node:zlib: keep zstd frame state across writes and ignore input after the last frame

[robobun]

Problem

• zlib.zstdDecompressSync(frame + junk) returns the data of the frame. If that data fills the 16384-byte output chunk exactly, it throws Unknown frame descriptor (ZSTD_error_prefix_unknown). A stream raises the same error for junk in a later write. Node v26.10.0 ignores the junk in each case.
• The cause is next_input_is_frame (src/runtime/node/zlib/NativeZstd.rs:510). It reads one write, and the decoder keeps no frame state between writes.

Fix

• Context keeps the frame state of Node's ZstdDecompressContext. do_work_decompress is a port of its DoThreadPoolWork.
• Correct because Node is the reference: test/js/node/zlib/zstd-frames.test.ts uses node:test, and the same file passes on Node v26.10.0. On main, 15 of its 25 tests fail.
• Verified: that file, all of test/js/node/zlib/, compression.test.ts, and 67 vendored Node tests.
• Self-reviewed: 15 concerns raised, 15 addressed.

Background

• A zstd stream is a sequence of frames. A frame starts with a 4-byte magic number. ZSTD_decompressStream returns 0 when a frame is complete.
• A write is one call of the native handle. The JS drivers make one for each chunk, and one more while the output chunk is full.
• Considered one flag for the end of a frame: it cannot hold a magic number that arrives split across writes.
• node:zlib: zstd decoder follows Node v26.10.0 across writes and at the end of the input #33413 is on top of this PR and waits for a decision.

Downsides

• 9 of 113 measured call orders change, all from ZSTD_error_prefix_unknown to success. Node v24.21.0 and v26.6.0 still raise that error.
• A zstd decode write inside a frame runs 24 more instructions and 5 more conditional branches (342 to 366). .text grows 256 bytes. Context stays 88 bytes.
• options.rejectGarbageAfterEnd stays unimplemented.

Notes

Which Node version this follows

The stream is write(frame), write("not valid compressed data"), write(frame), end(). Each Node binary is the linux-x64 build from nodejs.org, and its sha256 matches SHASUMS256.txt.

runtime	output	then
Node v24.19.0, v24.21.0, v26.5.0, v26.6.0	"first\n"	'error', ZSTD_error_prefix_unknown
Node v26.10.0	"first\n"	'end'
main (a4f1429)	"first\n"	'error', ZSTD_error_prefix_unknown
this PR	"first\n"	'end'

The upstream change is nodejs/node#65865 (7055c484ff, "zlib: improve zstd decoding across chunk boundaries"), first released in v26.10.0. With frame and junk in one write, all of these runtimes give "first\n" and no error.

Upstream tests

nodejs/node#65865 changed three test files.

• test-stream-iter-transform-sync.js: replaced with the copy from v26.10.0. It passes before and after this PR, because its new zstd case has no junk.
• test-stream-iter-transform-roundtrip.js: not replaced. The copy from v26.10.0 fails at line 50, in testValidatedTransformReceiver, because pull() calls a transform without its receiver (src/js/internal/streams/iter/pull.ts:642). That is a different defect with its own fix in progress. Its testZstdConcatenatedFrames has the junk case of the table above.
• test-zlib-reject-garbage-after-end.js: not vendored. 9 of its 12 tests need options.rejectGarbageAfterEnd, which bun does not have. zstd-frames.test.ts has its five zstd tests with the default options.

So this PR adds 0 passing vendored Node tests. It removes two skipIf from zlib.test.js (the zstd variants 1 and 2 of "premature end handles bytesWritten properly"). Their comment said "fails in node too", which is not true for Node v26.10.0.

Why a new test file

zlib.test.js imports bun:test, so Node cannot run it. zstd-frames.test.ts uses node:test and node:assert and no harness import. Run it on Node with node --test test/js/node/zlib/zstd-frames.test.ts. The zlib/iter cases run in a child of the same runtime, because the module needs --experimental-stream-iter. The file starts that child before its tests.

Test runs

$ node --test test/js/node/zlib/zstd-frames.test.ts     # Node v26.10.0
ℹ tests 25
ℹ pass 25
ℹ fail 0

$ bun bd test test/js/node/zlib/zstd-frames.test.ts     # this PR
 25 pass
 0 fail

$ bun bd test test/js/node/zlib/zstd-frames.test.ts     # src/ of main at a4f1429148
 10 pass
 15 fail

$ bun bd test test/js/node/zlib/zlib.test.js            # src/ of main at a4f1429148
 418 pass
 2 fail

The 2 tests of zlib.test.js are the ones that lose their skipIf. Other suites on this PR: test/js/node/zlib/ (516 pass), test/js/web/streams/compression.test.ts (73 pass), test/js/bun/util/zstd.test.ts, test/js/web/fetch/fetch-compress.test.ts, test/regression/issue/23314/, and the 67 vendored files test-zlib* and test-stream-iter-transform-* (66 exit 0 as scripts, test-zlib-type-error.js passes under bun test).

