module loader: fetch a module again after an earlier load of it failed to build by robobun · Pull Request #39204 · oven-sh/bun

robobun · 2026-08-15T19:45:03Z

Problem

After an import() of a module with two or more build errors rejects with a proper AggregateError ("2 errors building ...", errors, cause), every later load of the same module in the process rejects or throws an AggregateError whose only own property is message: a second import(), a require(), or any other module that imports it. bun test hits the last form whenever two test files import the same broken module: the second file's report has no build errors in it, and the native error printer then crashes on the errors-less AggregateError (panic(main thread): Segmentation fault at address 0x5, bun testrunner crash #36963; the printer side of that is hardened separately in error printer: print the AggregateError header, label [cause]/[errors] blocks, and guard the .errors walk #36602).
A plugin module whose build.module() / onLoad callback rejects asynchronously loses the error's own properties (code, ...) the same way on the second import, and the callback is never run again.
Cause: the first failure leaves a ModuleRegistryEntry in status FetchFailed in JSC's module registry. Every later load of the key is settled from that entry, and for an ErrorInstance the registry does not hand back the stored error but JSModuleLoader::duplicateError's copy of it (vendor/WebKit/Source/JavaScriptCore/runtime/JSModuleLoader.cpp:116, reached from hostLoadImportedModule at :696 for static imports and from ModuleRegistryEntry::error() at ModuleRegistryEntry.cpp:149 for import() and require()). The copy is built from the error's type and message only. JSC does this for WPT, which wants distinct error objects for failed network fetches; Bun's fetch errors carry their information in own properties, so the copy is useless here. BuildMessage / ResolveMessage (the single-error case) are not ErrorInstances and pass through unchanged, which is why only the multi-error and plugin cases were visible.
The module is also never fetched again: fixing the file and importing it again in the same process still failed, where Node loads it.

Fix

evictFetchFailedModuleRegistryEntry() (src/jsc/bindings/ModuleLoader.cpp): if every registry entry under a key is in status FetchFailed, remove the key (JSModuleLoader::removeEntry, the same call mock.module() and delete require.cache[...] use). It is called right before each place the loader would consult the entry:
- GlobalObject::moduleLoaderResolve (ZigGlobalObject.cpp): resolution is the host hook JSC runs immediately before the registry lookup, for static imports (hostLoadImportedModule), import() (requestImportModule) and entry points (loadAndEvaluateModule) alike. The existing body moved into resolveModuleSpecifier() unchanged; the hook now calls it and then evicts. Bun.plugin: don't leave a failing onLoad cached in the module registry #33419 puts a plugin-only version of this eviction at the same spot; this change covers that case too (see the plugin test), without having to record the failures first.
- fetchCommonJSModule (ModuleLoader.cpp) and builtinLoader (JSCommonJSExtensions.cpp), the two require() entry points. They do not resolve through the loader, and every branch of them ends in provideFetch() (a no-op on an entry that is not New) or $requireESM (which returns the stored error), so a failed entry has to go before them. fetchCommonJSModule already built this Identifier further down; it is built once now.
Why it is correct:
- A FetchFailed entry has no module record: no module is linked against it and LoadedModules never refer to it. Loads that were in flight when it failed hold the entry's promises directly and are unaffected by its removal from the map. The only thing the entry does is replay the failure, so removing it just makes the next load fetch (transpile, or run the plugin) again and reject with the complete error of that attempt, or load the module if the file was fixed. This is Node's behavior: a module whose load failed is loaded again by the next import() or require() (verified with node v26: the second import() of a file with a syntax error gets a new error, and the third one succeeds after the file is rewritten), while a module that threw during evaluation stays failed.
- Only FetchFailed entries are removed. Fetching/Fetched entries may be shared by an in-flight or completed load, and InstantiationFailed/EvaluationFailed entries own a record whose failure the spec requires to be cached (that is also what require(): run a module again after an earlier load of it threw #38645 relies on for its require()-only retry of evaluation failures; the two changes compose, require(): run a module again after an earlier load of it threw #38645's helper would just take over the FetchFailed case on the require() paths).
- removeEntry() drops every (key, type) variant of the key at once (the registry is keyed by specifier plus import type). The helper therefore checks all five types and does nothing if any variant is in another state, so a failed import("./x") never unloads an import("./x", { with: { type: "text" } }) that succeeded (and vice versa). In that mixed case the failed variant keeps replaying the copied error as before; it needs a per-variant removal that the loader does not expose.
- Cost: resolve() does one pointer-keyed hash lookup per import of an already loaded module (the JavaScript bucket is probed first and any non-failed entry ends the check), and five misses for a module's first load, next to the filesystem resolution it just did. fetchCommonJSModule already did an equivalent lookup per require().
Verified with test/js/bun/resolve/build-error.test.ts, describe("loading a module again after it failed to build"): second import() and require() of a module with two build errors, a second module importing it, the bun test two-file case from bun testrunner crash #36963 (both files print the errors, no crash), import() / require() / require.extensions['.js'] loading the module after the file is fixed (each exercises one of the three call sites), the mixed text variant keeping its module identity, a plugin module being run again and delivering the object it threw, and, as a control that passes either way, an evaluation error still being cached. The six new tests fail on bun 1.4.0 and on a debug build of main without the src/ changes (the "fixed file" one shows all three paths still returning the stripped AggregateError), and pass with them.
Also run with the debug build: test/js/bun/resolve/ (one pre-existing timeout in load-same-js-file-a-lot.test.ts, identical on the unmodified build: the test has no debug/ASAN timeout allowance), test/js/bun/plugin/, test/js/bun/test/mock/, test/js/node/module/, test/cli/test/isolation.test.ts, test/cli/hot/hot.test.ts, the module-related files in test/cli/run/ (the RSS-threshold leak fixtures fail the same way on the unmodified build under this ASAN build), and the repro under BUN_JSC_validateExceptionChecks=1.

