node:zlib: zstd decoder follows Node v26.10.0 across writes and at the end of the input by robobun · Pull Request #33413 · oven-sh/bun

robobun · 2026-07-06T03:39:41Z

Problem

zlib.zstdDecompressSync() returns a short or an empty buffer, and no error, for zstd input that ends inside a frame. Node ≥ 26.6.0 and ≥ 24.20.0 throw unexpected end of file (Z_BUF_ERROR).
Junk after a complete frame raises Unknown frame descriptor (ZSTD_error_prefix_unknown) when it arrives in a later write. Node v26.10.0 ignores it.
The cause: the decoder (src/runtime/node/zlib/NativeZstd.rs) keeps no frame state between writes.

Fix

Context keeps the frame state of Node's ZstdDecompressContext. The decode step and the end-of-input check are ports of its DoThreadPoolWork and GetErrorInfo.
finishFlush: ZSTD_e_flush turns the check off, as in Node.
Verified: test/js/node/zlib/zstd-frames.test.ts uses node:test and passes on Node v26.10.0 too. On main, 36 of its 70 tests fail. test-zlib-truncated.js from Node v26.10.0 exits 1 before and 0 after.
Self-reviewed: 15 concerns raised, 15 addressed.

Background

A zstd stream is a sequence of frames. Each starts with a 4-byte magic number. A write is one call of the native handle, and a frame can span writes.
History: on 2026-07-06 the request here was "Scope the strictness to DecompressionStream" (comment). Node added this check 15 days later (zlib: reject truncated zstd input nodejs/node#64593). This PR now does the opposite of that request.
Considered one flag for the end of a frame: it cannot hold a magic number that arrives split across writes.

Downsides

51 of 113 measured call orders change: 42 from success to Z_BUF_ERROR, 9 from ZSTD_error_prefix_unknown to success. zstdDecompressSync(Buffer.alloc(0)) now throws.
A zstd decode write inside a frame runs 30 to 36 more instructions. .text grows 512 bytes. Context stays 88 bytes.
options.rejectGarbageAfterEnd stays unimplemented (node:zlib: the rejectGarbageAfterEnd option is ignored #44136). A caller that sets it no longer gets the accidental ZSTD_error_prefix_unknown for junk in a later write.

Notes

History of this PR

2026-07-06: the PR made the zstd decoder strict for all of node:zlib. A maintainer asked to scope the strictness to DecompressionStream. The PR did that: head 0349be1 set the default finishFlush of ZstdDecompress to ZSTD_e_continue. At that time Node accepted truncated zstd input, so node:zlib stayed equal to Node.
2026-07-21: zlib: reject truncated zstd input nodejs/node#64593 ("zlib: reject truncated zstd input") merged. It is in v26.6.0 and v24.20.0 (LTS).
2026-08-04: webstreams: native CompressionStream/DecompressionStream/TextEncoderStream/TextDecoderStream #36695 made DecompressionStream native. DecompressionStream("zstd") rejects a frame that ends mid-stream since then, so the bug in the first description of this PR is gone on main.
v26.10.0: zlib: improve zstd decoding across chunk boundaries nodejs/node#65865 ("zlib: improve zstd decoding across chunk boundaries") added the rules for input after a complete frame.
Now: the branch starts again from main. Nothing of the old head remains. For some hours the first half of this change was a PR of its own (node:zlib: keep zstd frame state across writes and ignore input after the last frame #44133). It is part of this PR again.

Which Node version this follows

runtime	frame without its last 10 bytes	no input	frame, then such a frame	junk in a later write after a frame
Node v24.19.0, v26.5.0	0 bytes	0 bytes	6000 bytes	`ZSTD_error_prefix_unknown`
Node v24.21.0 (LTS), v26.6.0	throws	throws	6000 bytes	`ZSTD_error_prefix_unknown`
Node v26.10.0	throws	throws	throws	`'end'`
`main` (`a4f1429`)	0 bytes	0 bytes	6000 bytes	`ZSTD_error_prefix_unknown`
this PR	throws	throws	throws	`'end'`

The first three columns are zlib.zstdDecompressSync(input) with default options. "throws" is Error: unexpected end of file, code: "Z_BUF_ERROR", errno: -5. The last column is a stream: write(frame), write(junk), write(frame), end(). Each Node binary is the linux-x64 build from nodejs.org, and its sha256 matches SHASUMS256.txt.

zlib.zstdDecompressSync(frame + junk) returns the data of the frame on all of these runtimes. On main and on Node before v26.10.0 it throws ZSTD_error_prefix_unknown when the data of the frame fills the output chunk exactly (16384 bytes, or a multiple).

bun reports process.version v26.3.0, and this PR does not change that. Merged changes that follow a Node release after v26.3.0: #42977 (Node v26.8.2, with its vendored test synced to that tag) and #42128 (Node v26.4.0).

