From 0d4ae13fef7fa9037e5777aed204b7024d1e9367 Mon Sep 17 00:00:00 2001
From: Archkon <180910180+Archkon@users.noreply.github.com>
Date: Mon, 20 Jul 2026 16:31:47 +0800
Subject: [PATCH] zlib: restore unzip auto-detection on reset

Restore the initial UNZIP mode and clear partial gzip header state when
resetting an Unzip stream. Preserve GUNZIP mode for internal resets
between concatenated gzip members.

Signed-off-by: Archkon <180910180+Archkon@users.noreply.github.com>
---
 src/node_zlib.cc                       | 17 +++++++++--
 test/parallel/test-zlib-unzip-reset.js | 39 ++++++++++++++++++++++++++
 2 files changed, 54 insertions(+), 2 deletions(-)
 create mode 100644 test/parallel/test-zlib-unzip-reset.js

diff --git a/src/node_zlib.cc b/src/node_zlib.cc
index 95201624cfa2fa..2dbfa27de9f1ea 100644
--- a/src/node_zlib.cc
+++ b/src/node_zlib.cc
@@ -188,7 +188,10 @@ class ZlibContext final : public MemoryRetainer {
   void SetFlush(int flush);
   void GetAfterWriteOffsets(uint32_t* avail_in, uint32_t* avail_out) const;
   CompressionError GetErrorInfo() const;
-  inline void SetMode(node_zlib_mode mode) { mode_ = mode; }
+  inline void SetMode(node_zlib_mode mode) {
+    mode_ = mode;
+    initial_mode_ = mode;
+  }
   CompressionError ResetStream();
 
   // Zlib-specific:
@@ -212,6 +215,7 @@ class ZlibContext final : public MemoryRetainer {
 
  private:
   CompressionError ErrorForMessage(const char* message) const;
+  CompressionError ResetStream(node_zlib_mode target_mode);
   CompressionError SetDictionary();
   bool InitZlib();
 
@@ -222,6 +226,7 @@ class ZlibContext final : public MemoryRetainer {
   int level_ = 0;
   int mem_level_ = 0;
   node_zlib_mode mode_ = NONE;
+  node_zlib_mode initial_mode_ = NONE;
   int strategy_ = 0;
   int window_bits_ = 0;
   bool reject_garbage_after_end_ = false;
@@ -1144,7 +1149,7 @@ void ZlibContext::DoThreadPoolWork() {
         // Trailing zero bytes are okay, though, since they are frequently
         // used for padding.
 
-        ResetStream();
+        ResetStream(mode_);
         err_ = inflate(&strm_, flush_);
       }
       break;
@@ -1209,6 +1214,10 @@ CompressionError ZlibContext::GetErrorInfo() const {
 
 
 CompressionError ZlibContext::ResetStream() {
+  return ResetStream(initial_mode_);
+}
+
+CompressionError ZlibContext::ResetStream(node_zlib_mode target_mode) {
   bool first_init_call = InitZlib();
   if (first_init_call && err_ != Z_OK) {
     return ErrorForMessage("Failed to init stream before reset");
@@ -1225,6 +1234,7 @@ CompressionError ZlibContext::ResetStream() {
     case INFLATE:
     case INFLATERAW:
     case GUNZIP:
+    case UNZIP:
       err_ = inflateReset(&strm_);
       break;
     default:
@@ -1234,6 +1244,9 @@ CompressionError ZlibContext::ResetStream() {
   if (err_ != Z_OK)
     return ErrorForMessage("Failed to reset stream");
 
+  mode_ = target_mode;
+  if (mode_ == UNZIP) gzip_id_bytes_read_ = 0;
+
   return SetDictionary();
 }
 
diff --git a/test/parallel/test-zlib-unzip-reset.js b/test/parallel/test-zlib-unzip-reset.js
new file mode 100644
index 00000000000000..0b20b40c51d027
--- /dev/null
+++ b/test/parallel/test-zlib-unzip-reset.js
@@ -0,0 +1,39 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('node:assert');
+const zlib = require('node:zlib');
+
+function testReset(prefix, input, expected) {
+  const output = [];
+  const unzip = zlib.createUnzip()
+    .on('error', common.mustNotCall())
+    .on('data', (chunk) => output.push(chunk))
+    .on('end', common.mustCall(() => {
+      assert.strictEqual(Buffer.concat(output).toString(), expected);
+    }));
+
+  unzip.write(prefix, common.mustCall(() => {
+    unzip.reset();
+    unzip.end(input);
+  }));
+}
+
+// Reset while gzip auto-detection is halfway through.
+testReset(
+  Buffer.from([0x1f]),
+  zlib.gzipSync('hello'),
+  'hello',
+);
+
+// Reset after auto-detection selected INFLATE. The new input must be detected
+// as GUNZIP so that every concatenated gzip member is processed.
+testReset(
+  zlib.deflateSync('discarded').subarray(0, 2),
+  Buffer.concat([
+    zlib.gzipSync('abc'),
+    zlib.gzipSync('def'),
+    zlib.gzipSync('ghi'),
+  ]),
+  'abcdefghi',
+);