Fixes #36963

Background

Module registry: JSC's JSModuleLoader keeps a ModuleRegistryEntry per (resolved key, import type). An entry goes New -> Fetching -> Fetched (it then holds the module record) or ends in FetchFailed; a failure while linking or evaluating the record is stored as InstantiationFailed / EvaluationFailed. Since the loader was ported to C++ it also caches failures: a later load of a key with a stored error is settled from the entry without calling the host again.
Fetch: the host hook (moduleLoaderFetch -> fetchESMSourceCode -> Bun__transpileFile, or a plugin) that turns a key into source code. A transpile error, a plugin rejection or a JSON parse error are all fetch failures; single-message transpile failures reject with a BuildMessage/ResolveMessage, several messages with an AggregateError of them (process_fetch_log in src/jsc/VirtualMachine.rs).
JSModuleLoader::duplicateError: creates a new error of the same ErrorType with the same message and copies five loader-private properties; nothing else. JSC's own callers outside the registry only apply it to errors carrying a private "fetch failure kind" marker that WebCore sets; the two registry paths above apply it to every ErrorInstance.
require() of a non-CommonJS file: fetchCommonJSModule hands the source to the registry with provideFetch() and returns -1, and the builtin $requireESM then loads the key synchronously through the same registry (loadModuleSync). builtinLoader is the Module._extensions['.js'] implementation, used when require.extensions is called or overridden.

Repro

bad.js:    const dup = 1; const dup = 2; const dup = 3;
main.js:
  for (let i = 1; i <= 2; i++)
    try { await import("./bad.js"); } catch (e) { console.log(i, Object.getOwnPropertyNames(e), e.errors?.length); }
  try { require("./bad.js"); } catch (e) { console.log("require", Object.getOwnPropertyNames(e), e.errors?.length); }

bun 1.4.0:

1 [ "message", "cause", "errors" ] 2
2 [ "message" ] undefined
require [ "message" ] undefined

With this change all three lines carry errors (length 2). Two test files import "./bad.js" under bun test print the two build errors for each file and exit 1 instead of crashing on the second file.

…d to build JSC keeps the registry entry of a module whose fetch was rejected and settles every later load of that key with JSModuleLoader::duplicateError's copy of the stored error, which keeps only the type and message. The second import() (or a require(), or another module importing it) of a module with several build errors therefore got an AggregateError without errors, and a plugin error lost its own properties. Drop an entry whose every variant is FetchFailed right before the loader looks the key up: in the resolve hook for static imports, import() and entry points, and at the two require() entry points, which do not resolve through the loader. The module is fetched again and each importer gets the complete error of its own attempt, or the module once the file is fixed, as in Node. Entries that failed to link or evaluate are left cached.

coderabbitai · 2026-08-15T19:45:11Z

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Reviewing files that changed from the base of the PR and between b97b9ea and 114c35e.