Who sees a change

A caller with the default finishFlush (ZSTD_e_end) whose input ends inside a frame, or who gives no input at all. undici and minizlib pass finishFlush: ZSTD_e_end, so they get the error on bun as they do on Node. axios passes finishFlush: ZSTD_e_flush, so it sees no change.
A caller that relies on ZSTD_error_prefix_unknown to detect junk after a frame in a later write.
Not Bun.zstdDecompressSync(), fetch() or DecompressionStream. They do not use this decoder.

Upstream tests

test-zlib-truncated.js is replaced with the copy from Node v26.10.0. On main that copy exits 1 (AssertionError: Missing expected exception., line 46). On this PR it exits 0. The old copy had no zstd case.
test-stream-iter-transform-sync.js is 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 is 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.
test-zlib-reject-garbage-after-end.js is 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.
zlib.test.js loses two skipIf (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, with a deadline of 60 seconds.

Test runs

$ node --test test/js/node/zlib/zstd-frames.test.ts     # Node v26.10.0
ℹ tests 70
ℹ pass 69
ℹ fail 0
ℹ skipped 1

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

$ bun test test/js/node/zlib/zstd-frames.test.ts        # release build of main at a4f1429148
 34 pass
 36 fail

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

The test that Node skips drives a handle before init(). Node aborts there (node_zlib.cc:559, init_done_ && "write before init"), so it is for bun only. The 2 tests of zlib.test.js are the ones that lose their skipIf.

Other suites on a debug build of this PR: test/js/node/zlib/ (561 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 and test/regression/issue/23314/ (135 pass), 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).

The four clauses of the end-of-input check

The check is flush == ZSTD_e_end, frame not complete, input.pos == input.size, output.pos < output.size.

The output clause decides alone when the write of the input has ZSTD_e_end and fills output chunks: without it the stream gives the error while it still has output to give. The test "the blocks that are complete come out before the error" holds it.
The input clause changes no result: zstd stops with room in the output only when it has no input left. Node has the clause, so the port keeps it.
A compressor keeps no frame state, and a handle that init() did not set up decoded nothing, so the check does not run for them. Node aborts on a write before init(), so it has no such case.

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. A test that timed out on the loaded host is not counted. After this measurement the zlib/iter tests changed how they start their child, and one search loop of a test got a bound. The assertions did not change.

clause broken on purpose	`zstd-frames.test.ts`	`zlib.test.js`	`test-zlib-truncated.js`	`test-stream-iter-transform-sync.js`
input after a tail is decoded again	12	0	exit 0	exit 0
a write with no input after a complete frame starts a frame	0	0	exit 0	exit 0
the count of magic-number bytes starts again at each write	25	0	exit 0	exit 0
a write decodes one frame at most	26	1	exit 0	exit 1
input that cannot start a frame goes to zstd	15	4	exit 0	exit 0
frame-complete stays set when zstd asks for more input	26	0	exit 0	exit 0
a frame end does not clear the count of magic-number bytes	8	0	exit 0	exit 0
a frame end does not set the possible frame types again	34	1	exit 0	exit 1
init() and reset() keep the frame state	4	0	exit 0	exit 0
no byte rules out a zstd frame	15	4	exit 0	exit 0
no byte rules out a skippable frame	15	4	exit 0	exit 0
a skippable magic number must start with 0x50	15	0	exit 0	exit 0
each first byte can start a skippable frame	4	0	exit 0	exit 0
byte 1 of the zstd magic number is not compared	4	0	exit 0	exit 0
byte 2 of the zstd magic number is not compared	1	0	exit 0	exit 0
byte 3 of the zstd magic number is not compared	1	0	exit 0	exit 0
byte 4 of the zstd magic number is not compared	1	0	exit 0	exit 0
byte 2 of the skippable magic number is not compared	1	0	exit 0	exit 0
byte 3 of the skippable magic number is not compared	1	0	exit 0	exit 0
byte 4 of the skippable magic number is not compared	1	0	exit 0	exit 0
the check runs on a handle that init() did not set up	1	0	exit 0	exit 0
the check runs on a compressor	the file fails at load	the file fails at load	exit 1	exit 1
the check runs with each flush value	34	0	exit 1	exit 1
the check runs after a complete frame	12	20	exit 1	exit 1
the check runs with input left in the buffer	0	0	exit 0	exit 0
the check runs with a full output buffer	5	0	exit 0	exit 0
the check runs with input left and with a full output buffer	17	3	exit 1	exit 0
1 to 3 bytes after a frame are an error	8	0	exit 0	exit 0
4 bytes after a frame are not an error	7	0	exit 0	exit 0
3 bytes after a frame are an error	4	0	exit 0	exit 0

