zlib.zstd: `createZstdCompress` and `zstdCompressSync` produce different outputs

[willfarrell]

Version

v24.0.2

Platform

Darwin usernames-Mac-Studio.local 24.5.0 Darwin Kernel Version 24.5.0: Tue Apr 22 19:54:49 PDT 2025; root:xnu-11417.121.6~2/RELEASE_ARM64_T6000 arm64

Subsystem

zlib

What steps will reproduce the bug?

import test from 'node:test'
import { deepEqual } from 'node:assert'
import { Readable, pipeline } from 'node:stream'
import { createZstdCompress, zstdCompressSync } from 'node:zlib'

test(`zstd`, async (t) => {
  const input = JSON.stringify(new Array(1024).fill(0))
  const expectedOutput = zstdCompressSync(input)
  const readableStream = Readable.from(input)
  const compressStream = createZstdCompress()

  const chunks = []
  for await (const chunk of readableStream.pipe(compressStream)) {
	chunks.push(Buffer.from(chunk))
  }
  const output = Buffer.concat(chunks)
  deepEqual(output, expectedOutput)
})

Run node --test

How often does it reproduce? Is there a required condition?

Always

What is the expected behavior? Why is that the expected behavior?

Outputs to be the same.

What do you see instead?

▶ node --test packages/compress/node.test.js
✖ zstd (3.136875ms)
ℹ tests 1
ℹ suites 0
ℹ pass 0
ℹ fail 1
ℹ cancelled 0
ℹ skipped 0
ℹ todo 0
ℹ duration_ms 52.920791

✖ failing tests:

test at packages/compress/node.test.js:6:1
✖ zstd (3.136875ms)
  AssertionError [ERR_ASSERTION]: Expected values to be loosely deep-equal:
  
  Buffer(20) [Uint8Array] [
    40,
    181,
    47,
    253,
    0,
    88,
    93,
    0,
    0,
    32,
    91,
    48,
    44,
    93,
    1,
    0,
    250,
    247,
    187,
    33
  ]
  
  should loosely deep-equal
  
  Buffer(21) [Uint8Array] [
    40,
    181,
    47,
    253,
    96,
    1,
    7,
    93,
    0,
    0,
    32,
    91,
    48,
    44,
    93,
    1,
    0,
    250,
    247,
    187,
    33
  ]
      at TestContext.<anonymous> (file:///Users/username/Development/github/datastream/packages/compress/node.test.js:15:4)
      at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
      at async Test.run (node:internal/test_runner/test:1069:7)
      at async startSubtestAfterBootstrap (node:internal/test_runner/harness:332:3) {
    generatedMessage: true,
    code: 'ERR_ASSERTION',
    actual: <Buffer 28 b5 2f fd 00 58 5d 00 00 20 5b 30 2c 5d 01 00 fa f7 bb 21>,
    expected: <Buffer 28 b5 2f fd 60 01 07 5d 00 00 20 5b 30 2c 5d 01 00 fa f7 bb 21>,
    operator: 'deepEqual'
  }

Additional information

Not sure if this is a bug in zstd or if this is expect (metadata indicating how the compression happened maybe). Both variants of the output decompress correctly.

I noticed that zstdCompressSync(<string>) returns a buffer instead of a string like other algorithm convenience methods such as brotliCompressSync(<string>).

[bjohansebas]

cc: @nodejs/zlib