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WalkthroughThe change adds arena-backed UTF-8 repair and applies it to text and Markdown loaders. Malformed sequences become U+FFFD, valid bytes remain unchanged, and tests cover loader variants, Markdown rendering, and bundled output. ChangesUTF-8 loader decoding
Priority: ➖ Normal Severity of issue fixed: Medium Merge Risk: ⚪ Minimal · up to Text and Markdown imports now normalize ill-formed UTF-8 across the covered loader paths. No supported merge-blocking runtime risk remains. 🚥 Pre-merge checks | ✅ 4✅ Passed checks (4 passed)
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Updated 4:48 PM PT - Sep 14th, 2026
✅ @robobun, your commit f70c626eabf3fa338904c252871c70def6510a87 passed in 🧪 To try this PR locally: bunx bun-pr 38253That installs a local version of the PR into your bun-38253 --bun |
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I reviewed this PR and didn't find any bugs. Because it picks the loader-level approach over the printer-level fix in #35306 and adds a new bun_core::strings helper, a maintainer sign-off on that placement would still be worthwhile.
Checked that to_well_formed_utf8_alloc reuses the exact convert_utf8_bytes_into_utf16 / .len.max(1) advance loop that to_utf16_alloc already uses, so U+FFFD placement matches TextDecoder.
Checked the four loader sites (runtime + bundler, text + md) — well-formed files still return the original slice with no copy, and the repaired copy uses the same arena the md loader already allocates into.
Checked that non_ascii_sequence_length returning 0 for 0x80–0xC1 / 0xF5–0xFF is handled by .max(1), matching the unit tests for lone continuation bytes and invalid lead bytes.
Extended reasoning...
Overview
Adds strings::to_well_formed_utf8_alloc (simdutf validate → None fast path; otherwise the same per-codepoint decode loop to_utf16_alloc already uses, emitting EF BF BD for each failed maximal subpart) and a thin decode_utf8_file_contents wrapper that arena-copies the repaired bytes. Applied at four sites: the text and md loaders in src/bundler/transpiler.rs (runtime) and src/bundler/ParseTask.rs (bun build). Rust unit tests plus three JS test files cover the runtime imports (.txt, type: "text", ?raw, require), the bundler for bun and browser targets (asserts output decodes with fatal: true and evaluates to the expected code points), and the .md import.
Security risks
None identified. This narrows what bytes can reach the printer's E::String path (ill-formed input becomes U+FFFD instead of passing through), which if anything reduces the surface for malformed-bundle output. No auth, crypto, or path handling.
Level of scrutiny
Medium. The mechanical change is small and mirrors an existing decoder loop line-for-line, and the well-formed case is a no-op after a simdutf validate. What warrants a human look is the placement decision: the description explicitly argues against the competing printer-level approach in #35306 (printer can't tell a file's stray ED A0 80 from a JSON5-preserved surrogate). That argument reads sound to me, but choosing between two open PRs for the same bug class is a maintainer call.
Other factors
Test coverage is thorough — a self-verifying test asserts the fixture table matches TextDecoder output, the Rust unit tests cover every WHATWG maximal-subpart shape (overlong, surrogate, >U+10FFFF, truncated at each width, truncated at EOF, invalid lead, lone continuation), and the well-formed cases assert byte-exact pass-through. Arena lifetime for the repaired copy matches the md loader's existing alloc_slice_copy pattern in both files. No prior reviews on the PR beyond bot noise; CI is still building.
Jarred-Sumner
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This should be an arena-scoped Vec?
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Done in 26b7b7a: the helper is now Re-ran the three test files against the debug build (153 pass), plus a 100 KB file with a bad byte in every other position to exercise the arena vector growing, which imports and bundles to the same string |
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I reviewed this PR and didn't find any bugs. Because it changes user-visible import semantics for text/md files and takes a different design approach than the open #35306 (loader-level decode vs printer-level replacement), a maintainer should confirm which approach to land.
Checked: to_well_formed_utf8_in mirrors to_utf16_alloc's slow path exactly — same convert_utf8_bytes_into_utf16 stepping and .len.max(1) advance, so U+FFFD placement matches TextDecoder.
Checked: output can exceed the bytes.len() initial capacity (single stray bytes → 3 bytes each); BabyVec::extend_from_slice reserves and grows.
Checked: arena lifetime — bump in ParseTask and arena in transpiler.rs are the per-parse arenas that own the AST, so into_bump_slice() lives as long as the E::String that borrows it.
Checked: valid-UTF-8 files hit the is_valid_utf8 fast path and return None with no copy.
