PYTHON-5906 Raise ProtocolError for malformed OP_COMPRESSED messages in sync receive_message (#2937)

Author: aclark4life

pymongo/network_layer.py

@@ -775,6 +775,10 @@ def receive_message(
         )
     data: memoryview | bytes
     if op_code == 2012:
+        if length <= 25:
+            raise ProtocolError(
+                f"Message length ({length!r}) not longer than standard OP_COMPRESSED message header size (25)"
+            )
         op_code, _, compressor_id = _UNPACK_COMPRESSION_HEADER(receive_data(conn, 9, deadline))
         data = decompress(receive_data(conn, length - 25, deadline), compressor_id)
     else:

test/test_network_layer.py

@@ -80,6 +80,21 @@ def test_length_exceeds_max_raises(self):
         with self.assertRaisesRegex(ProtocolError, "larger than server max"):
             network_layer.receive_message(conn, request_id=None)
 
+    def test_compressed_length_too_small_raises(self):
+        # An OP_COMPRESSED frame with 16 < length <= 25 is malformed: it lacks
+        # room for the 9-byte compression sub-header plus payload. It must raise
+        # ProtocolError (not ValueError from a negative receive_data count).
+        for length in (17, 24, 25):
+            conn = _make_conn()
+            _mock_recv_into(
+                conn,
+                pack_msg_header(length=length, request_id=0, response_to=0, op_code=2012),
+            )
+            with self.assertRaisesRegex(
+                ProtocolError, "not longer than standard OP_COMPRESSED message header"
+            ):
+                network_layer.receive_message(conn, request_id=None)
+
     def test_unknown_opcode_raises(self):
         conn = _make_conn()
         _mock_recv_into(