[dylan-conway]

What does this PR do?

Fixes a regression from #42695 (not released). A bun build --splitting bundle with two entry points and an import cycle can throw at load, from both entry points, where the unbundled program and the 1.4.2 bundle load.

// index.js
import("./m5.js").then(m => console.log("ok", m.w5));
// worker.js
import "./flag.js"; // sets globalThis.IS_WORKER
import "./m6.js";
// m2.js
export { v5 as x } from "./m5.js";
export function v2() {}
// m5.js
import { v2 } from "./m2.js";
import { v6 } from "./m6.js";
export function v5() { return v2; }
export const w5 = globalThis.IS_WORKER ? null : v6.toUpperCase();
// m6.js
import { v2 } from "./m2.js";
export let v6 = "six";

m2, m5 and m6 are one chunk. index.js enters the cycle at m5, so m6 runs before it. worker.js enters it at m6, so m5 runs first. A chunk prints its files in one order, the order of one entry point, so the other one reads v6 before it has a value. Which side loses was picked by name in 1.4.2 and is picked by load_rank on main. No order serves both.

	unbundled	1.4.2	main	this PR
out/index.js	ok SIX	ok SIX	TypeError	ok SIX
out/worker.js, value of w5	null	"SIX"	TypeError	null

The change

The files that no single order can serve get the __esm wrapper that a require()d ES module already gets. The chunk only declares them, and each entry point runs them where it imports them:

// worker.js
import { init_m6 } from "./index-xzcj92tz.js";
globalThis.IS_WORKER = true; // flag.js
init_m6();

A new pass, wrapContestedFiles.rs, runs after tree shaking:

• Which files. A file on an import cycle, in a chunk that two entry points can each load first (EntryLoadGraph::load_class, the rule that chunk folding uses, moved out of merge_small_chunks). The code that it runs at load can read a binding of another file of the chunk, directly or through the functions and values that it names (part.dependencies). That other file runs before it under one entry point and after it under another.
• What else is wrapped. A wrapped file runs when some code calls it, and an unwrapped one when its chunk loads, which is earlier. Two rules keep each load in order, and apply until nothing is added:
  • a file that imports a wrapped file is wrapped. That includes an export * barrel with "sideEffects": false that tree shaking dropped: an import can read a wrapped file through it, so the import prints init_index(), which calls init_a(). What else the barrel re-exports stays out. A file with side effects that only such a dropped file imports comes back with it, wrapped.
  • a live file that some entry point runs after a wrapped file is wrapped, unless loading it runs nothing ("sideEffects": false, data): when such a file runs cannot be seen.
• What is not. An entry point that no file imports, and no page loads as a script: it is alone in its chunk and the last file of its load, and prints each init_x() in place.
• Unused imports. The order of a load counts an import statement that nothing uses, as the unbundled program runs it (main orders a chunk the same way). One that names a wrapped file is kept, and prints init_x().
• How. The pass does what scan_imports_and_exports does for a file that it wraps, then link() marks live parts and entry bits once more. The two manifest modules of a server-components build had no wrapper symbol, and AstBuilder now gives every file one. No test reaches that: it takes a contested cycle in such a build.

A build without wrappers prints the same bytes as main, except that the JS chunk of an HTML page gets another hash in its name. A build that has wrappers already can print them in another order (both below).

Behavior changes

• A build with a contested cycle has __esm wrappers in its shared chunks, about 45 bytes for each wrapped file plus one init_x() for each import of it.
• The read is judged by syntax: a function or an object that is only stored at load counts as a read of what it names. So some cycles are wrapped that would have run in either order.
• Two import() calls that are in flight at once run such a cycle in the order in which the runtime evaluates them, as the source does. Bun does not fix that order (the source of splitting/SharedChunkFollowsTheImportCallThatRunsFirst prints either result, 14 and 16 of 30 runs; Node prints one). That test now awaits the first import() before the second. Two new tests keep the rest: one runs the two calls in the opposite order from the source, and one has both in flight and checks the cycle against whichever ran first. main prints one result whichever runs first, so it fails the first test every time and the second in 7 of 30 runs.

