From cdde8e653babf5d705b5e56b8662b7bb842b1fce Mon Sep 17 00:00:00 2001
From: Serhiy Storchaka <storchaka@gmail.com>
Date: Wed, 20 May 2026 12:28:56 +0300
Subject: [PATCH] gh-150144: Speed up reading gzip files with long filename or
 comment

---
 Lib/gzip.py                                   | 16 ++++++-
 Lib/test/test_gzip.py                         | 42 +++++++++++++++++++
 ...-05-20-12-27-06.gh-issue-150144.m6dAoT.rst |  1 +
 3 files changed, 58 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-05-20-12-27-06.gh-issue-150144.m6dAoT.rst

diff --git a/Lib/gzip.py b/Lib/gzip.py
index 1e05f43c0c9e24..c354270f308120 100644
--- a/Lib/gzip.py
+++ b/Lib/gzip.py
@@ -488,7 +488,21 @@ def _read_until_null(fp, crc=None):
     '''Read until the first encountered null byte in fp.
     If crc is not None, update and return the CRC.
     '''
-    if crc is None:
+    if isinstance(fp, _PaddedFile):
+        size = 1
+        while True:
+            s = fp.read(size)
+            if not s:
+                break
+            i = s.find(0) + 1
+            if crc is not None:
+                crc = zlib.crc32(s[:i] if i else s, crc)
+            if i:
+                fp.prepend(s[i:])
+                break
+            if size < 2**20:
+                size *= 2
+    elif crc is None:
         while True:
             s = fp.read(1)
             if not s or s == b'\000':
diff --git a/Lib/test/test_gzip.py b/Lib/test/test_gzip.py
index b3b7c8f87e4f9f..6adaaafa614704 100644
--- a/Lib/test/test_gzip.py
+++ b/Lib/test/test_gzip.py
@@ -10,6 +10,7 @@
 import sys
 import unittest
 from subprocess import PIPE, Popen
+from test import support
 from test.support import catch_unraisable_exception
 from test.support import force_not_colorized_test_class, import_helper
 from test.support import os_helper
@@ -824,6 +825,47 @@ def test_corrupted_gzip_header(self):
                          f"Corrupted gzip header. Checksums do not "
                          f"match: {true_crc:04x} != {corrupted_crc:04x}")
 
+    def _test_long_header(self, flags):
+        with_crc = flags & 0x02
+        prefix = b'\x1f\x8b\x08' + bytes([flags]) + b'\x00\x00\x00\x00\x00\xff'
+        suffix = b'\x03\x00\x00\x00\x00\x00\x00\x00\x00\x00'
+        with open(self.filename, 'wb') as f:
+            f.write(prefix)
+            if with_crc:
+                crc = zlib.crc32(prefix)
+            block = b'ABCDEFGHIJKLMNOP' * 2**16  # 1 MiB
+            for i in range(1024):
+                f.write(block)
+                if with_crc:
+                    crc = zlib.crc32(block, crc)
+            f.write(b'\x00')
+            if with_crc:
+                crc = zlib.crc32(b'\x00', crc)
+                f.write(struct.pack("<H", crc & 0xFFFF))
+            f.write(suffix)
+        with gzip.GzipFile(self.filename, 'rb') as f:
+            f.read(1)
+
+    @support.requires_resource('largefile')
+    @support.requires_resource('cpu')
+    def test_long_filename(self):
+        self._test_long_header(0x08)  # FNAME
+
+    @support.requires_resource('largefile')
+    @support.requires_resource('cpu')
+    def test_long_filename_with_crc(self):
+        self._test_long_header(0x0a)  # FNAME | FHCRC
+
+    @support.requires_resource('largefile')
+    @support.requires_resource('cpu')
+    def test_long_comment(self):
+        self._test_long_header(0x10)  # FCOMMENT
+
+    @support.requires_resource('largefile')
+    @support.requires_resource('cpu')
+    def test_long_comment_with_crc(self):
+        self._test_long_header(0x12)  # FCOMMENT | FHCRC
+
     def test_read_truncated(self):
         data = data1*50
         # Drop the CRC (4 bytes) and file size (4 bytes).
diff --git a/Misc/NEWS.d/next/Library/2026-05-20-12-27-06.gh-issue-150144.m6dAoT.rst b/Misc/NEWS.d/next/Library/2026-05-20-12-27-06.gh-issue-150144.m6dAoT.rst
new file mode 100644
index 00000000000000..70463c6aa477a9
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-05-20-12-27-06.gh-issue-150144.m6dAoT.rst
@@ -0,0 +1 @@
+Sped up reading :mod:`gzip` files with long embedded filename or comment.