Mutation check

Each row breaks one clause of the new code on purpose, in a debug build of this PR. The numbers are the tests that then fail. After this measurement the zlib/iter test changed how it starts its child. Its assertions did not change.

clause broken on purpose	zstd-frames.test.ts	zlib.test.js	test-stream-iter-transform-sync.js
input after a tail is decoded again	12	0	exit 0
a write with no input after a complete frame starts a frame	0	0	exit 0
the count of magic-number bytes starts again at each write	10	0	exit 0
a write decodes one frame at most	16	1	exit 1
input that cannot start a frame goes to zstd	15	4	exit 0
frame-complete stays set when zstd asks for more input	10	0	exit 0
a frame end does not clear the count of magic-number bytes	2	0	exit 0
a frame end does not set the possible frame types again	16	1	exit 1
init() and reset() keep the frame state	3	0	exit 0
no byte rules out a zstd frame	15	4	exit 0
no byte rules out a skippable frame	15	4	exit 0
a skippable magic number must start with 0x50	6	0	exit 0
each first byte can start a skippable frame	4	0	exit 0
byte 1 of the zstd magic number is not compared	4	0	exit 0
byte 2 of the zstd magic number is not compared	1	0	exit 0
byte 3 of the zstd magic number is not compared	1	0	exit 0
byte 4 of the zstd magic number is not compared	1	0	exit 0
byte 2 of the skippable magic number is not compared	1	0	exit 0
byte 3 of the skippable magic number is not compared	1	0	exit 0
byte 4 of the skippable magic number is not compared	1	0	exit 0

One row has no failure. A write with no input after a complete frame changes no result, because fewer than 4 bytes after a frame are never an error. The clause saves one ZSTD_decompressStream call for each write with no input after a complete frame, for example the last write of each stream (see the table of calls below). rejectGarbageAfterEnd needs it, and Node has it.

Measurements

Release builds (bun scripts/build.ts --profile=release, bun-profile) of main at a4f1429 and of this PR. Instructions and branches come from a gdb single-step of each native write, with callees run at full speed.

Own code of a native write is the host function (or the worker task), do_work and get_error_info. zstd itself runs at full speed and is not counted.

native write	main: instructions / conditional branches	this PR	difference
zstdDecompressSync, a write inside a frame	342 / 41	366 / 46	+24 / +5
zstdDecompressSync, the write that completes a frame	346 / 42	368 / 47	+22 / +5
zstdDecompressSync, a 6000-byte frame in one write	355 / 43	368 / 47	+13 / +4
zstdDecompressSync, two frames in one write	389 / 48	464 / 70	+75 / +22
zstdDecompressSync, a frame and junk in one write	372 / 45	461 / 67	+89 / +22
zstdDecompressSync, a frame and 2 bytes of a magic number	376 / 46	441 / 62	+65 / +16
a write with no input after a complete frame	342 / 41, 1 zstd call	343 / 42, no zstd call	+1 / +1
stream, worker thread, a write inside a frame	79 / 10	103 / 15	+24 / +5
stream, worker thread, the write that completes a frame	83 / 11	105 / 16	+22 / +5
stream, JS thread, get_error_info	23 / 1	23 / 1	0
zstd compress write	337 / 39	336 / 39	-1 / 0

• zstd decode write, do_work + check_error: +24 instructions and +5 conditional branches per native write inside a frame (gdb single-step on bun-profile, main 342 and 41, this PR 366 and 46). A write that goes on after a complete frame reads up to 4 bytes one by one, and costs +65 to +89 instructions.
• zstd compress write, do_work + check_error: -1 instruction, +0 conditional branches per native write.
• ZSTD_decompressStream calls for each public call, main -> this PR. No row is higher.