Five fixes that wrapped files need

All five fail on main without --splitting. This PR puts more files behind wrappers, so it would make them easier to reach.

• Call order. The imports of a wrapper printed in three zones: the sync init_x() calls, one await for the async ones, then everything else. So a sync ES module ran ahead of a CommonJS or an async import that the source names first (bundler: evaluate a wrapped module's dependencies in source order and await every async one #41601). The statements now stay in source order, with one await at the end:

  var init_w = __esm(async () => {
    init_t(); // async: starts here
    init_u();
    await init_t(); // __esm keeps the promise

  An async call gets a statement of its own only when another statement follows it, so await __promiseAll([init_a(), init_b()]) prints as before. Nothing is awaited until every import has started: a file of an import cycle that calls back into the wrapper after an await would wait for it forever. One new edge: when a sync import throws after an async one has started, and that one rejects as well, its rejection has no handler.
• export *. export * from an async wrapped file printed init_a() without await, so the importer went on before a.js had run. It now takes the path of an import statement.
• Pages. A <script src> whose file has a wrapper never ran: a CommonJS script, or an ES module that something require()s. The AST of a page had the import records and no statements, so nothing printed require_x() or init_x(). A page now has one part per record, in order, and a script that is bundled is an import statement. The part ranges of a chunk go into its hash, so the JS chunk of every page gets another name, with the same bytes. Three snapshots that pin such a hash change.
• An unwrapped file that imports two async wrapped files printed await __promiseAll(...) without the helper: ReferenceError. (bundler: define __promiseAll when an unwrapped file awaits two async ESM wrappers #42046 and bundler: evaluate a wrapped module's dependencies in source order and await every async one #41601 fix it another way.)
• propagate_async_dependencies missed files on an import cycle, so a wrapper that is not async held an await: "await" can only be used inside an "async" function. It now goes backward from the async files. (bundler: evaluate a wrapped module's dependencies in source order and await every async one #41601 fixes it another way.)

These change builds that this PR is not about. Without --splitting, where import() and require() make the wrappers:

graphs, no --splitting	output differs	runs better	runs worse	runs that lose more values
D below, 1,500	279	28	0	0
B below, 800	0	0	0	0
H below, 1,200	529	64	5	0
I below (small graphs), 800	379	117	16	5

In H the base commit crashes in 19 runs, and this branch in none. Of the 5 in the last row, 1 is traced: the cycle is entered at another file than in the source, through an unused import that both bundles drop, and this branch prints what the specification gives for that entry. The base has the value because its export * calls are not awaited. The other 4 are not traced.

Not fixed

• On main, without --splitting too, a require()d ES module that another file reads through an export * barrel with "sideEffects": false is never initialized: the read gives undefined. This PR handles that shape for the files that it wraps, not for the ones that scan_imports_and_exports wraps.
• Top-level await in or under a contested cycle. main prints such files flat in the chunk, and this PR puts them behind async wrappers. A file with no await of its own, on a cycle that reaches a file with one, gets an async wrapper. It yields where the source runs it in one go, so a sibling can start before it has assigned its exports. That can swap two effects, and it can make a read give undefined where main has the value: 6 of 2,034 runs in column I below, none in D or H.
• Two entry points that run shared files off a cycle in different orders. The chunk still follows one of them. No binding is read early there, only side effects change places.
• An entry point's own earlier import still runs after a shared chunk that is not wrapped (bundler(splitting): keep import order when an entry point's chunk takes no fold #44028, bundler(splitting): run an entry's earlier imports before shared code #44495).

