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

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

Start with the createUnzip() and reset() entry points and run the provided reproduction, confirming that resetting after the partial 0x1f input produces Z_DATA_ERROR. Trace the reset behavior for auto-detection and verify that the stream can then process gzipSync('hello') and emit 'hello'.