Extended reasoning...
Overview
Adds bun_core::strings::to_well_formed_utf8_in(bytes, arena) -> Option<&[u8]>, a WHATWG-UTF-8 repair helper that returns None for already-valid input and otherwise arena-allocates a copy with each maximal ill-formed subpart replaced by U+FFFD. A thin wrapper decode_utf8_file_contents in src/bundler/transpiler.rs applies it at four loader sites: the text and md loaders in both the runtime transpiler and the bundler's ParseTask. Three test files gain coverage (15 byte-sequence cases through .txt/type: "text"/?raw/require; bundler output for bun and browser targets validated with TextDecoder({fatal:true}); runtime .md import), plus Rust unit tests for the helper.
Security risks
None. This narrows previously-undefined behavior (raw bytes leaking into E::String and the bundle output) into WHATWG-compliant replacement, matching Bun.file().text(), fs.readFileSync(_, 'utf8'), and TextDecoder. No new attack surface; the repaired bytes go into the same E::String slot as before.
Level of scrutiny
Medium-high. The implementation itself is small and closely modeled on the existing to_utf16_alloc slow path in the same file (same decoder, same advance logic), so the algorithmic risk is low. But it touches the runtime module loader and the bundler — both critical paths — and changes user-visible semantics for a whole class of inputs (any text/md file that isn't valid UTF-8). More importantly, the PR description explicitly frames this as one of two competing designs: #35306 fixes the same bug inside the printer instead. The PR argues convincingly why loader-level is correct (the printer can't distinguish a file's stray ED A0 80 from a JSON5 "\\ud800" that must round-trip), but choosing between two open PRs for the same issue is a maintainer call.
Other factors
- I traced the decoder: on
fail == false,convert_utf8_bytes_into_utf16_with_lengthalways returns.len ∈ {2,3,4}matching a well-formed sequence, so copying&remaining[..len]through is byte-exact. Onfail == true,.lenis the maximal-subpart length (1-3), so one U+FFFD is emitted and the loop advances correctly — matches the test expectations for surrogates (3×FFFD), overlongs (per-byte FFFD), and truncated sequences (1×FFFD). ArenaVec(=BabyVec) grows viareserveinextend_from_slice, so thewith_capacity_in(bytes.len())under-estimate for stray-byte-heavy inputs is fine; the author also load-tested a 100 KB alternating-bad-byte file.into_bump_slice()leaks into the parse arena (reclaimed on arena reset), and both call sites pass the arena that already owns the surrounding AST — no separate free path, consistent with the md loader's existing rendered-HTML allocation.- The comment-cop bot flagged long comments in earlier commits; those were shortened to single lines and the threads are resolved.
- Test coverage is thorough: exact code-point assertions cross-checked against
TextDecoderat runtime, bundle output validated as strict UTF-8, and the well-formed case is asserted to be byte-exact.
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LGTM — decodes ill-formed UTF-8 at the loader layer so E::String always holds well-formed WTF-8, matching TextDecoder/Bun.file().text().
What was reviewed:
to_well_formed_utf8_inreusesconvert_utf8_bytes_into_utf16(same decoder asto_utf16_allocjust above it), so U+FFFD placement is identical to TextDecoder; the non-fail branch copies through only sequences the decoder validated as well-formed.- Valid files hit the simdutf validate-only path and return the original slice with no copy; the repair path allocates in the parse arena via
ArenaVec/into_bump_slice, which grows when replacements expand the output pastbytes.len(). - All four loader sites covered (runtime + bundler × text + md); tests exercise
.txt,?raw,type: "text",require, and bothbun/browserbundle targets, plus Rust unit tests for the maximal-subpart cases.
Extended reasoning...
Overview
Adds strings::to_well_formed_utf8_in(bytes, arena) in src/bun_core/string/immutable.rs and a thin wrapper decode_utf8_file_contents in src/bundler/transpiler.rs, applied at four loader sites: parse_text_loader and parse_md_loader in the runtime transpiler, and the Loader::Text / Loader::Md arms in src/bundler/ParseTask.rs. The rest of the diff is tests (Rust unit tests plus three JS test files).
Security risks
None. The change only affects how ill-formed bytes in imported .txt/.md files are decoded into a JS string: replacing them with U+FFFD is strictly safer than the previous behavior, which emitted raw invalid bytes into bundle output (making the bundle itself non-UTF-8) or lone surrogates into runtime strings. No new unsafe blocks; the arena owns the repaired buffer.
