Fix ZLibDecompressor dropping data past the first gzip member

When a response body contains concatenated gzip members (RFC 1952 §2.2),
zlib sets eof and moves the remaining bytes to unused_data once the
first member is fully consumed. decompress_sync() was not checking
unused_data, so every member after the first was silently discarded.

Apply the same while-eof-and-unused_data loop that ZSTDDecompressor
already uses for multi-frame zstd streams. Add unused_data to
ZLibDecompressObjProtocol so the attribute is typed. Include three
tests mirroring the existing ZSTD multi-frame test suite.

Fixes #7157

Signed-off-by: Ashutosh Kumar Singh <ahutoshhjp1067@gmail.com>

Author: Ashutosh-177

CHANGES/7157.bugfix.rst

@@ -0,0 +1,6 @@
+Fixed :class:`~aiohttp.ZLibDecompressor` silently dropping data past the first
+member when decompressing concatenated gzip/deflate streams. Each subsequent
+member is now handed to a fresh decompressor, matching the behaviour already
+implemented for ZSTD multi-frame streams.
+
+-- by :user:`Ashutosh-177`

CONTRIBUTORS.txt

@@ -49,6 +49,7 @@ Andrew Top
 Andrew Zhou
 Andrii Soldatenko
 Anes Abismail
+Ashutosh Kumar Singh
 Antoine Pietri
 Anton Kasyanov
 Anton Zhdan-Pushkin

aiohttp/compression_utils.py

@@ -55,6 +55,9 @@ def eof(self) -> bool: ...
     @property
     def unconsumed_tail(self) -> bytes: ...
 
+    @property
+    def unused_data(self) -> bytes: ...
+
 
 class ZLibBackendProtocol(Protocol):
     MAX_WBITS: int
@@ -284,6 +287,24 @@ def decompress_sync(
         )
         # Only way to know that isal has no further data is checking we get no output
         self._last_empty = result == b""
+
+        # Handle concatenated gzip/deflate streams (multi-member).
+        # After a member ends, unused_data holds the start of the next member.
+        # Create a fresh decompressor for each subsequent member.
+        while self._decompressor.eof and self._decompressor.unused_data:
+            unused = self._decompressor.unused_data
+            self._decompressor = self._zlib_backend.decompressobj(wbits=self._mode)
+            remaining = (
+                max_length - len(result)
+                if max_length != ZLIB_MAX_LENGTH_UNLIMITED
+                else ZLIB_MAX_LENGTH_UNLIMITED
+            )
+            if max_length != ZLIB_MAX_LENGTH_UNLIMITED and remaining <= 0:
+                break
+            chunk = self._decompressor.decompress(unused, remaining)
+            self._last_empty = chunk == b""
+            result += chunk
+
         return result
 
     def flush(self, length: int = 0) -> bytes:

tests/test_compression_utils.py

@@ -1,5 +1,6 @@
 """Tests for compression utils."""
 
+import gzip
 import sys
 
 import pytest
@@ -87,3 +88,27 @@ def test_zstd_multi_frame_max_length_exhausted_preserves_unused_data() -> None:
     assert result1 == b"AAAA"
     result2 = d.decompress_sync(frame3)
     assert result2 == b"BBBBCCCC"
+
+
+def test_zlib_gzip_multi_member_unlimited() -> None:
+    d = ZLibDecompressor(encoding="gzip")
+    member1 = gzip.compress(b"AAAA")
+    member2 = gzip.compress(b"BBBB")
+    result = d.decompress_sync(member1 + member2)
+    assert result == b"AAAABBBB"
+
+
+def test_zlib_gzip_multi_member_max_length_partial() -> None:
+    d = ZLibDecompressor(encoding="gzip")
+    member1 = gzip.compress(b"AAAA")
+    member2 = gzip.compress(b"BBBB")
+    result = d.decompress_sync(member1 + member2, max_length=6)
+    assert result == b"AAAABB"
+
+
+def test_zlib_gzip_multi_member_max_length_exhausted() -> None:
+    d = ZLibDecompressor(encoding="gzip")
+    member1 = gzip.compress(b"AAAA")
+    member2 = gzip.compress(b"BBBB")
+    result = d.decompress_sync(member1 + member2, max_length=4)
+    assert result == b"AAAA"