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[JSC] Synchronous module load: use a fetch the host already delivered, and do not make import() wait for another load's promise - #675
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…eLoading A require(esm) nested inside the host load hook drains the synchronous module queue: Bun evaluates CommonJS module bodies during makeModule, and such a body can require an ESM sibling of the graph that is currently loading. The drain then runs pending ModuleLoadStep reactions for other requests of the same referrer module, so module.loadedModules() can grow across the hostLoadImportedModule call without the current request having completed, and can grow by more than one. The size-delta check then misidentifies whether this request still needs a ModuleGraphLoadingError reaction (a missed reaction loses the rejection, leaving the graph load pending forever), and the growth assertion fires on debug builds: ASSERTION FAILED: module->loadedModules().size() <= loadedModulesCountBefore + 1 Decide by membership of the request itself instead.
…livered hostLoadImportedModule() connects the host's fetch promise to the entry's fetch promise with pipeFrom(), which is a microtask. Beneath loadModuleSync (Bun's require(esm)) the host fetches synchronously, so its promise is already fulfilled, but the job that hands the source over is parked in VM::m_synchronousModuleQueue. Until that job runs, the entry is Fetching with a pending fetch promise, as if the fetch were still in flight. Two callers read it that way: 1. A second static request for the module (two siblings import it) ran the host's fetch again. In Bun the module was transpiled twice, its macros ran twice, and an onLoad plugin saw it twice. When the second request came from a macro's module, the second transpile was the macro-mode one, and it replaced the source that the program runs. 2. A top-level load that has to complete before the queue drains again waited for the promise, and nothing could settle it. A require() from a CommonJS module that is evaluated while another module's imports are requested reported `require() async module ... is unsupported`. A Bun macro's import() of its own file spun forever. The entry now keeps the host's promise. Both callers take its result when it is already settled: the static request in place of the second fetch, at the same point, so the order of the loader's jobs does not change, and the top-level loadModule() before it chains on the fetch promise. A fetch that is really in flight is handled as before. * Source/JavaScriptCore/runtime/JSModuleLoader.cpp: (JSC::JSModuleLoader::loadModule): (JSC::JSModuleLoader::hostLoadImportedModule): * Source/JavaScriptCore/runtime/ModuleRegistryEntry.cpp: (JSC::ModuleRegistryEntry::visitChildrenImpl): (JSC::ModuleRegistryEntry::setFetchSource): (JSC::ModuleRegistryEntry::takeSettledFetchSource): (JSC::ModuleRegistryEntry::fetchComplete): * Source/JavaScriptCore/runtime/ModuleRegistryEntry.h:
…other load's promise For an entry that has its record, hostLoadImportedModule() calls FinishLoadingImportedModule at once, but returns the entry's load promise, which only the load that created the entry fulfills. moduleLoadTopSettled AND-joins that promise into the result of a dynamic import. Beneath loadModuleSync (Bun's require(esm)) the reactions that fulfill it can be parked in VM::m_synchronousModuleQueue. A Bun macro on the main thread waits for `await import(<the macro's file>)` while require(esm) transpiles a dependency. When an ancestor in the require()'s graph had already imported the macro's file, the import() linked and evaluated the file, then waited for that promise, and the wait spun forever. Beneath a synchronous load a dynamic import now gets a fulfilled promise there. It does not need the entry's promise: ContinueDynamicImport runs LoadRequestedModules on the record itself, then links and evaluates it. The second import() of the same file takes the [[LoadedModules]] fast path, which returned the same promise, so that path does the same. A top-level graph load keeps the entry's promise, because it links when that promise is fulfilled. * Source/JavaScriptCore/runtime/JSModuleLoader.cpp: (JSC::JSModuleLoader::hostLoadImportedModule):
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Included review availability: Your plan provides up to 5 included reviews per hour; 0 remain after this review. WalkthroughSynchronous module loading now reuses delivered fetch results, avoids unnecessary refetches, adapts cached module promises, and detects synchronous completion by checking the requested module in ChangesModule loading
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Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.
Beyond the inline findings, I also checked: the new m_fetchSource barrier is visited in visitChildrenImpl and cleared in fetchComplete, and every hunk is under USE(BUN_JSC_ADDITIONS) so the upstream #else paths are unchanged; the membership-based needsErrorReaction is a strict superset of the old size-delta test (a load that completed synchronously always inserts the key). The macro-import-cycle recursion and the missing ModuleGraphLoadingError reaction for a shared module were examined and traced to pre-existing behavior rather than to this change.
Extended reasoning...
Two confirmed findings are posted inline, so the review already signals that a human should look. This note only records what else was examined: GC safety of the new WriteBarrier member (visited, set via .set(vm, this, ...), cleared when the record arrives), preservation of the non-Bun paths, and the equivalence direction of the needsErrorReaction rewrite. The three ruled-out candidates (macro-mode source substitution at the second fetch, unbounded recursion of a macro import() into the file being transpiled, and the absent error reaction for a module shared with a nested require) were traced and attributed to pre-existing behavior of the synchronous loader rather than to this diff. The PR itself is a complex change to Bun's synchronous module loading, with no test in this repository; it should not be approved without a human.
…host's fetch promise The top-level loadModule() settled the entry's fetch promise from the delivered fetch. That queued ModuleRegistryFetchSettled ahead of ModuleLoadTopSettled. The first ran makeModule(). The second reached the inline replay in hostLoadImportedModule() while the module promise was still pending, and ran makeModule() again, so a synthetic module's generator ran twice. The same order occurs without this change when a parked pipeFrom() job runs ahead of a top-level load's reaction: an import() of a CommonJS module that the enclosing require(esm) has just fetched read the module's exports twice. loadModule() now chains ModuleLoadTopSettled on the host's promise and leaves the entry alone. hostLoadImportedModule() settles the entry from the delivered fetch, as it does for a static request, and the module is built once. * Source/JavaScriptCore/runtime/JSModuleLoader.cpp: (JSC::JSModuleLoader::loadModule): * Source/JavaScriptCore/runtime/ModuleRegistryEntry.cpp: (JSC::ModuleRegistryEntry::deliveredFetch const): (JSC::ModuleRegistryEntry::takeSettledFetchSource): * Source/JavaScriptCore/runtime/ModuleRegistryEntry.h:
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Inline comments:
In `@Source/JavaScriptCore/runtime/ModuleRegistryEntry.cpp`:
- Line 275: Update ModuleRegistryEntry terminal completion paths to clear
m_fetchSource after transferring its result: ensure fetchComplete,
provideModule, and terminal error setters release the retained promise on both
success and rejection, including entries completed while in Status::Fetching.
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oven-sh/WebKit#675 got one more commit: a top-level load of a module that the enclosing require(esm) has just fetched continues from the host's fetch promise, so the module record is built once. The size check compares with the latest build of main, which pins a newer WebKit (c28156899e, built with LLVM 23). This preview is 65513e295c plus the fix, because the base branch is still on the LLVM 21 toolchain. That difference puts bun-freebsd-aarch64 1.6 MB over, and it goes away when the pin moves to the merged commit.
… path fetchComplete() was the only place that cleared m_fetchSource. An entry that fails (setFetchError, setInstantiationError, setEvaluationError), an entry that provideModule() completes, and an entry whose delivered fetch was taken kept the host's promise for the lifetime of the entry. setStatus() now clears it for every status past Fetching, and provideModule() and takeSettledFetchSource() clear it themselves. * Source/JavaScriptCore/runtime/ModuleRegistryEntry.cpp: (JSC::ModuleRegistryEntry::setStatus): (JSC::ModuleRegistryEntry::provideModule): (JSC::ModuleRegistryEntry::takeSettledFetchSource):
Problem
loadModuleSync(Bun'srequire(esm)) the host fetches synchronously, but the result reaches the entry's fetch promise throughpipeFrom, a job inVM::m_synchronousModuleQueue. Until that job runs, the entry looks like a fetch in flight.require()reportsrequire() async module ... is unsupported, and a Bun macro'simport()of its own file spins (macros: do not spin when a macro runs beneath require() of an ES module bun#42778).import()also spins when the entry has its record:moduleLoadTopSettledjoins the entry's load promise, which only therequire()'s parked reactions fulfill.Fix
loadModulecontinues from it, and the module is built once.import()of an entry with a record gets a fulfilled promise:ContinueDynamicImportloads, links and evaluates the record itself.Background
loadModuleSyncpointsVM::m_synchronousModuleQueueat a stack frame. The loader's reactions go there, and only that call drains it.ModuleRegistryEntryhas a fetch promise (source), a module promise (record) and a load promise (record plus requested modules).import().Notes
Commits. (1) #396, cherry-picked unchanged. (2) The entry keeps the host's fetch promise, and a synchronous load takes a settled one. (3)
import()beneath a synchronous load does not wait for another load's promise. (4) From the review: the top-levelloadModulechains on the host's promise and leaves the entry tohostLoadImportedModule, somakeModuleruns once. Before,ModuleRegistryFetchSettledand the inline replay each ran it. The same double build exists without this PR:import()of a CommonJS module that the enclosingrequire(esm)has just fetched reads the module's exports twice on 1.4.3, and once on 1.3.13 and with commit 4. (5) From the review: the entry releases the host's promise on every terminal path, not only infetchComplete. oven-sh/bun#42915 pins the preview of commit 4, and commit 5 does not change behavior. Commit 3 has one consumer today: a Bun macro on the main thread (oven-sh/bun#42778). oven-sh/bun#40059 moves macros to their own thread and removes that consumer. Commit 3 can be dropped without the other two.Repro without a macro (commit 2, top-level load).
bun index.cjson Bun 1.4.3 throwsrequire() async module "leaf.mjs" is unsupported. Bun 1.3.13 and this build print["leaf","second","esm","first","root"].innerModuleLoading(esm.mjs)requestsleaf.mjs(fetched,pipeFromjob parked in the queue of therequire("./esm.mjs")), thensecond.cjs. That entry isFetchingwith a fulfilled fetch promise, somakeModuleruns inline and evaluates the CommonJS body. Itsrequire("./leaf.mjs")is a newloadModuleSyncwith its own queue. The top-levelloadModulechains on the pending fetch promise, the drain has nothing to run, and Bun reports the pending result as an async module.Repro without a macro (commit 2, static request).
root.mjsimportsa.mjsandb.mjs, and both importshared.mjs. With aBun.pluginonLoadthat records each path,require("./root.mjs")recordsshared.mjstwice on 1.4.3 and once on 1.3.13 and on this build.BUN_JSC_dumpModuleLoadingState=1shows two[fetch]lines for it.Repro A (commit 2), with oven-sh/bun#42778.
bun run index.tsspins. Expected:from-macro!helper.Repro B (commit 3). The same files, but
importer.tsimportsm.tsandchild.ts, andchild.tsimportswith-macro.ts. When the macro runs,m.tshas its record and itsModuleLoadStep(Requested) reaction is parked in therequire()'s queue. The trace ends with[evaluate] m.ts:statePromisesettles,loadPromisedoes not.Which shape needs which change (each change disabled in turn, debug build with ASAN):
require()of a module the enclosing graph has just fetched (no macro)node:builtinASSERTION FAILED: needsErrorReaction != module->loadedModules().contains(...)ReferenceError: the macro-mode transpile replaces the program's source[[LoadedModules]]fast path)Why the static request takes the result where it fetched again. A first version settled the fetch promise at once in the new-entry path and dropped the
pipeFromjob. That is one job less per module, and it changed plainrequire(esm): a CommonJS body now ran one pass earlier than theRequestedstep of a sibling, so arequire()of that sibling in it reported a false async module, the inlinemakeModulereplay ran more often (and hit the unguardedfetchCompletethat #492 fixes), and one static graph changed its CommonJS evaluation order. This version keeps every job where it was. The second fetch and the take settle the fetch promise at the same point.Differential test. 2,800 random module graphs (ES and CommonJS mixed,
require()of ES modules from CommonJS bodies,import(), modules that throw), each run on a build with and without this change and on 1.3.13. No graph is worse with the change. Three graphs that report a false async module without it load as on 1.3.13. The rest is identical, including nine graphs that hitASSERT(m_status == Status::Fetching)infetchCompleteon both builds (#492).#662. It adds a second fetch to the top-level
loadModuleunder the same condition (Fetching, pending fetch promise, synchronous queue). The two compose: this change takes a delivered fetch first, and the second fetch of #662 then runs only for a fetch that is really in flight on another thread. For a macro the difference matters: the second fetch runs in macro mode (no macro expansion), and its result would become the source that the program runs. TheloadModulehunks touch adjacent lines.#492. It rewrites the new-entry path and moves the inline steps into the entry.
setFetchSourcebelongs next to itspipeFrom, in the Bun arm, andtakeSettledFetchSourcecan use itssettleFetch. A textual auto-merge can putsetFetchSourceinto the non-Bun#elsearm.Why a dynamic import does not need the entry's load promise. The promise says that the load that created the entry finished
LoadRequestedModulesfor the record.ContinueDynamicImportcallsLoadRequestedModuleson the same record with its ownGraphLoadingState, and links only when that completes. A request that the other load has in flight is forced (Fetching) or joined through aCachedstep. A failure in the other load reaches theimport()through the same walk: the failed entry, or the cached resolution failure. About 45 hand-writtenimport()beneathrequire()fixtures without macros give the same output with and without commit 3.Why a top-level graph load keeps it.
loadModulewith aModuleGraphLoadingStatelinks and evaluates fromModuleLoadLinkEvaluateSettled, a reaction of the promise thathostLoadImportedModulereturns. A fulfilled promise there would link before the graph is loaded.Not changed by this PR.
require()(a graph load) of a module that has its record while its load promise is pending still reports a false async module, for example from a CommonJS body two levels below a sibling import, or from a macro's body. Part 1 of [JSC] Run a CommonJS module's generator before its importing graph links, not when its fetch completes #662 moves the CommonJS case.import()still spins. Bun routes that completion to the regular event loop, which the macro's wait does not run.status() != New(JSModuleLoader: hand each synchronous load step to the registry entry, and register a fetch before its hook runs #492).Local check, before the preview build existed.
UnifiedSource-runtime-26.cppand-33.cpp(the bundles withJSModuleLoader.cppandModuleRegistryEntry.cpp) recompiled with this change on9b02218df6, with the compile command that its linux debug-asan tarball ships, swapped into itslibJavaScriptCore.a, and a bun debug build linked against it. The new field is the last member, and the cell size stays 96 bytes, so no other object changed.JSModuleLoader.cppdiffers between9b02218df6andmainonly in the prelinked-edge code. On that build:test/bundler/transpiler/macro-test.test.ts,test/js/bun/resolve/,test/js/node/module/,test/js/bun/plugin/plugins.test.ts,test/js/bun/test/mock/, and the repros withBUN_JSC_validateExceptionChecks=1andBUN_JSC_collectContinuously=1.