📒 Files selected for processing (3)

src/jsc/bindings/ModuleLoader.cpp
src/jsc/bindings/ModuleLoader.h
src/jsc/bindings/ZigGlobalObject.cpp

Walkthrough

Changes

The module loader now evicts stale fetch-failed registry entries before retrying module resolution and CommonJS loads. New tests cover repeated failures, retries after source fixes, plugin failures, and cached evaluation errors.

Module fetch retry handling

Layer / File(s)	Summary
Failed-entry eviction API `src/jsc/bindings/ModuleLoader.h`, `src/jsc/bindings/ModuleLoader.cpp`	Adds `evictFetchFailedModuleRegistryEntry`, which removes a module key only when all present registry variants are fetch-failed.
Resolution and loading integration `src/jsc/bindings/ZigGlobalObject.cpp`, `src/jsc/bindings/ModuleLoader.cpp`, `src/jsc/bindings/JSCommonJSExtensions.cpp`	Module resolution and loading evict stale failed entries before retrying. CommonJS loading reuses the computed module key.
Retry regression coverage `test/js/bun/resolve/build-error.test.ts`	Adds concurrent tests for repeated build failures, error propagation, reloads after source fixes, plugin retries, alternate module types, and cached evaluation errors.

Possibly related PRs

oven-sh/bun#37185: Both changes modify module registry behavior in ModuleLoader.cpp.
oven-sh/bun#37765: Both changes cover retrying failed module loads.
oven-sh/bun#38273: Both changes modify module resolution failure handling.

Suggested reviewers: jarred-sumner

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: retrying module fetches after an earlier build failure.
Description check	✅ Passed	The description explains the problem, fix, rationale, affected paths, and extensive verification results.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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robobun · 2026-08-15T19:45:14Z

^{Updated 3:05 PM PT - Aug 15th, 2026}

❌ @robobun, your commit 114c35e has some failures in Build #98669 (All Failures)

🧪 To try this PR locally:

bunx bun-pr 39204

That installs a local version of the PR into your bun-39204 executable, so you can run:

bun-39204 --bun

robobun · 2026-08-15T19:45:52Z

Status: reproduced on bun 1.4.0 and on a debug build of main with the two-file repro in the PR description (second import() and require() of a module with two build errors come back as an AggregateError with only message; the bun test variant is #36963). The new tests in test/js/bun/resolve/build-error.test.ts fail without the src/ changes and pass with them.

CI on 114c35e (build 98669): 178 of 179 jobs passed. The one red lane is macOS 14 arm64, where test/js/third_party/grpc-js/test-tonic.test.ts failed with a gRPC CANCELLED status; that test does not involve module loading and has been reported separately. The other annotations in that build are tests that passed on retry.

Related open PRs: #33419 (plugin-only eviction at the same hook, covered by this change) and #38645 (require() retry of evaluation failures, composes with this change).

claude

I reviewed this PR and didn't find any bugs. Because it changes hot-path module-loader behavior (adding eviction of FetchFailed registry entries at every moduleLoaderResolve call and both require() entry points), a human look would still be worthwhile.

What was reviewed:

evictFetchFailedModuleRegistryEntry follows the existing cellLock() + removeEntry() pattern used in BunPlugin.cpp, bindings.cpp, and ZigGlobalObject.cpp.
The moduleLoaderResolve → resolveModuleSpecifier extraction preserves behavior; the added outer throw scope + RETURN_IF_EXCEPTION returns the same empty identifier the inner path already produced.
moduleKey in fetchCommonJSModule is now built once and reused for the pre-existing registryEntry(moduleKey) lookup — no double-construction or lifetime change.
Tests cover all three call sites, the mixed-type-variant guard, the plugin path, and the negative case (evaluation failures stay cached).

Extended reasoning...

Overview

This PR fixes #36963: after a module's fetch fails (transpile errors, plugin rejection), JSC caches a FetchFailed registry entry and every subsequent load returns duplicateError's type-and-message-only copy — losing errors on an AggregateError and causing the bun test crash when a second test file imports the same broken module. The fix adds evictFetchFailedModuleRegistryEntry() and calls it from the three places the loader is about to consult the entry: moduleLoaderResolve (refactored to extract its body into resolveModuleSpecifier), fetchCommonJSModule, and builtinLoader. Five files touched: three C++ bindings, one header, and build-error.test.ts with seven new tests.

Security risks

None identified. This is module-loader error-handling behavior; no auth, crypto, or untrusted-input parsing is involved. The change makes failed modules re-fetch, which matches Node's behavior.

