/vfs/setup.js`	Heart of the PR. Installs overrides, reimplements `legacyMainResolve` + package.json ops with parity to `src/node_modules.cc` / `src/node_file.cc`, scope-purges caches on unmount.
9	`doc/api/vfs.md`	Mounting section + Module loader integration section.

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