Two rows have 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 such write, for example the last write of each stream (see the table of calls below). rejectGarbageAfterEnd needs it, and Node has it.
The input clause of the check, see above.

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. 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	378 / 51	+36 / +10
`zstdDecompressSync`, the write that completes a frame	346 / 42	377 / 51	+31 / +9
`zstdDecompressSync`, a 6000-byte frame in one write	355 / 43	377 / 51	+22 / +8
`zstdDecompressSync`, two frames in one write	389 / 48	473 / 74	+84 / +26
`zstdDecompressSync`, a frame and junk in one write	372 / 45	470 / 71	+98 / +26
`zstdDecompressSync`, a frame and 2 bytes of a magic number	376 / 46	460 / 70	+84 / +24
a write with no input after a complete frame	342 / 41, 1 zstd call	352 / 46, no zstd call	+10 / +5
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`, a write with `ZSTD_e_continue`	23 / 1	29 / 4	+6 / +3
stream, JS thread, `get_error_info`, the write with `ZSTD_e_end`	23 / 1	32 / 5	+9 / +4
zstd compress write	337 / 39	338 / 40	+1 / +1

zstd decode write, do_work + check_error: +30 (stream) to +36 (one-shot) instructions and +8 to +10 conditional branches per native write inside a frame. A write that goes on after a complete frame reads up to 4 bytes one by one, and costs +84 to +98 instructions.
zstd compress write, do_work + check_error: +1 instruction, +1 conditional branch 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: +463 bytes in the NativeZstd symbols (18365 -> 18828), +512 bytes of .text (58143477 -> 58143989). 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, 36 of the 70 tests of zstd-frames.test.ts fail. With frame for a complete frame and junk for input that cannot start a frame:

From ZSTD_error_prefix_unknown to success (15 tests):

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.

From success to Z_BUF_ERROR (21 tests), each input through zstdDecompressSync, zstdDecompress, _processChunk and a stream:

no input, and end() with no write
a frame without its last 10 bytes
2 bytes of a magic number at the start of the stream
a skippable frame without its last 2 bytes
a checksummed frame without the last byte of its checksum, and without its checksum
the output of a compressor that got a flush and no end
a frame, and then a frame without its last 10 bytes
a frame, and then the 4 bytes of a magic number, in one write or split across writes
flush(ZSTD_e_end) inside a frame, and reset() followed by end()
zlib/iter, async and sync: no chunk, an empty chunk, a frame without its last byte, a frame and then 4 bytes of a magic number

A stream gives the output that the input produced before it ended, and then 'error'. A write that fails gives no output, as for the other decoders.

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 all 113.

Not in this PR

options.rejectGarbageAfterEnd is not implemented for any decoder (node:zlib: the rejectGarbageAfterEnd option is ignored #44136). Node v24.19.0 and v26.5.0 have it. On main, a caller that sets it gets ZSTD_error_prefix_unknown for junk in a later write, by accident and with a code that Node does not use for it. With this PR that caller gets no error, as for junk in the same write and as for the other decoders.
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 raised 15 concerns. 12 asked for tests or evidence, 2 asked for a simpler guard in the end-of-input check, and 1 asked for a separate task. What changed:

This PR does the opposite of a maintainer's request. The description says so.
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 a section above names the versions.
Comments in src/ had more than one line. Each is one line now.
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.
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.
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.
No test reached a second frame boundary with a tail, or with 1 to 3 bytes of a magic number. Both are tested now, after two frames and after a skippable frame.
reset() after a tail had no test. Three tests cover it.
The output clause of the check had no test, and a note said that it only repeats the input clause. That was wrong. See the section on the four clauses.
The inputs did not vary the decoder. The table of inputs now has checksummed frames and the output of a compressor that got a flush and no end.
Two teardown tests could not fail: destroy() and close() ran before the handle saw the write. They now wait for the write.
The body did not say what the PR does to the upstream tests. See "Upstream tests".
A flag bit recorded that the decode step ran. The check now reads the mode and the state of the handle, which say the same.
The same concern from the side of the shared mixin: no change there.
The drift of the vendored Node tests from Node's releases is a separate task.

