From c2af718454a4e997b0cf90026955488006da06a0 Mon Sep 17 00:00:00 2001
From: Varun Chawla <varun_6april@hotmail.com>
Date: Sat, 7 Feb 2026 21:50:32 -0800
Subject: [PATCH] Fix GzipDecompressor to handle concatenated gzip members

The GzipDecompressor class now properly handles concatenated gzip members
by checking for unused_data after each decompression and creating a new
decompressor to continue processing subsequent members. This prevents silent
data loss when decompressing multi-member gzip streams.

Changes:
- Added loop in decompress() to process all concatenated gzip members
- Added _flushed flag to prevent decompress() calls after flush()
- Added comprehensive test cases for single, double, and triple member streams
- Added test to verify RuntimeError after flush()

Fixes #3560
---
 tornado/test/util_test.py | 56 +++++++++++++++++++++++++++++++++++++++
 tornado/util.py           | 36 +++++++++++++++++++++++--
 2 files changed, 90 insertions(+), 2 deletions(-)

diff --git a/tornado/test/util_test.py b/tornado/test/util_test.py
index baa2ff2bda..22f441a2c5 100644
--- a/tornado/test/util_test.py
+++ b/tornado/test/util_test.py
@@ -1,3 +1,4 @@
+import gzip
 import re
 import sys
 import datetime
@@ -14,6 +15,7 @@
     timedelta_to_seconds,
     import_object,
     re_unescape,
+    GzipDecompressor,
 )
 
 from typing import cast, Dict, Any
@@ -371,3 +373,57 @@ def test_version_info_compatible(self):
 
     def test_current_version(self):
         self.assert_version_info_compatible(tornado.version, tornado.version_info)
+
+
+class GzipDecompressorTest(unittest.TestCase):
+    def test_concatenated_gzip_members(self):
+        """Test that concatenated gzip members are fully decompressed."""
+        data1 = b"First gzip member content."
+        data2 = b"Second gzip member content."
+
+        member1 = gzip.compress(data1)
+        member2 = gzip.compress(data2)
+
+        concatenated = member1 + member2
+        decompressor = GzipDecompressor()
+        result = decompressor.decompress(concatenated)
+
+        expected = data1 + data2
+        self.assertEqual(
+            result, expected, "Concatenated gzip members should be fully decompressed"
+        )
+
+    def test_single_gzip_member(self):
+        """Test that single gzip member is decompressed correctly."""
+        data = b"This is some example data that will be compressed using gzip."
+        compressed = gzip.compress(data)
+
+        decompressor = GzipDecompressor()
+        result = decompressor.decompress(compressed)
+
+        self.assertEqual(result, data)
+
+    def test_multiple_concatenated_members(self):
+        """Test that three or more concatenated gzip members are fully decompressed."""
+        data1 = b"First member."
+        data2 = b"Second member."
+        data3 = b"Third member."
+
+        concatenated = gzip.compress(data1) + gzip.compress(data2) + gzip.compress(data3)
+        decompressor = GzipDecompressor()
+        result = decompressor.decompress(concatenated)
+
+        expected = data1 + data2 + data3
+        self.assertEqual(result, expected)
+
+    def test_decompress_after_flush_raises(self):
+        """Test that decompress() raises RuntimeError after flush()."""
+        data = b"Test data"
+        compressed = gzip.compress(data)
+
+        decompressor = GzipDecompressor()
+        decompressor.decompress(compressed)
+        decompressor.flush()
+
+        with self.assertRaises(RuntimeError):
+            decompressor.decompress(gzip.compress(b"More data"))
diff --git a/tornado/util.py b/tornado/util.py
index 19df491323..73d9f205de 100644
--- a/tornado/util.py
+++ b/tornado/util.py
@@ -77,6 +77,7 @@ def __init__(self) -> None:
         # http://stackoverflow.com/questions/1838699/how-can-i-decompress-a-gzip-stream-with-zlib
         # This works on cpython and pypy, but not jython.
         self.decompressobj = zlib.decompressobj(16 + zlib.MAX_WBITS)
+        self._flushed = False
 
     def decompress(self, value: bytes, max_length: int = 0) -> bytes:
         """Decompress a chunk, returning newly-available data.
@@ -89,7 +90,36 @@ def decompress(self, value: bytes, max_length: int = 0) -> bytes:
         in ``unconsumed_tail``; you must retrieve this value and pass
         it back to a future call to `decompress` if it is not empty.
         """
-        return self.decompressobj.decompress(value, max_length)
+        if self._flushed:
+            raise RuntimeError("Cannot call decompress() after flush()")
+
+        data = value
+        out = bytearray()
+        remaining = max_length
+
+        while True:
+            if remaining:
+                chunk = self.decompressobj.decompress(data, remaining)
+            else:
+                chunk = self.decompressobj.decompress(data)
+
+            out.extend(chunk)
+
+            if remaining:
+                remaining = max(0, max_length - len(out))
+                if remaining == 0:
+                    break
+
+            # Handle concatenated gzip members
+            unused = getattr(self.decompressobj, "unused_data", b"")
+            if unused:
+                data = unused
+                self.decompressobj = zlib.decompressobj(16 + zlib.MAX_WBITS)
+                continue
+
+            break
+
+        return bytes(out)
 
     @property
     def unconsumed_tail(self) -> bytes:
@@ -102,7 +132,9 @@ def flush(self) -> bytes:
         Also checks for errors such as truncated input.
         No other methods may be called on this object after `flush`.
         """
-        return self.decompressobj.flush()
+        result = self.decompressobj.flush()
+        self._flushed = True
+        return result
 
 
 def import_object(name: str) -> Any: