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bundler: list the chunks and assets reached through dynamic imports in the HTML import manifest - #38347
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WalkthroughThe manifest writer now tracks reachable chunks through cross-chunk imports and JavaScript CSS dependencies. Tests cover splitting, page-specific assets, shared CSS, large entry counts, and HTTP serving. ChangesHTML manifest reachability
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Reproduced on the released 1.4.0 and on main with a server build that imports one HTML file and is built with Fix: #38347 (this PR). Note that #38337 touches the same function; the two compose, whichever lands second needs a small rebase. Rebased onto main at 467e741. The only conflict was in the import lines of |
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Beyond the inline nit, I also checked that css_chunks is populated in computeChunks.rs independently of code_splitting, so the non---splitting shared-CSS test reaches the deduplicated chunk through the new traversal even though cross_chunk_imports is empty there; that the traversal terminates on cycles (the chunks_to_write bitset gates re-visits before enqueuing neighbours); and that Flags::IS_BROWSER_CHUNK_FROM_SERVER_BUILD remains in use elsewhere so dropping the import here is correct. Given this replaces the chunk-selection heuristic with a graph walk and robobun flagged an overlap with #38337 in the same function, a maintainer look at the walk's server-chunk exclusion argument and merge ordering would still be worthwhile.
Extended reasoning...
The Rust change is a principled rewrite of manifest chunk selection from entry-bit intersection (plus a single-HTML-import fallback) to a BFS over cross_chunk_imports + css_chunks seeded from the page's own entry chunks. I traced the non-splitting path: compute_cross_chunk_dependencies early-returns when !code_splitting, leaving cross_chunk_imports empty, but css_chunks is assigned in computeChunks.rs:244 regardless, so the deduplicated-CSS case still resolves. The traversal is bounded by the chunks_to_write bitset. The removed Flags import is not dead code elsewhere. The five new tests use exact toEqual on sorted inputs and an end-to-end 200/404 assertion, and the existing snapshot tests in the same file constrain the non-splitting output. Deferring rather than approving because the argument that the walk cannot reach the server entry chunk (and that dropping the single-HTML fallback loses nothing) rests on linker invariants a bundler maintainer should confirm, and #38337 touches the same function.
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Updated 9:05 PM PT - Sep 2nd, 2026
✅ @robobun, your commit 61ac5d7b219afc4fae6600cada73affbdc04f267 passed in 🧪 To try this PR locally: bunx bun-pr 38347That installs a local version of the PR into your bun-38347 --bun |
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In `@test/js/bun/http/bun-serve-html-manifest.test.ts`:
- Line 375: Update the readdirSync call in the directory-iteration loop to use
an absolute path derived from import.meta.dir, targeting the built server
directory explicitly instead of the current working directory ".".
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src/bundler/HTMLImportManifest.rstest/bundler/html-import-manifest.test.tstest/js/bun/http/bun-serve-html-manifest.test.ts
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Review follow-ups, all test or comment only (the manifest change itself is unchanged since the first commit):
Both test files and The one red lane in the first CI run ( |
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I reviewed this PR and didn't find any bugs; the earlier stdout-drain nit and the comment-length/readdirSync notes have all been addressed. Because this replaces the manifest's chunk-selection heuristic with a graph traversal in the bundler and composes with #38337 (whichever lands second needs a rebase), a human look from someone familiar with the bundler would still be worthwhile.
What was reviewed:
- The
cross_chunk_imports+css_chunkstraversal — visited-set DFS matches the existingappend_isolated_hashes_for_imported_chunkspattern, cycles are guarded. - Bitset sizing switch to
linker_graph.entry_points.len()— checked thatfile_entry_bitsare sized from the same list, sohas_intersectioncompares like representations. - Verified the removed
Flagsimport is not the last user ofIS_BROWSER_CHUNK_FROM_SERVER_BUILD(still used incomputeChunks.rsand elsewhere). - New tests: each targets a distinct failure mode (lazy asset, per-page attribution, shared CSS, >127 entry points, end-to-end serve), and the existing manifest snapshots are unchanged.
Extended reasoning...
Overview
The PR changes one function, write() in src/bundler/HTMLImportManifest.rs, which generates the JSON manifest embedded in server bundles for each HTML import. The old logic selected chunks by intersecting each chunk's entry bits with the HTML entry point's bit, plus a fallback that added every browser chunk when the build had exactly one HTML import. The new logic seeds from the page's own entry chunks (HTML + JS + CSS created for the HTML entry point) and walks Chunk.cross_chunk_imports and each JS chunk's css_chunks transitively, recording every chunk reached in a visited bitset and adding each reached entry-point chunk's entry-point id to the asset-selection bitset. It also fixes the bitset sizing to match the linker's entry-point list (which includes dynamic-import entry points), and removes the single-HTML-import fallback and the now-unused Flags import. Four new itBundled tests and one new Bun.serve end-to-end test are added; existing manifest snapshots are unchanged.
Security risks
None identified. The manifest is build-time metadata derived from the chunk graph the bundler already computed; there's no untrusted input, no path handling change, and no new allocation on external data. The traversal is bounded by chunks.len() via the visited bitset.
Level of scrutiny
Medium-high. The bundler is a critical code path and this replaces a selection heuristic with a graph traversal, so the correctness argument depends on invariants about cross_chunk_imports (that it is exactly what emitted chunks import/import(), so the walk never enters the server entry chunk, and the runtime chunk contributes no entry bits). The PR description argues this precisely and the traversal pattern mirrors the existing append_isolated_hashes_for_imported_chunks in LinkerContext.rs, which walks the same edges with the same visited-set shape. Still, someone with bundler ownership should confirm the invariants hold for all chunk shapes (e.g., HTML chunks' cross_chunk_imports) and coordinate with #38337, which touches the same function.
Other factors
All prior review feedback has been addressed in follow-up commits: the build subprocess now drains stdout (d1c669c), the two multi-line explanatory comments were reduced to one line each, and the serve fixture reads import.meta.dir instead of "." (d550678). Test coverage is strong — five new tests each pinning a distinct failure mode, including the negative contract (server-only asset excluded, each page gets only its own lazy chunk), plus an end-to-end Bun.serve fetch that 404'd before the fix. The one red CI lane on the first run (dev-server teardown segfault on Windows) does not go through this code path per the author's note. robobun independently reproduced the bug on 1.4.0 and confirmed the tests fail without the src/ change.
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@robobun rebase |
…mport manifest With code splitting, a dynamically imported module is an entry point of its own, so the files only it reaches carry its entry bit rather than the HTML entry's. The manifest selected assets by the HTML entry's bit alone, so an asset imported only from a lazy chunk was copied to the outdir but left out of html.files, and Bun.serve had no route for it. Lazy chunks themselves were only listed through a heuristic that applied when the build had a single HTML import, so with two or more HTML imports they were missing as well. Walk cross_chunk_imports (and the JS chunks' CSS chunks) from the page's own chunks, list every chunk reached, and select assets with the entry points reached on the way. The walk also picks up a CSS chunk that was deduplicated into another page's, and sizing the bitset from the linker's entry point list makes it comparable with the files' bits once there are more than 127 entry points.