Two more findings came from the bot review after the push: the helper child of the test file had no deadline, and one search loop of a test had no bound. Both are fixed.

coderabbitai · 2026-07-06T03:39:52Z

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

Walkthrough

Zstd decompression now tracks frame prefixes and completion across writes, processes concatenated frames, and reports incomplete frames on final flush. The changes add regression tests for frame handling, truncated input, and decompression behavior across APIs.

Changes

Zstd decompression

Layer / File(s)	Summary
Frame-prefix processing `src/runtime/node/zlib/NativeZstd.rs`	The decompression context tracks frame completion and classifies frame prefixes incrementally. It processes subsequent recognized frames and leaves unrecognized trailing input unconsumed.
End-of-input handling `src/zstd/lib.rs`, `src/runtime/node/zlib/NativeZstd.rs`, `test/js/node/test/parallel/test-zlib-truncated.js`	The Zstd end directive is defined. Final flush reports an unexpected end of file when input ends inside a frame. Truncated-input tests cover Zstd flush and stream behavior.
Decompression API regression coverage `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 trailing input, concatenated and skippable frames, malformed and truncated input, resets, byte counts, and results across decompression APIs.

Suggested reviewers: jarred-sumner, cirospaciari

Priority: ⬇️ Low

Merge Risk: 🔵 Low · up to 52f6d

This change makes zstd decompression match Node v26.10.0 when handling concatenated, trailing, and truncated frames. Truncated input now produces Z_BUF_ERROR under the default end directive, which is an intentional behavior change for maintainers to accept. The only remaining issue is that the new test file uses a test runner that does not follow repository convention. It should either follow that rule or receive a documented exception.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
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.
Title check	✅ Passed	The title clearly identifies the main change: making the node:zlib Zstandard decoder match Node v26.10.0 across writes and at end of input.
Description check	✅ Passed	The description explains the problem, implementation, behavior changes, scope, limitations, and verification results. It does not use the exact template headings, but it provides the required informat…

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robobun · 2026-07-06T03:39:52Z

^{Updated 10:02 AM PT - Sep 27th, 2026}

✅ @robobun, your commit 52f6ddede4162d74f36a27e4fcc2cc40a81e26d7 passed in Build #121270! 🎉

🧪 To try this PR locally:

bunx bun-pr 33413

That installs a local version of the PR into your bun-33413 executable, so you can run:

bun-33413 --bun

robobun · 2026-07-06T03:43:45Z

@coderabbitai review

coderabbitai · 2026-07-06T03:43:50Z

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Review finished.

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claude

I didn't find any bugs, but this deliberately diverges from Node's zstd behavior and relies on subtle latching against the JS driver's re-drive loop, so a maintainer should sign off on both the compat decision and the mechanism.

Extended reasoning...

Overview

Adds a frame_ended latch to the zstd decompression Context in src/runtime/node/zlib/NativeZstd.rs so that get_error_info can raise Z_BUF_ERROR / "unexpected end of file" when the finishing flush arrives without ever having seen ZSTD_decompressStream return 0. Also adds the ZSTD_e_end constant to src/zstd/lib.rs, a new test file test/js/node/zlib/zstd-truncated-frame.test.ts, and a parametrized truncation suite in test/js/web/streams/compression.test.ts covering all five formats.

Security risks

None identified. This is purely error-reporting; no new parsing of untrusted input, no allocation sizing, no memory-lifetime changes. The added bool field is initialized in Default, reset in init() (and thus reset()), and only read/written on the same paths that already touch the Context.

Level of scrutiny

Medium-high. The native change is small but the correctness argument is subtle: it depends on (a) zstd's contract that a zero-length input always returns a nonzero hint, (b) the JS driver's re-drive condition being exactly output.pos == output.size, so output.pos < output.size is the terminal iteration, and (c) the latch surviving the finishing flush's empty write because input.size == 0 skips the update. The tests do cover the negative contract (multi-chunk output, zero-byte frames, trailing bytes, finishFlush: ZSTD_e_continue escape hatch), which is reassuring.