Cost

Release build, time inside the new pass, 5,000 files each. Measured before the rules for what else is wrapped took their present form: the three contested rows are not current.

build	whole build	new pass	marking again
1 entry point, 50 import()s, no cycle	190 ms	0.8 ms
2 entry points, cycles that only declare	200 ms	2.8 ms
300 entry points, cycles that only declare	470 ms	3.7 ms
2 entry points, two 2,500-file cycles that read each other, same order under both	165 ms	5.6 ms
2 entry points, contested, 88 files wrapped	285 ms	14 ms	40 ms
2 entry points, 1,000 contested cycles, 3,046 files wrapped	210 ms	36 ms	3 ms
300 entry points, contested, 3,287 files wrapped	860 ms	78 ms	229 ms

How did you verify your code works?

bun bd test on 31 bundler test files (esbuild/*, bundler_splitting, bundler_compile_splitting, bundler_html, bundler_edgecase, bundler_cjs2esm, bundler_barrel, bundler_promiseall_deadcode and others): 1,874 pass. The 7 that fail, fail the same way on a build of the base commit here (2 need a package install, 3 time out on --compile with a debug build). No snapshot changes but the three above. The dev server and Bun.serve HTML tests pass (bake/dev/html, bake/dev/css, bun-serve-html*).

40 new tests: 27 splitting/ContestedCycle*, 5 in bundler_html, 7 in bundler_promiseall_deadcode, 1 with --compile and a Worker. On a build of the base commit, 37 fail. ContestedCycleConcurrentImportCalls depends on a race there and fails in 7 of 30 runs. html/external-script and the export * form of the CommonJS call order pass there: they hold two regressions that review found in earlier commits of this branch.

Each clause has a test. 21 clauses of the new logic deleted one at a time: 20 make a test fail. The other one is the wrapper symbol of AstBuilder (see above).

The code that moved. EntryLoadGraph and bind_wrapped_import are moves out of merge_small_chunks and scan_imports_and_exports. A build with only those, against the base commit: 9,900 random graphs in 8 configurations, 4 of them with --min-chunk-size, give identical files.

Random module graphs, each built by the base commit and by this branch and compared with the unbundled program. Every module logs when it runs and what it reads at load. The script is not in the repo.

	A	B	C	D	E	F	G	H	I
graphs	6,000	3,000	800	4,000	4,800	2,600	4,500	2,500	500
graphs whose output differs	737	1,163	491	151	549	all (names)	1,188	316	214
values that the source reads and the bundle does not, base	767	2,383	2,942	n/c	591	1,981	n/c	355	n/c
the same, this branch	0	0	0	n/c	0	1	n/c	0	n/c
pairs of effects in the wrong order, base	3,079	14,753	59,796	1,152	3,699	32,122	79,970	2,825	19,612
the same, this branch	1,568	2,643	5,299	1,055	1,356	5,725	36,397	1,412	9,272
entry point runs that are worse than the base	0	1	0	8	0	5	14	21	46
entry point runs that are better	781	2,355	2,149	93	803	2,552	3,390	447	707

n/c: not counted. Tree shaking drops modules in those columns, so a missing value says nothing.

• A: ES modules with named imports, up to 12 files and 3 entry points, many cycles.
• B: also namespace, default and bare imports, export * as, classes with static blocks, objects that hold closures, JSON, and entry points that other files import. Up to 20 files and 5 entry points.
• C: the same with up to 73 files and 9 entry points.
• D: A with CommonJS, top-level await, barrels, namespaces and "sideEffects": false packages.
• E: B where about half of the files are one "sideEffects": false package with cycles of its own, which the rest imports by file or through its export * barrel. Up to 53 files and 7 entry points.
• F: B and C with HTML pages as entry points, several <script src> each. Some pages have an external script, and some scripts are entry points of their own as well.
• G: E where the files of the package import files of the app, which have side effects.
• H: E with top-level await in the package.
• I: G and H together, up to 53 files and 7 entry points.

The runs that are worse. Those of A to G, and the ones that lose a value, are each traced, or classified by which of the two files of a swapped pair has a wrapper. The rest of H and I are not looked at one by one:

• Top-level await: 7 of the 8 in D, and the graphs of H and I. See "Not fixed". 6 runs of I lose a value: in each, the file that is read has an async wrapper and no await of its own.
• Two unwrapped files in two chunks swap, in G and 3 of the 5 in F: an entry point's chunk imports the chunk that holds a binding ahead of a chunk that it imports for its side effects only. The base commit and 1.4.2 have that bug too, and it does not take a cycle.
• Another import() order than the source, because of that same bug: the 1 in B, 2 in F and 1 under Node below. Against a reference that runs its import() calls in the order that the bundle follows, none of them is worse.
• CommonJS, 1 in D: this branch reads a value that the base loses, and the unbundled program runs a CommonJS file ahead of an ES module that is imported before it.
• 5 runs of one graph in G read a value where the source and the base read none: a "sideEffects": false file runs earlier than in the source.

The one value lost in F is lost by the base commit too.

No value is lost in 1,000 more graphs of B each with [hash] in the entry names, with --target=node under Node, and with --target=browser --minify, nor in 1,000 of E with --min-chunk-size.

Two entry points that share hono, svelte or axios (49, 105 and 87 modules): every output file is identical to the base commit.

--compile --splitting --bytecode on the graph above prints the same as unbundled.

Not run: Windows and Linux locally, a real browser, the bake production tests (they install packages).

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Walkthrough

Code-split builds now detect shared module cycles whose evaluation order differs by entry point and wrap affected files. The linker updates wrapper imports, async initialization, HTML import handling, and chunk-load analysis. Tests cover cycle behavior, async modules, and HTML script execution.

Changes

Entry-order-aware cycle wrapping

Layer / File(s)	Summary
Compute entry load relationships src/bundler/linker_context/mergeSmallChunks.rs, src/bundler/LinkerGraph.rs, src/collections/bit_set.rs	EntryLoadGraph computes dynamic-entry relationships, synchronous loads, dominators, and load classes. Async dependency propagation follows static statement importers.
Detect contested files src/bundler/linker_context/wrapContestedFiles.rs	Cycle analysis compares entry-point evaluation orders and shared load-time dependencies to identify files for wrapping.
Bind wrappers and HTML imports src/bundler/LinkerContext.rs, src/bundler/linker_context/scanImportsAndExports.rs, src/bundler/linker_context/convertStmtsForChunk.rs, src/bundler/linker_context/findAllImportedPartsInJSOrder.rs, src/bundler/ParseTask.rs, src/bundler/AstBuilder.rs, src/bundler/ServerComponentParseTask.rs, src/bundler/lib.rs	The linker binds wrapped imports and runtime helpers, centralizes sync and async wrapper initialization, and creates import-specific HTML loader parts and namespace symbols.
Recalculate linking state src/bundler/LinkerContext.rs, src/bundler/linker_context/README.md, docs/bundler/index.mdx	When wrapping changes the graph, the linker reruns tree shaking and code splitting. Each pass resets liveness and entry-point state.
Validate cycle, async, and HTML behavior test/bundler/bundler_compile_splitting.test.ts, test/bundler/bundler_splitting.test.ts, test/bundler/bundler_promiseall_deadcode.test.ts, test/bundler/bundler_html.test.ts, test/bundler/html-import-manifest.test.ts, test/js/bun/http/bun-serve-html.test.ts	Tests cover contested cycles across entry points, HTML script order, async wrapper initialization, and compiled splitting. Snapshots record regenerated output.

Suggested reviewers: jarred-sumner

Priority: ➖ Normal

Merge Risk: 🔵 Low · up to 3c257

This change adds wrapping for shared import cycles in code-split builds, along with broad test coverage. The remaining issue is a documentation wording fix, so the change is mergeable after that small follow-up.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Title check		The title clearly and concisely describes the main change: wrapping files in import cycles when entry points use different execution orders.
Description check		The description includes both required sections, explains the regression and implementation, and documents verification results, limitations, and unrun tests.

