Prevent invalid window bit sizes

This is specifically to deal with the case whereby the window size is
set to 8. As zlib dropped support for 256 (8 bit) window sizes in
1.2.9, https://www.zlib.net/ChangeLog.txt, onwards. 8 bit windows were
broken before as well, madler/zlib#171. As
autobahn tests 8 bit windows the tox test would fail based on the zlib
version installed - hence the relevant autobahn tests are also
disabled.

8 bit windows are rarely used in practice, so this should be ok
practically. If not zlib needs to be fixed, see
madler/zlib#87 (comment)

compliance/run-autobahn-tests.py

@@ -25,7 +25,7 @@
         }
     ],
     "cases": ["*"],
-    "exclude-cases": [],
+    "exclude-cases": ["13.3.*", "13.5.*", "13.7.*"],
     "exclude-agent-cases": {},
 }
 
@@ -35,7 +35,7 @@
     "outdir": "./reports/clients",
     "webport": 8080,
     "cases": ["*"],
-    "exclude-cases": [],
+    "exclude-cases": ["13.3.*", "13.5.*", "13.7.*"],
     "exclude-agent-cases": {},
 }
 

src/wsproto/extensions.py

@@ -71,13 +71,13 @@ def __init__(
         server_max_window_bits: Optional[int] = None,
     ) -> None:
         self.client_no_context_takeover = client_no_context_takeover
-        if client_max_window_bits is None:
-            client_max_window_bits = self.DEFAULT_CLIENT_MAX_WINDOW_BITS
-        self.client_max_window_bits = client_max_window_bits
         self.server_no_context_takeover = server_no_context_takeover
-        if server_max_window_bits is None:
-            server_max_window_bits = self.DEFAULT_SERVER_MAX_WINDOW_BITS
-        self.server_max_window_bits = server_max_window_bits
+        self._client_max_window_bits = self.DEFAULT_CLIENT_MAX_WINDOW_BITS
+        self._server_max_window_bits = self.DEFAULT_SERVER_MAX_WINDOW_BITS
+        if client_max_window_bits is not None:
+            self.client_max_window_bits = client_max_window_bits
+        if server_max_window_bits is not None:
+            self.server_max_window_bits = server_max_window_bits
 
         self._compressor: Optional[zlib._Compress] = None  # noqa
         self._decompressor: Optional[zlib._Decompress] = None  # noqa
@@ -90,6 +90,26 @@ def __init__(
 
         self._enabled = False
 
+    @property
+    def client_max_window_bits(self) -> int:
+        return self._client_max_window_bits
+
+    @client_max_window_bits.setter
+    def client_max_window_bits(self, value: int) -> None:
+        if value < 9 or value > 15:
+            raise ValueError("Window size must be between 9 and 15 inclusive")
+        self._client_max_window_bits = value
+
+    @property
+    def server_max_window_bits(self) -> int:
+        return self._server_max_window_bits
+
+    @server_max_window_bits.setter
+    def server_max_window_bits(self, value: int) -> None:
+        if value < 9 or value > 15:
+            raise ValueError("Window size must be between 9 and 15 inclusive")
+        self._server_max_window_bits = value
+
     def _compressible_opcode(self, opcode: Opcode) -> bool:
         return opcode in (Opcode.TEXT, Opcode.BINARY, Opcode.CONTINUATION)
 
@@ -146,25 +166,27 @@ def _parse_params(self, params: str) -> Tuple[Optional[int], Optional[int]]:
 
         return client_max_window_bits, server_max_window_bits
 
-    def accept(self, offer: str) -> Union[bool, str]:
+    def accept(self, offer: str) -> Union[bool, None, str]:
         client_max_window_bits, server_max_window_bits = self._parse_params(offer)
 
-        self._enabled = True
-
         parameters = []
 
         if self.client_no_context_takeover:
             parameters.append("client_no_context_takeover")
-        if client_max_window_bits is not None:
-            parameters.append("client_max_window_bits=%d" % client_max_window_bits)
-            self.client_max_window_bits = client_max_window_bits
         if self.server_no_context_takeover:
             parameters.append("server_no_context_takeover")
-        if server_max_window_bits is not None:
-            parameters.append("server_max_window_bits=%d" % server_max_window_bits)
-            self.server_max_window_bits = server_max_window_bits
-
-        return "; ".join(parameters)
+        try:
+            if client_max_window_bits is not None:
+                parameters.append("client_max_window_bits=%d" % client_max_window_bits)
+                self.client_max_window_bits = client_max_window_bits
+            if server_max_window_bits is not None:
+                parameters.append("server_max_window_bits=%d" % server_max_window_bits)
+                self.server_max_window_bits = server_max_window_bits
+        except ValueError:
+            return None
+        else:
+            self._enabled = True
+            return "; ".join(parameters)
 
     def frame_inbound_header(
         self,

test/test_permessage_deflate.py

@@ -45,12 +45,12 @@ class TestPerMessageDeflate:
         },
         {
             "client_no_context_takeover": True,
-            "client_max_window_bits": 8,
+            "client_max_window_bits": 9,
             "server_no_context_takeover": True,
             "server_max_window_bits": 9,
         },
         {"client_no_context_takeover": True, "server_max_window_bits": 9},
-        {"server_no_context_takeover": True, "client_max_window_bits": 8},
+        {"server_no_context_takeover": True, "client_max_window_bits": 9},
         {"client_max_window_bits": None, "server_max_window_bits": None},
         {},
     ]