Other factors

The PR explicitly states this is a deliberate divergence from Node, which currently accepts truncated zstd frames silently. That is a user-facing behavior change to zlib.zstdDecompress / zstdDecompressSync / createZstdDecompress and a Node-compat policy call — exactly the kind of decision a maintainer should confirm rather than a bot. The argument (silent truncation is worse; matches the other four formats; finishFlush is the documented escape hatch) is well-made, but it's still a policy decision.

robobun · 2026-07-06T04:06:30Z

Both points are fair. Answering each with something checkable.

The mechanism: `output.pos < output.size` is the driver's own re-drive predicate

It reads as a heuristic in isolation, but it is the exact negation of the condition every caller uses to decide whether to call the handle again.

NativeZstd::update_write_result writes output.size - output.pos into writeState[0] (node_zlib_binding.rs#L236 maps it to slot 0), and every driver loops on exactly writeState[0] === 0:

driver	re-drive condition
`processChunkSync`	zlib.ts#L380 `if (availOutAfter === 0)`
`processCallback`	zlib.ts#L466 `if (availOutAfter === 0)`
`makeZlibTransform`	transform.ts#L328 `if (availOut === 0)`
`makeZlibTransformSync`	transform.ts#L550 `if (availOut === 0)`

Those four are the whole set: NativeZstd is reachable only from node:zlib and internal/streams/iter/transform, sync and async halves each. So output.pos < output.size means precisely "the caller is about to stop re-driving this write", which is the one moment the decoder can decide.

The failure mode if that assumption ever broke is also the safe direction: a caller that stopped early on a full output buffer would make the check under-report (a truncated frame decodes quietly, today's behavior), never over-report a valid frame. I did hit the over-report case while writing this, before the predicate was there, and it is what the "output spans many chunks" test pins.

The divergence: `node:zlib` was the only zstd decoder in Bun that accepted a truncated frame

This is the part I should have put in the description. Every other zstd decode path in Bun already rejects the same bytes:

Bun.zstdDecompressSync(truncated)          throws  Decompression failed: InvalidZstdData
fetch(content-encoding: zstd, truncated)   throws  ZstdDecompressionError
zlib.zstdDecompressSync(truncated)         returns 0 bytes, no error   <-- before this PR

(the fetch one is already asserted in test/regression/issue/18413-truncation.test.ts)

So it lines node:zlib up with the rest of Bun, not only with the other four formats inside node:zlib. That is the argument for doing it here; it is still a maintainer call, and I am not going to pretend otherwise.

If you would rather keep zlib.* byte-compatible with Node and confine the strictness to DecompressionStream("zstd"), that is a small change and no native code moves: the decoder already keys off flush == ZSTD_e_end, so it reduces to giving ZstdDecompress a lenient default finishFlush and having DecompressionStream opt in. The trade-off is that ZstdDecompress.prototype._finishFlushFlag would then differ from Node's. Happy to push that version instead, just say which you prefer.

claude

No bugs found — the latch mechanism and its coupling to the driver's availOut === 0 re-drive predicate check out. Deferring only because the deliberate divergence from Node's zlib.zstdDecompress* behavior (vs. scoping strictness to DecompressionStream) is the maintainer call you already flagged.

Extended reasoning...

Overview

This PR adds truncated-frame detection to the zstd decompression path in node:zlib. The native change is confined to src/runtime/node/zlib/NativeZstd.rs: a new frame_ended: bool field on Context, latched in do_work when ZSTD_decompressStream returns 0 (and cleared only by non-empty input), then read in get_error_info to raise Z_BUF_ERROR at the finishing flush once the driver stops re-driving (output.pos < output.size). src/zstd/lib.rs gains only the ZSTD_e_end constant. Tests cover both the DecompressionStream and node:zlib surfaces plus the negative contract (multi-chunk output, zero-byte payloads, trailing bytes, finishFlush: ZSTD_e_continue escape hatch).

Security risks

None identified. This tightens error reporting on malformed input rather than loosening it; no new allocation, no new user-controlled sizes, no unsafe blocks added. The failure direction if the output.pos < output.size invariant ever broke is under-reporting (today's silent-truncation behavior), not a false positive on valid data.