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Review comments at @test/bundler/bundler_splitting.test.ts:
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• src/bundler/linker_context/mergeSmallChunks.rs
• src/bundler/linker_context/scanImportsAndExports.rs
• src/bundler/linker_context/wrapContestedFiles.rs
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• test/bundler/bundler_splitting.test.ts

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[claude]

Beyond the inline findings, I also checked two refactors for behavior drift: the bind_import_of_wrapped_file extraction in src/bundler/LinkerContext.rs keeps the same conditions, flag writes and helper counts as the block it replaces in scanImportsAndExports.rs, and the rerun of tree_shaking_and_code_splitting() after wrapping resets files_live and reinitializes every entry bitset before marking again, so the second pass does not accumulate on top of the first.

Extended reasoning...

The change adds an 830-line linker pass (wrapContestedFiles.rs) that wraps import-cycle files with __esm when two entry points load them in different orders, hoists LoadClasses out of merge_small_chunks, extracts bind_import_of_wrapped_file, and reruns tree shaking when anything was wrapped; it touches no security-sensitive surface. Four confirmed findings are posted inline (a missing wrapper_ref validity check reachable from Bake server-components builds, a sideEffects:false barrel losing its init call, a weakened concurrent-import() test, and a top-level-await ordering change), so this is not an approve. The extraction and the rerun reset were read directly from the diff and found behavior-preserving.

Findings marked 🟡 are optional suggestions and need no follow-up push.

[robobun]

^{Updated 1:40 AM PT - Oct 9th, 2026}

❌ @dylan-conway, your commit 3c2570b has 1 failures in Build #124093 (All Failures):

• test/bundler/bundler_bytecode_portable.test.ts - code 1 on 🐧 3.23 x64
• test/bundler/bundler_bytecode_portable.test.ts - code 1 on 🐧 25.04 aarch64
• test/bundler/bundler_bytecode_portable.test.ts - code 1 on 🐧 13 aarch64
• test/bundler/bundler_bytecode_portable.test.ts - code 1 on 🐧 25.04 x64
• test/bundler/bundler_bytecode_portable.test.ts - code 1 on 🐧 13 x64
• test/bundler/bundler_bytecode_portable.test.ts - code 1 on 🐧 3.23 aarch64
• test/bundler/bundler_bytecode_portable.test.ts - code 1 on 🐧 13 x64-asan
• test/bundler/bundler_bytecode_portable.test.ts - code 1 on 🪟 2019 x64
• test/bundler/bundler_bytecode_portable.test.ts - code 1 on 🪟 11 aarch64
• test/bundler/bundler_bytecode_portable.test.ts - code 1 on 🍎 any aarch64
• test/bundler/bundler_bytecode_portable.test.ts - code 1 on 🍎 any x64

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🧪   To try this PR locally:

bunx bun-pr 44823

That installs a local version of the PR into your bun-44823 executable, so you can run:

bun-44823 --bun

[claude]

Still open from earlier reviews (1):

• Unresolved: 1 minor or pre-existing.

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Actionable comments posted: 1

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Inline comments:
Review comments at @src/bundler/linker_context/mergeSmallChunks.rs:
- Line 1028: Rename the local `load_classes` variable in `merge_small_chunks` to
`load_graph` to reflect that it holds an `EntryLoadGraph`, and update its
references within the function. Leave the computed load-class collections and
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• src/bundler/linker_context/convertStmtsForChunk.rs
• src/bundler/linker_context/findAllImportedPartsInJSOrder.rs
• src/bundler/linker_context/mergeSmallChunks.rs
• src/bundler/linker_context/scanImportsAndExports.rs
• src/bundler/linker_context/wrapContestedFiles.rs
• test/bundler/bundler_compile_splitting.test.ts
• test/bundler/bundler_html.test.ts
• test/bundler/bundler_promiseall_deadcode.test.ts
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[claude]

Still open from earlier reviews (1):

• Unresolved: 1 minor or pre-existing.

