zlib: createUnzip().reset() does not restore auto-detection state #64619

Version

latest main branch

Platform

7.1.3-arch1-3

Subsystem

zlib

What steps will reproduce the bug?

'use strict';

const zlib = require('node:zlib');

const output = [];
const unzip = zlib.createUnzip();

unzip.on('data', (chunk) => output.push(chunk));
unzip.on('error', console.error);
unzip.on('end', () => {
  console.log(Buffer.concat(output).toString());
});

unzip.write(Buffer.from([0x1f]), () => {
  unzip.reset();
  unzip.end(zlib.gzipSync('hello'));
});

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

Every

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

The reset stream should behave like a newly created Unzip stream and output:

hello

This is consistent with the documented behavior that zlib.reset() resets the compressor/decompressor to factory defaults.

What do you see instead?

The stream emits Z_DATA_ERROR.

After reading 0x1f, gzip_id_bytes_read_ is set to 1, while mode_ remains UNZIP. The reset implementation does not handle UNZIP, so the underlying inflater and the gzip magic-byte counter are not reset.

The same state leak also occurs after auto-detection completes: mode_ remains INFLATE or GUNZIP instead of returning to UNZIP, so the next stream does not start with fresh format detection.

Additional information

No response