Level of scrutiny

Moderate-to-high. The native change is small and well-reasoned — the latch semantics match zstd's documented "0 = frame complete and flushed" contract, and the author verified the output.pos < output.size predicate is the exact negation of the re-drive condition in all four JS drivers. The subtlety is real (the finishing flush's empty write returning a nonzero hint is why a naive port of the brotli check doesn't work), but the tests pin both the false-positive case (multi-chunk output) and the false-negative cases.

Other factors

The reason I'm not approving is the user-facing behavioral change to zlib.zstdDecompress / zstdDecompressSync / createZstdDecompress: input that previously decoded to a (possibly empty/short) buffer now throws. The author explicitly calls this out as a deliberate Node divergence, argues it aligns with every other zstd path in Bun and every other format in node:zlib, and offers a scoped alternative (lenient default finishFlush on ZstdDecompress, strict opt-in for DecompressionStream). That's a product/compat decision a maintainer should sign off on, not something an automated reviewer should resolve.

robobun · 2026-07-06T05:39:52Z

Status

One PR again. It holds both parts of the change: the zstd decoder of node:zlib keeps its frame state across writes, and it reports input that ends inside a frame as Z_BUF_ERROR. #44133 held the first part for some hours and is closed.

The second part is the opposite of the request of 2026-07-06 on this PR. The description says why.

How to reproduce

const zlib = require("node:zlib");
const frame = zlib.zstdCompressSync(Buffer.alloc(6000, "hello world "));

// 1. Input that ends inside a frame.
try {
  console.log(zlib.zstdDecompressSync(frame.subarray(0, frame.length - 10)).length);
} catch (err) {
  console.log(err.code);
}

// 2. Junk after a complete frame, in a later write.
const decoder = zlib.createZstdDecompress();
decoder.on("data", chunk => console.log("data", chunk.length));
decoder.on("end", () => console.log("end"));
decoder.on("error", err => console.log("error", err.code));
decoder.write(frame);
decoder.write(Buffer.from("junk"));
decoder.end();

runtime	1	2
`main` (`a4f1429`)	`0`	`data 6000`, `error ZSTD_error_prefix_unknown`
Node v26.10.0, this PR	`Z_BUF_ERROR`	`data 6000`, `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.

Jarred-Sumner · 2026-07-06T05:42:35Z

Scope the strictness to DecompressionStream

robobun · 2026-07-06T06:40:52Z

Done in 0349be1, scoped to DecompressionStream.

ZSTD_e_end is the directive that asks the decoder to verify the frame ended, so the scoping falls out of which finishFlush each entry point uses. No native code moved:

ZstdDecompress and zlib/iter's decompressZstd / decompressZstdSync finish with ZSTD_e_continue and stay lenient.
DecompressionStream("zstd") passes finishFlush: ZSTD_e_end and gets the check.

Verified against node 26 on every observable surface:

	node 26	bun, this PR
`zstdDecompressSync(truncated)`	0 bytes, no error	0 bytes, no error
`zstdDecompressSync(empty)`	0 bytes, no error	0 bytes, no error
`zstdDecompressSync(valid + trailing junk)`	6000 bytes	6000 bytes
`createZstdDecompress(truncated)`	ends cleanly	ends cleanly
`DecompressionStream("zstd")` truncated	(format unsupported)	`Z_BUF_ERROR`

One thing to flag since it is the knob doing the scoping: ZstdDecompress.prototype._finishFlushFlag now defaults to ZSTD_e_continue instead of node's ZSTD_e_end. Nothing reads that field, and every behavior it drives is unchanged, but it does mean a caller who passes finishFlush: ZSTD_e_end explicitly opts into the check (the mirror of finishFlush: Z_SYNC_FLUSH opting out of gzip's). If you would rather that field match node exactly, the alternative is an explicit opt-in flag plumbed through NativeZstd::init, which costs a native parameter. Say the word and I will switch.

Tests moved with the scope: test/js/web/streams/compression.test.ts now covers truncation and empty input across all five formats, plus the negative contract (a frame whose output spans many 16 KiB chunks must not read as truncated) and the scoping itself (zlib.zstdDecompressSync stays lenient, ZSTD_e_end is what turns the check on). test/js/node/zlib/zstd-truncated-frame.test.ts is gone, since its premise was the strictness that is no longer there.