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Actionable comments posted: 1

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Review comments at @src/bundler/linker_context/README.md:
- Line 758: Clarify the wrapper-propagation rule in the README by splitting the
combined clause into separate sentences: state that files importing wrapped
files are wrapped, then separately explain that tree-shaken files brought back
are wrapped along with the side-effectful files they import exclusively. Use
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📐 Maintainability & Code Quality | 🟡 Minor | ⚡ Quick win

Clarify the wrapper-propagation rule.

Line 758 joins two rules with a comma, and “which brings it back” has no clear antecedent. Split the rule into separate sentences. For example: “Wrap a file that imports a wrapped file. Also wrap a file dropped by tree shaking, along with the side-effectful files that it imports exclusively.” LanguageTool also flags the wording.

🧰 Tools

🪛 LanguageTool

[grammar] ~758-~758: Use a hyphen to join words.
Context: ...d file is wrapped, also a file that tree shaking dropped (an export * barrel wi...

(QB_NEW_EN_HYPHEN)

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Review comment at @src/bundler/linker_context/README.md at line 758:
Clarify the wrapper-propagation rule in the README by splitting the combined
clause into separate sentences: state that files importing wrapped files are
wrapped, then separately explain that tree-shaken files brought back are wrapped
along with the side-effectful files they import exclusively. Use clear wording
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Source: Linters/SAST tools

[claude]

Nothing blocking. The comments below are optional suggestions. There is no need to push a fix for them before merging.

[claude]

🟡 (optional) Users whose sideEffects: false module reads global state at load, after a contested cycle, get a stale value from an entry the base bundle served correctly. The after rule skips such a file through has_no_load_effects at wrapContestedFiles.rs:591, so it runs when the shared chunk loads, before the entry calls init_a(). Fix: keep a pure follower unwrapped only when its live parts run nothing at load (every initializer can_be_moved()); a file with a load-time part (is_load_time_part) is wrapped like any other follower. The PR says when such a file runs cannot be seen; a globalThis read at load is seen.

Why this was flagged

Splitting build, entries e1.js and e2.js. a.js: import { b } from "./b.js"; globalThis.READY = true; export const a = globalThis.ENTRY === "e1" ? "a+" + b : "a". node_modules/pure/package.json has "sideEffects": false; pure/index.js: export const p = globalThis.READY ? "ready" : "not ready". a.js and b.js are a contested cycle, so both are wrapped. The after rule at wrapContestedFiles.rs:585-595 reaches pure/index.js with seen_wrapped true, but skips it because has_no_load_effects (wrapContestedFiles.rs:496-498) is true: loading_file_side_effects (mergeSmallChunks.rs:53-112) sets declared_pure from the annotation and the file has no import records. pure/index.js is printed unwrapped in the shared {e1,e2} chunk, so p is computed when that chunk loads, before out/e1.js runs init_a(), and e1 prints e1 a+b not ready. On the base branch nothing is wrapped and the chunk prints b, a, pure in e1's order, so out/e1.js prints e1 a+b ready. The importer rule does not catch it because the file imports nothing wrapped.

Verification: Triggering condition: a --splitting build with a contested cycle where a "sideEffects": false file reads, at load, global state that a wrapped file sets. The after rule at src/bundler/linker_context/wrapContestedFiles.rs:588-595 skips the file when has_no_load_effects(source_index) (line 591). The base chunk runs a.js before pure, so base prints the ready value and the PR prints the stale one.

[dylan-conway]

Closing in favor of #44876, which fixes the reported regression by choosing a different owner for the shared chunk and leaves every other build unchanged.