Level of scrutiny
Medium. This touches the bundler/transpiler parse path for two loaders, but the fast path for well-formed files is a single simdutf validation that returns the original slice unchanged, so the common case is unaffected. The repair path is only reached for genuinely invalid input. I traced convert_utf8_bytes_into_utf16 through non_ascii_sequence_length and convert_utf8_bytes_into_utf16_with_length to confirm that (a) replacement.len is always ≥ 1 so the loop terminates, (b) the non-fail branch only fires when the sequence's continuation bytes were validated (E0/ED/F0/F4 special ranges included), so copying remaining[..len] through is well-formed, and (c) the maximal-subpart semantics match WHATWG (surrogate/overlong sequences produce one U+FFFD per byte, truncated valid prefixes produce one).
Other factors
Tests are thorough and assert exact code-point sequences against TextDecoder output (not just "doesn't crash"), covering the full variant matrix the review guidelines call for. The PR description explains why the fix belongs at the loader rather than the printer (WTF-8 surrogates from JSON5 "\\ud800" must survive printing). The comment-cop bot's feedback about long doc comments was addressed in follow-up commits and all threads are resolved. ArenaVec::extend_from_slice handles the case where U+FFFD expansion makes the output longer than the input (confirmed by the author's 100 KB alternating-bad-byte test).
The text loader (.txt, type: "text", ?raw) and the md loader put the raw file bytes into an E.String, whose UTF-8 form the printer requires to be well-formed WTF-8. For a file that is not valid UTF-8 the printer then dropped the bytes following a bad lead byte, emitted stray continuation and 0xF5..0xFF bytes as Latin-1 code points (or wrote them into bun build output verbatim, producing a bundle that is not valid UTF-8) and kept UTF-8 encoded surrogates. Validate the contents with simdutf and, only when they are ill-formed, re-encode them through the same decoder TextDecoder uses, so each ill-formed subsequence becomes one U+FFFD and the imported string matches Bun.file().text() / fs.readFileSync(path, "utf8"). Well-formed files are still used in place without a copy.
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@Jarred-Sumner this is ready for another look. The one open request (build the repaired text in an arena-scoped vector) is done in the "Build the repaired text in the parse arena" commit: Rebased onto main today (f70c626) with no conflicts. On the rebased branch the three test files pass on the debug build (162 tests), the five Relation to #41789: that PR makes the printer emit U+FFFD instead of NUL for every producer (JSON and YAML strings under |
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Actionable comments posted: 1
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In `@test/bundler/bundler_loader.test.ts`:
- Around line 589-613: Replace the target iteration around the loader tests with
a describe.each() parameterized suite, passing “bun” and “browser” as cases
while preserving the existing test names, targets, fixtures, and assertions.
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Problem
import t from "./f.txt"(alsowith { type: "text" },?raw,require()) and.mdimports do not decode the file as UTF-8. On a file with invalid bytes the imported string differs fromBun.file(f).text()/fs.readFileSync(f, "utf8")/TextDecoder, which all agree with each other:E2 41 42imports asU+0000("AB"is gone); expectedU+FFFD A BF5 41 FF 42imports asU+0000; expectedU+FFFD A U+FFFD B80 41imports asU+0080 A; expectedU+FFFD AED A0 80 41imports as a lone surrogateU+D800 A; expectedU+FFFD U+FFFD U+FFFD Abun buildof the same imports writes the stray bytes (80..BF,F5..FF) into the output string literal as-is, so the emitted.jsis not valid UTF-8 and node and bun read different strings from the same bundle.parse_text_loader(src/bundler/transpiler.rs:2131 on main, runtime) and the bundler'sLoader::Textarm (src/bundler/ParseTask.rs:973), plus the matchingmdloader sites, wrap the raw file bytes in anE::String. The printer's UTF-8 path (write_pre_quoted_string_inner, src/js_printer/lib.rs:1097) requires that data to be well-formed WTF-8 (print_string_literal_utf8at lib.rs:3128 debug-asserts it): on a bad lead byte it prints\x00and skips the lead byte's whole implied width, and a byte that is not a lead byte is treated as a one-byte Latin-1 code point and copied through. The "decoding" users see is that skipping logic, not a UTF-8 decoder.Uncaught SyntaxErrorwhen importing certain PDFs via text loader #12981. printer: print ill-formed UTF-8 in 8-bit strings as U+FFFD instead of NUL #41789 is the complementary printer-side change: it makes the same printer arm emit U+FFFD instead of NUL for any producer (JSON, JSONC, JSON5 and YAML strings underbun build, and the package.json editors throughprint_json), while this PR gives text and md imports the exactTextDecoderresult and keeps the invalid bytes out of the AST. Neither covers the other's cases.Fix
bun_core::strings::to_well_formed_utf8_in(bytes, arena):simdutfvalidates the bytes; when they are well-formed it returnsNonewithout allocating and the caller keeps using the file buffer as before. Otherwise it re-encodes into anArenaVecon the given arena throughconvert_utf8_bytes_into_utf16, the decoderTextDecoderandto_utf16_allocalready use, writingEF BF BDfor each failed maximal subpart and copying well-formed sequences through unchanged, so the U+FFFD positions are the same onesBun.file().text()produces.decode_utf8_file_contents(src/bundler/transpiler.rs) applies that to the four loader sites: the text and md loaders in the runtime transpiler and in the bundler'sParseTask. The repaired text is built directly in the parse arena that owns the rest of the AST (the same arena the md loader's rendered HTML already goes into), so nothing is allocated on the heap and freed separately. The md loader decodes its input before rendering, so it renders whatBun.markdown.html(await file.text())would.E::Stringvalues that legitimately carry WTF-8 surrogates still print as before.bun bd test test/js/bun/import-attributes/import-attributes.test.ts(15 byte sequences through.txt,type: "text",?rawandrequire(), asserted against theTextDecoderresult; fails on main)bun bd test test/bundler/bundler_loader.test.ts(text and md bundles fortarget: "bun"and"browser": output must decode withTextDecoder({ fatal: true })and evaluate to the expected code points; all 4 fail on main, the browser ones because the bundle is not valid UTF-8)bun bd test test/js/bun/md/md-edge-cases.test.ts(runtime.mdimport; fails on main)to_well_formed_utf8_ininsrc/bun_core/string/immutable.rs(same byte sequences as the JS tests);cargo check -p bun_core --tests,cargo clippy -p bun_core -p bun_bundler,cargo fmt --checkclean.Background
bun buildturn a non-JS file into a small JS module. Fortextandmdthat module isexport default "<contents>", built as anE::StringAST node and printed back to JS source, which JSC (or the bundle's consumer) parses again. So what the importer gets is whatever the printer emitted for that node.E::Stringstores either UTF-16 or 8-bit data. The 8-bit form is WTF-8: UTF-8 that may additionally contain encoded surrogates, which parsers use to preserve lone surrogates from sources such as"\ud800"in JSON5; the printer escapes those back to\uD800. The printer therefore cannot tell an encoded surrogate that must be preserved from one that came out of a file and should be U+FFFD. Only the loader knows its bytes are a file to be decoded, which is why the decoding is done at the loader. A strict UTF-8 replacement inside the printer (the approach of the now closed printer: decode ill-formed UTF-8 as U+FFFD when quoting byte strings #35306) would also turn those preserved surrogates into U+FFFD; printer: print ill-formed UTF-8 in 8-bit strings as U+FFFD instead of NUL #41789 avoids that by keeping the WTF-8 decode and replacing only undecodable bytes, one U+FFFD per byte, which is why it cannot produce theTextDecoderresult for text files on its own.TextDecoder, Node and Bun's file readers implement, replaces each "maximal subpart" of an ill-formed sequence with one U+FFFD: a lead byte plus any valid continuation bytes that follow it is one U+FFFD, and the byte that broke the sequence is then decoded on its own. That is what givesE2 82 41one U+FFFD andED A0 80three:A0is not a valid second byte afterED(it would encode a surrogate), so all three bytes fail individually.simdutfis the SIMD UTF-8 library Bun already uses forstrings::is_valid_utf8; validating a file is much cheaper than transcoding it, which keeps the common well-formed case at a single pass with no allocation.Output of the ledger repro before and after
Rebase onto #40177 (bun build --compile text embedding)
#40177 split the bundler's
Loader::Textarm in two: a standalone executable now registers the text file as an embedded asset andencode_text_module(src/standalone_graph/StandaloneModuleGraph.rs) turns it into a string body withto_utf16_alloc, which already replaces ill-formed bytes with U+FFFD. The conflict was in that arm. Resolution: keep the compile branch as main has it, and applydecode_utf8_file_contentsonly to the other branch, the one that still builds anE::Stringforbun build/Bun.buildoutput. The runtime loader and the md loader did not conflict. The fivecompile/TextImport*tests from #40177 pass on the rebased branch together with the tests in this PR.[human-review] gate passed · iteration 0 · 6 files touched
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