public call	main	this PR
zstdDecompressSync, 6000-byte frame	1	1
zstdDecompressSync, two frames	2	2
zstdDecompressSync, frame and junk	1	1
zstdDecompressSync, frame and 2 bytes of a magic number	2	2
zstdDecompressSync, frame that fills the 16384-byte chunk	2	1
zstdDecompressSync, that frame and junk	2, then ZSTD_error_prefix_unknown	1
zstdDecompressSync, 1 MiB frame	65	64
zstdDecompressSync, 1 MiB frame, chunkSize: 64	16385	16384
stream: frame, end()	2	1
stream: frame, frame, end()	3	2
stream: frame, junk, frame, end()	2, then ZSTD_error_prefix_unknown	1
stream: each byte of a frame, end()	31	30
stream: frame that fills the 16384-byte chunk, end()	3	1
stream: 1 MiB frame, end()	66	64

• release binary: +379 bytes in the NativeZstd symbols (18365 -> 18744), +256 bytes of .text (58143477 -> 58143733). The file size of the stripped bun does not change (80827976 bytes).
• size_of::<Context>() for NativeZstd: 88 -> 88 bytes. A const assertion holds it.
• zlib and brotli symbols: 0 bytes changed, 0 disassembly lines differ (35 symbols each).
• Calls inside a native write: the same as on main, less one memcmp. No allocation and no syscall is added.

Call orders that change

On main, 15 of the 25 tests of zstd-frames.test.ts fail. Each of them has a call order that changes from ZSTD_error_prefix_unknown to success. The call orders, with frame for a complete frame and junk for input that cannot start a frame:

• stream: write(frame), write(junk), write(frame), end(). The output is that of the first frame, then 'end'.
• stream: write(frame), then each byte of junk in a write of its own. bytesWritten counts the bytes of junk that a magic number can begin with.
• stream: two frames and then junk, and a frame, a skippable frame and then junk.
• one-shot: zstdDecompressSync(frame + junk) when the output of the frame fills the output chunk exactly.
• _processChunk: frame, junk, reset(), frame.
• zlib/iter, async and sync: [frame, junk, frame] and [frame, frame, junk].
• stream with awaited writes: frame, junk, reset(), frame, end(). The result is 'end' and then ERR_STREAM_PUSH_AFTER_EOF, as on Node v26.10.0. main gives one ZSTD_error_prefix_unknown.

A probe script outside the tree ran 113 call orders of the decoder on Node v26.10.0 and on release builds. main equals Node on 62 of 113, and this PR on 71. 9 change between main and this PR, all from error to success. The 42 that still differ are the ones of #33413.

Not in this PR

• options.rejectGarbageAfterEnd (Node ≥ 26.5.0) is not implemented for any decoder.
• reset() of a zstd stream drops the dictionary and the parameters. Node v26.10.0 keeps them. A separate fix is in progress.
• Open PRs on the same files: Deduplicate zlib/brotli/zstd binding helpers #31996, node:zlib: one failure path for zstd and brotli init(): close, call onerror, throw #38684, node:zlib: expose the flat ZSTD_* constant aliases on the module #41555, DecompressionStream: deliver the output decoded ahead of trailing junk before the error #43719.

Self-review

The review read this PR and #33413 as one stack and raised 15 concerns. The ones that apply to this PR, and what changed:

1. The evidence was not in the tree, and no text said which Node version the change follows. zstd-frames.test.ts now runs on Node, and the first section above names the versions.
2. Comments in src/ had more than one line. Each is one line now.
3. The tests had two stream helpers, and one could not see an event after 'end'. There is one helper, decode. It resolves at 'close' and returns the output, the events and bytesWritten.
4. No tail had a first byte that fits a magic number and a later byte that does not. The table of tails has one row for each of bytes 2, 3 and 4 of both magic numbers, and rows for the edges of the first byte.
5. No test made the decoder report an error in a later frame. Three kinds of bad frame are tested, in the same write and in a later write.
6. No test reached a second frame boundary with a tail. Two do now: after two frames, and after a skippable frame.
7. reset() after a tail had no test. Three tests cover it, and one records that the output of the next frame comes after 'end'.
8. The body did not say what the PR does to the upstream tests. See "Upstream tests".
9. Some tests could not fail. The mutation table shows what each clause of the new code breaks.
10. The drift of the vendored Node tests is a separate task, not part of this PR.

The other concerns are about the check in #33413.

[robobun]

Status

Ready for review. #33413 is on top of this PR and waits for a decision. This PR does not depend on it.