Level of scrutiny

High. moduleLoaderResolve is the host hook JSC calls for every static import, import(), and entry point — this adds up to five hash lookups on that path (one in the already-loaded common case, since Type::JavaScript is probed first and any non-failed entry short-circuits). The PR description addresses cost, but a maintainer should confirm this is the right layer for the fix versus alternatives (patching duplicateError in the WebKit fork, or not caching FetchFailed at all). The all-variants-or-nothing removal is a documented design compromise (a failed JS variant next to a succeeded text variant means neither is evicted); the test explicitly locks in that limitation.

Other factors

The cellLock() + removeEntry() pattern exactly matches four existing call sites in the codebase.
The moduleLoaderResolve refactor was verified line-by-line: the extracted body is unchanged, and the new wrapper's RETURN_IF_EXCEPTION(scope, resolved) returns emptyIdentifier on failure, identical to before.
Tests are thorough: they exercise each of the three call sites separately (import/require/require.extensions), the bun test two-file crash repro, the type-variant guard, plugin retry, and the control that EvaluationFailed stays cached per spec. describe.concurrent keeps the seven subprocess tests fast.
The PR description notes verification under BUN_JSC_validateExceptionChecks=1 and against the wider resolve/plugin/mock/module test directories.
This is a behavior change (retrying failed fetches, per Node) rather than a pure bugfix, which is another reason a human should sign off.

…Module() new Module("/path") trips JSC's exception check validation on the asan lanes (tracked separately); the helper passes its own module object to require.extensions['.js'], which exercises the same loader path.

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Inline comments:
In `@src/jsc/bindings/ModuleLoader.cpp`:
- Around line 669-672: Correct the comment adjacent to the static types array to
reflect the lookup loop’s actual behavior: it returns early only for a present
entry whose status is not FetchFailed, while registry misses continue through
all five types. Do not add control-flow changes unless explicitly needed; update
the misleading “stops at the first lookup” description.

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📒 Files selected for processing (5)

src/jsc/bindings/JSCommonJSExtensions.cpp
src/jsc/bindings/ModuleLoader.cpp
src/jsc/bindings/ModuleLoader.h
src/jsc/bindings/ZigGlobalObject.cpp
test/js/bun/resolve/build-error.test.ts

claude

Additional findings (outside current diff — PR may have been updated during review):

🟡 src/jsc/bindings/ModuleLoader.cpp:717-718 — The hoisted moduleKey local is only reused at the registryEntry() lookup; the three provideFetch() calls in the same function (lines 756, 821, 855) still rebuild JSC::Identifier::fromString(vm, specifierWtfString) inline. Passing moduleKey there would finish the dedup the PR description claims ("it is built once now").
Extended reasoning...

What the finding is

The PR hoists auto moduleKey = JSC::Identifier::fromString(vm, specifierWtfString) to the top of fetchCommonJSModule (src/jsc/bindings/ModuleLoader.cpp:717) so it can be passed to both evictFetchFailedModuleRegistryEntry() and, further down, registryEntry(). The PR description states: "fetchCommonJSModule already built this Identifier further down; it is built once now."

However, only the registryEntry(moduleKey) site (line 833) was updated. Three identical JSC::Identifier::fromString(vm, specifierWtfString) constructions remain in the same function scope, all within reach of moduleKey:
- Line 756 — the provideFetch() call in the isBunTest virtual-module Fulfilled branch
- Line 821 — the provideFetch() call in the !isBunTest virtual-module Fulfilled branch
- Line 855 — the provideFetch() call in the IsolatedModuleCache hit branch
(The fifth occurrence at line 953 is inside fetchCommonJSModuleNonBuiltin, a separate function where moduleKey is not in scope, so it is correctly left alone.)

Why existing code doesn't prevent it

moduleKey is declared before any of the branches that reach these three provideFetch() calls, so it is in scope and holds exactly the value being reconstructed. Nothing between line 717 and any of the three sites mutates specifierWtfString, so the reconstructed identifiers are byte-identical to moduleKey.

Impact

None on correctness. Identifier::fromString on a WTF::String is a cheap ref-bump into the atom table, and each of the three sites lies on a mutually exclusive branch, so at most one extra construction runs per call. This is purely a consistency/readability point: the PR description claims the identifier "is built once now," and REVIEW.md's dedup guideline says a hoisted local should be used "at EVERY parallel site." Leaving three of four sibling sites on the old spelling makes the dedup look incomplete.