… by import.meta.dir in the serve test
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Problem
--splitting, an asset (file loader) imported only by a dynamically imported module is copied to the outdir and referenced by the lazy chunk, but is missing from the manifest (html.files), soBun.serveregisters no route for it and the page's lazy code gets a 404. The lazy chunk itself is listed. Without--splittingthe asset is listed.--splitting, the lazy chunks are missing from every manifest too, along with their assets, soBun.serve404s theimport()itself.src/bundler/HTMLImportManifest.rs: the manifest selected chunks and assets by intersecting their entry bits with the HTML entry point's bit only. With code splitting, each dynamically imported module is an entry point of its own and the entry-bit traversal stops at dynamic imports (is_external_dynamic_import), so everything only a lazy module reaches carries the lazy entry point's bit, never the HTML's. Lazy chunks were only listed through a fallback that added every browser chunk to the manifest when the build had exactly one HTML import (fix(bundler): include lazy chunks in frontend.files for compiled fullstack builds #26024); assets had no such fallback, and builds with several HTML imports had none at all.graph.entry_points(user entries plus HTML files), while the files' and chunks' bits are sized from the linker's list, which also contains the dynamic imports.AutoBitSet::has_intersectionreturns false between the inline variant (up to 127 bits) and the heap variant, so once a split build had more than 127 entry points in total, every asset dropped out of the manifest, including ones the page's own script imports statically.<link href>points at.Fix
Chunk.cross_chunk_imports, plus each JS chunk'scss_chunks, and list every chunk reached. Every entry point chunk reached through a cross-chunk import (the dynamic imports, transitively) adds its entry point id to the bitset that selects assets, which is sized from the linker's entry point list. The single-HTML-import fallback is removed.cross_chunk_importsis exactly what the emitted chunk files import andimport(), so a chunk is in the manifest iff the browser can reach it from the page; an asset's entry bits say which entry points' module graphs contain it, and the entry points reached by the walk are the ones whose graphs the page can load. The old single-import fallback is a subset of this: an entry chunk statically imports every chunk carrying its bit (compute_cross_chunk_dependencies), and a lazy entry chunk is reached through theimport()that loads it. Only a chunk for animport()that tree shaking removed is no longer listed, and nothing references that file. Followingcss_chunksis how the HTML printer itself finds the page's stylesheet (Chunk::get_css_chunk_for_html), which is what makes the deduplicated CSS case come out right; CSS chunks reached that way do not contribute entry ids, since a deduplicated chunk carries the first page's id.--splittingtest still passes.entry_point_bits; with this change that bitset also covers the lazy entry points (and is sized correctly past 127 entries).test/bundler/html-import-manifest.test.ts:splitting-lists-assets-of-lazy-chunks(direct, nested and shared-chunk assets listed, server-only asset not),splitting-attributes-lazy-chunks-to-their-page(two pages, each manifest gets exactly its own lazy chunk and asset),shared-css-chunk-is-in-every-manifest,splitting-more-than-127-entry-points.test/js/bun/http/bun-serve-html-manifest.test.ts:serves assets that only a lazily imported chunk referencesbuilds with--splitting, runs the output and fetches the asset (404 before, 200 after).src/change; they pass with it.html-import-manifest,bun-serve-html-manifest,regression/issue/25628,regression/issue/23569,bundler_html,bundler_html_server,bundler_splitting,bundler_compile_splitting, the HTML import test inbundler_compile;cargo clippy -p bun_bundler --no-depsandcargo fmt --checkare clean.Background
import html from "./index.html", the bundler embeds a JSON list of that page's output files (HTML, JS and CSS chunks, assets, each with its path and headers) into the server bundle.Bun.serveturns every entry into a static route, so a file missing from the list is a 404 even though it exists in the outdir.--splitting, every dynamically imported module is appended to that list as an entry point of its own, the traversal does not cross dynamic imports, and chunks are formed from files with identical bits.Chunk.entry_bitsis the bitset a chunk was formed for.cross_chunk_imports: computed bycompute_cross_chunk_dependenciesfor split builds; for each JS chunk, the other chunks it statically imports (every chunk it needs symbols or side effects from, and for entry chunks, every chunk carrying the entry's bit) plus the chunks itsimport()calls were rewritten to. It is the chunk-level dependency graph, andappend_isolated_hashes_for_imported_chunksalready walks it to compute output hashes.AutoBitSet: stores up to 127 bits inline and switches to a heap allocation above that; the two representations never intersect, so two bitsets must be created from the same count to be comparable.Repro (release 1.4.0)
Before (
out/big-khdk8dkp.pngexists andout/lazy-*.jscontainsvar big_default = "./big-khdk8dkp.png"):After:
Two HTML imports (
home.html->import("./lazy.js")->big.png,about.html->import("./lazy2.js")->other.png),--splitting, output run from the outdir with both pages passed toBun.serve, fetching every file on disk, before:After: all 200, and each manifest lists only its own lazy chunk and asset.
One HTML import whose script has 130
import()s and one staticimport icon from "./icon.png",--splitting, before: the manifest lists the JS chunks and the HTML but nofileentry; after:icon.pngis listed. With 100import()s it was listed before as well.