How to reproduce

const zlib = require("node:zlib");
const junk = Buffer.from("junk");

// 1. One-shot: the result depends on the size of the output.
for (const size of [16383, 16384]) {
  const input = Buffer.concat([zlib.zstdCompressSync(Buffer.alloc(size, "a")), junk]);
  try {
    console.log(size, zlib.zstdDecompressSync(input).length);
  } catch (err) {
    console.log(size, err.code);
  }
}

// 2. Stream: junk in a later write.
const decoder = zlib.createZstdDecompress();
decoder.on("data", chunk => console.log("data", String(chunk)));
decoder.on("end", () => console.log("end"));
decoder.on("error", err => console.log("error", err.code));
decoder.write(zlib.zstdCompressSync("first"));
decoder.write(junk);
decoder.end();

runtime	one-shot 16383	one-shot 16384	stream
main (a4f1429), Node v24.21.0, Node v26.6.0	16383	ZSTD_error_prefix_unknown	data first, error ZSTD_error_prefix_unknown
Node v26.10.0, this PR	16383	16384	data first, end

test/js/node/zlib/zstd-frames.test.ts runs on bun and on Node: node --test test/js/node/zlib/zstd-frames.test.ts.

[coderabbitai]

Navigate logical layers of code changes, visualize relationships, and explore their blast radius.

Walkthrough

Zstd decompression now tracks frame completion and partial frame prefixes across writes. It processes concatenated frames and ignores unrecognized trailing input after a completed frame. New tests cover stream and iterator behavior, frame splits, resets, byte counts, and errors.

Changes

Zstd decompression

Layer / File(s)	Summary
Frame state and decoding src/runtime/node/zlib/NativeZstd.rs	The decompression context tracks frame completion and partial frame prefixes. Decompression classifies frame magics across writes, loops over recognized frames, and leaves unrecognized trailing input unconsumed after a completed frame.
Frame behavior validation test/js/node/zlib/zstd-frames.test.ts, test/js/node/test/parallel/test-stream-iter-transform-sync.js, test/js/node/zlib/zlib.test.js	Tests cover concatenated frames, trailing input, split prefixes, resets, errors, byte counts, and iterator results. The bytesWritten test now runs for all decompressor variants.

Suggested reviewers: jarred-sumner, cirospaciari

Priority: ⬇️ Low

Merge Risk: 🔵 Low · up to 99b45

Zstd decompression now keeps frame state across writes and ignores trailing input after a completed frame, matching Node v26.10.0. No runtime defect was found. The one remaining item is that the PR edits a mirrored upstream Node test file; revert that edit before merging, since the new Bun-owned test already covers the same case.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: preserving Zstd frame state across writes and ignoring trailing input after the final frame.
Description check	✅ Passed	The description explains the problem, implementation, verification steps, reference behavior, test results, limitations, and performance impact. It does not use the exact template headings, but it pro…
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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[coderabbitai]

Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
In @test/js/node/test/parallel/test-stream-iter-transform-sync.js:
- Line 22: Remove the Bun-specific `zstdCompressSync` import,
`testZstdConcatenatedFrames` function, and its invocation from the vendored
stream test; retain the concatenated-frame coverage in the existing Bun-owned
`zstd-frames.test.ts` test.

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📒 Files selected for processing (4)

• src/runtime/node/zlib/NativeZstd.rs
• test/js/node/test/parallel/test-stream-iter-transform-sync.js
• test/js/node/zlib/zlib.test.js
• test/js/node/zlib/zstd-frames.test.ts

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[robobun]

^{Updated 9:12 AM PT - Sep 27th, 2026}

❌ @robobun, your commit 99b45a2 has 1 failures in Build #121263 (All Failures):

• test/js/bun/spawn/spawn.test.ts - code 1: stdout reader of an unref'd child and process lifetime > an idle reader stopped at the highwater … on 🐧 13 x64-asan

---

🧪   To try this PR locally:

bunx bun-pr 44133

That installs a local version of the PR into your bun-44133 executable, so you can run:

bun-44133 --bun

[claude]

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 do_work_decompress loop for a no-progress spin: it only re-enters ZSTD_decompressStream after a completed frame when unconsumed input remains, output has room, and classify_frame_prefix still allows a frame magic, so every iteration either consumes header bytes or returns — the same shape as Node's DoThreadPoolWork. The size_of::<Context>() == 88 assertion holds on every 64-bit target Bun builds (no 32-bit targets).

Extended reasoning...

The change rewrites the zstd decompress path in src/runtime/node/zlib/NativeZstd.rs to keep frame state across writes and adds tests; it touches no auth, crypto, or injection surface, only a byte-classification of untrusted compressed input that is bounds-limited to 4 bytes. Inline findings are posted, so a human review is already signalled; this note only records the loop-termination and struct-size checks that were examined and ruled out.

[robobun]

Closing: this change is part of #33413 again, so that the work is one PR. The head of #33413 holds this commit's change and the two fixes from the review here (a deadline for the helper child of the test file, and the note on rejectGarbageAfterEnd).