Step-by-step example
1. require("./mod.js") enters fetchCommonJSModule with specifierWtfString = "/abs/mod.js".
2. Line 717 builds moduleKey = Identifier::fromString(vm, "/abs/mod.js") and passes it to evictFetchFailedModuleRegistryEntry().
3. isBunTest is false, no builtin matches, and runVirtualModule returns a plugin result whose promise fulfills synchronously.
4. Control reaches line 821, which calls provideFetch(globalObject, JSC::Identifier::fromString(vm, specifierWtfString), …) — reconstructing the exact identifier already sitting in moduleKey from step 2.
Fix

Replace the inline construction at lines 756, 821, and 855 with moduleKey:
```
globalObject->moduleLoader()->provideFetch(globalObject, moduleKey, JSC::ScriptFetchParameters::Type::JavaScript, jsSourceCode);
```
This is a mechanical three-line change with no behavioral effect.

…s in fetchCommonJSModule

claude

I reviewed this PR and the automated bug hunt found no issues. Because it changes module-loader behavior in the JSC bindings (registry eviction on every resolve/require, with acknowledged interactions with #33419 and #38645), a human look would still be worthwhile.

What was reviewed:

evictFetchFailedModuleRegistryEntry: only removes when every present type variant is FetchFailed; cellLock() matches the other four removeEntry() call sites.
moduleLoaderResolve refactor: body moved into resolveModuleSpecifier unchanged; wrapper adds RETURN_IF_EXCEPTION before eviction.
fetchCommonJSModule: hoisted moduleKey replaces four identical Identifier::fromString calls with the same string — no behavior change.
Tests cover all three call sites, the mixed type-variant guard, plugin retry, and the negative (evaluation failures stay cached).

Extended reasoning...

Overview

This PR fixes #36963: after a module fails to build (transpile error, async plugin rejection), later loads of the same key received a stripped AggregateError (only message, no errors) because JSC's module registry replayed the cached FetchFailed entry through JSModuleLoader::duplicateError. The fix adds evictFetchFailedModuleRegistryEntry() (ModuleLoader.cpp) and calls it from three entry points — moduleLoaderResolve (static imports, import(), entry points), fetchCommonJSModule (require()), and builtinLoader (require.extensions). The moduleLoaderResolve body is extracted unchanged into a static resolveModuleSpecifier helper so the eviction can run after resolution succeeds. Seven new tests in build-error.test.ts cover repeated import()/require(), static importers, the bun test two-file crash, reload-after-fix through each entry point, the mixed type-attribute guard, plugin retry, and a control that evaluation failures stay cached.

Security risks

None identified. The change reads and removes entries from JSC's in-process module registry using existing APIs (moduleMap().get, removeEntry) under the same cellLock() pattern used at four other call sites. No user input reaches new parsing or allocation paths.

Level of scrutiny

High. This is core module-loader C++ in the JSC bindings, on the hot path of every module resolution and every require(). It changes observable behavior (failed fetches are now retried, matching Node) and interacts with the module registry's per-type-variant keying. The PR itself notes a known limitation (mixed failed/succeeded type variants keep replaying the stripped error because removeEntry is per-key, not per-variant) and describes composition with two related open PRs. These are exactly the design tradeoffs a maintainer with JSC module-loader context should confirm.

Other factors

The eviction helper's all-variants-must-have-failed guard, the cellLock() usage, and the FetchFailed-only status check were verified against existing removeEntry() sites in ZigGlobalObject.cpp, BunPlugin.cpp, and bindings.cpp — the pattern is consistent.
The fetchCommonJSModule change hoists a single Identifier to replace four identical constructions; no semantic change beyond adding the eviction call.
Test coverage is thorough and each test exercises a distinct call site or guard. The PR description reports the suite fails on 1.4.0 and on main without the src/ changes.
Outstanding thread: the comment-cop bot re-fired on the 3-line function header comment after the author already trimmed and justified it in 48cec3f; no code change is pending there.
Performance: the description quantifies one hash lookup for a loaded module and five misses for a first load, next to filesystem resolution. Reasonable, but worth a maintainer's eye given this runs on every resolve.

robobun · 2026-08-15T20:39:02Z

114c35e also picks up the review note on fetchCommonJSModule: the three provideFetch() calls in that function now take the hoisted moduleKey as well, so the identifier really is built once there. No behavior change; the plugin, mock and --isolate suites that go through those branches pass on the debug build.

github-actions Bot added the claude label Aug 15, 2026

robobun mentioned this pull request Aug 15, 2026

Bun.plugin: don't leave a failing onLoad cached in the module registry #33419

Open

claude Bot reviewed Aug 15, 2026

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