When a zstd decoder finishes with ZSTD_e_end and the input stops inside a frame, the decoder reports "unexpected end of file" (Z_BUF_ERROR, errno -5). Before, it reported success, and the caller got a short or an empty result. Node does this since v26.6.0 and v24.20.0. This is the opposite of a maintainer's request of 2026-07-06 on oven-sh#33413 ("Scope the strictness to DecompressionStream"). Node did not have the check then. The change needs that maintainer's decision. The check is a port of ZstdDecompressContext::GetErrorInfo in Node v26.10.0, without rejectGarbageAfterEnd. It reads the frame state that the decoder keeps across writes. It applies when the write has ZSTD_e_end, the frame is not complete, the input is consumed and the output buffer has room. With a full output buffer the caller writes again, and that write gets the result. Fewer than 4 bytes after a complete frame do not show that a frame began, so they are not an error. finishFlush ZSTD_e_flush or ZSTD_e_continue turns the check off. A compressor and a handle that init() did not set up report nothing. test-zlib-truncated.js is the copy from Node v26.10.0.

robobun · 2026-09-27T15:49:19Z

@Jarred-Sumner this PR changed direction, so one part of it is yours to decide.

On 2026-07-06 you wrote here: "Scope the strictness to DecompressionStream". Node.js accepted truncated zstd input then. Since v26.6.0 and v24.20.0 (LTS), Node raises unexpected end of file (Z_BUF_ERROR, errno -5) for zstd input that ends inside a frame. bun reports process.version v26.3.0, which accepted it.

This PR makes the zstd decoder of node:zlib do what Node v26.10.0 does, by default. finishFlush: ZSTD_e_flush or ZSTD_e_continue is the opt-out, as in Node. If you do not want that part, say so, and I take the end-of-input check out. The other part (frame state across writes) does not depend on it.

The description has the measurements and the history. (Edited: #44133 is closed. Both parts are in this PR.)

claude

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

Beyond the inline finding, I also checked three things in the new input_ended_inside_frame check and ruled them out: the empty-input case (zstdDecompressSync(Buffer.alloc(0)) now throws) matches Node v26.10.0's own behaviour, so it is a deliberate compat change rather than a bug; zlib.iter.decompressZstd rejecting truncated input with no finishFlush escape hatch is the same contract Node exposes; and the "complete frame but zstd returned a nonzero hint while the output chunk was full" scenario cannot produce a false Z_BUF_ERROR because the output.pos == output.size clause bails before the frame state is consulted. The behavioural question the description itself raises (going strict for all of node:zlib after a maintainer asked to scope it to DecompressionStream) still needs a human call.

Extended reasoning...

The native change is a ~30-line addition to src/runtime/node/zlib/NativeZstd.rs plus a new ZSTD_e_end constant in src/zstd/lib.rs; the rest is a re-synced upstream test file and a large new bun:test block. It touches no auth, crypto, or injection surface. Not approved because the PR explicitly contradicts an earlier maintainer direction on scope and asks for a decision, and because one confirmed finding (an unbounded search loop in the new test) is posted inline.

…e end of the input The zstd decoder keeps Node's frame state between writes, and reads it in two places. After a complete frame, input that cannot start a frame ends the stream. The decoder ignores that input and all later input. Before, this was true only when the input came in the same write as the end of the frame and the output chunk had room. In a later write the same input raised "Unknown frame descriptor" (ZSTD_error_prefix_unknown). When a decoder finishes with ZSTD_e_end and the input stops inside a frame, the decoder reports "unexpected end of file" (Z_BUF_ERROR, errno -5). Before, it reported success, and the caller got a short or an empty result. Node does this since v26.6.0 and v24.20.0. finishFlush ZSTD_e_flush or ZSTD_e_continue turns the check off. Fewer than 4 bytes after a complete frame do not show that a frame began, so they are not an error. The second part is the opposite of a maintainer's request of 2026-07-06 on #33413 ("Scope the strictness to DecompressionStream"). Node did not have the check then. Both parts are a port of ZstdDecompressContext in Node v26.10.0 (DoThreadPoolWork and GetErrorInfo), without rejectGarbageAfterEnd. The state fits the tail padding of Context, which stays 88 bytes. init() clears it, so reset() clears it too. test/js/node/zlib/zstd-frames.test.ts uses node:test and node:assert. The same file passes under Node v26.10.0. test-zlib-truncated.js and test-stream-iter-transform-sync.js are the copies from Node v26.10.0.

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Inline comments:
In @test/js/node/zlib/zstd-frames.test.ts:
- Line 11: Update the test imports from node:test to bun:test, and replace the
node:test-only skip-options form in the test using test(name, { skip }, fn) with
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claude

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

Beyond the inline finding, I also checked that the new DecodeState/frame_prefix_size/possible_frame_types fields are cleared on every lifecycle path — reset() routes through init(), which zeroes them before re-creating the DCtx — and that the first-block search in zstd-frames.test.ts is now bounded by big.length with an assertion when no prefix matches, so the earlier hang concern no longer applies.

Extended reasoning...

The change ports Node's ZstdDecompressContext frame-state tracking into src/runtime/node/zlib/NativeZstd.rs and adds tests; it touches no auth, crypto, or injection surface. The remaining inline finding on rejectGarbageAfterEnd semantics is the item a human should weigh; the lifecycle-reset and test-loop-bound concerns were verified against the code and ruled out.

robobun · 2026-09-30T00:51:49Z

#44261 changes Context::reset in NativeZstd.rs. It no longer calls init(). It resets the zstd session on the same context, as Node v26.10.0 does.

This PR clears decode, frame_prefix_size and possible_frame_types in init() only. The two branches merge with no text conflict, but the merged tree does not compile. reset() in #44261 names every field of Context in a pattern with no .., so the three new fields give error E0027 there.

The PR that lands second must add the three fields to that pattern and clear them in reset() after ZSTD_DCtx_reset, as Node's ZstdDecompressContext::ResetStream does.

github-actions Bot added the claude label Jul 6, 2026

claude Bot reviewed Jul 6, 2026

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robobun changed the title ~~zlib: report a truncated zstd frame as Z_BUF_ERROR~~ DecompressionStream: error on a zstd frame that ends mid-stream Jul 6, 2026

robobun mentioned this pull request Jul 6, 2026

node:fs: apply cpSync errorOnExist per entry, not to the destination directory #33416

Closed

claude Bot mentioned this pull request Jul 27, 2026

DecompressionStream("zstd"): decode concatenated frames instead of rejecting as trailing junk #36021

Merged

This was referenced Sep 27, 2026

node:stream/iter: call transform() of a transform object with the object as this #44131

Open

node:zlib: keep zstd frame state across writes and ignore input after the last frame #44133

Closed

robobun changed the title ~~DecompressionStream: error on a zstd frame that ends mid-stream~~ node:zlib: report zstd input that ends inside a frame as Z_BUF_ERROR Sep 27, 2026

robobun changed the base branch from main to robobun/4d1aec15/zstd-frame-tracking September 27, 2026 15:47

robobun force-pushed the farm/4d1aec15/zstd-decompress-truncated-frame branch from 0349be1 to 5e6c80e Compare September 27, 2026 15:48

claude Bot reviewed Sep 27, 2026

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Comment thread test/js/node/zlib/zstd-frames.test.ts Outdated

robobun changed the title ~~node:zlib: report zstd input that ends inside a frame as Z_BUF_ERROR~~ node:zlib: zstd decoder follows Node v26.10.0 across writes and at the end of the input Sep 27, 2026

robobun changed the base branch from robobun/4d1aec15/zstd-frame-tracking to main September 27, 2026 16:34

robobun force-pushed the farm/4d1aec15/zstd-decompress-truncated-frame branch from 5e6c80e to 52f6dde Compare September 27, 2026 16:34

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claude Bot reviewed Sep 27, 2026

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Comment thread src/runtime/node/zlib/NativeZstd.rs

robobun mentioned this pull request Sep 27, 2026

node:zlib: the rejectGarbageAfterEnd option is ignored #44136

Closed

robobun requested a review from Jarred-Sumner September 27, 2026 17:12

robobun mentioned this pull request Sep 30, 2026

node:zlib: reset a zstd stream in place, keep its dictionary and parameters #44261

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robobun commented Jul 6, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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robobun commented Jul 6, 2026

The mechanism: output.pos < output.size is the driver's own re-drive predicate

The divergence: node:zlib was the only zstd decoder in Bun that accepted a truncated frame

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The mechanism: `output.pos < output.size` is the driver's own re-drive predicate

The divergence: `node:zlib` was the only zstd decoder in Bun that accepted a truncated frame

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