Clarify behavior of recv(timeout=0) behavior.

Refs #1552.

src/websockets/sync/connection.py

@@ -232,9 +232,10 @@ def recv(self, timeout: float | None = None, decode: bool | None = None) -> Data
         message stream.
 
         If ``timeout`` is :obj:`None`, block until a message is received. If
-        ``timeout`` is set and no message is received within ``timeout``
-        seconds, raise :exc:`TimeoutError`. Set ``timeout`` to ``0`` to check if
-        a message was already received.
+        ``timeout`` is set, wait up to ``timeout`` seconds for a message to be
+        received and return it, else raise :exc:`TimeoutError`. If ``timeout``
+        is ``0`` or negative, check if a message has been received already and
+        return it, else raise :exc:`TimeoutError`.
 
         If the message is fragmented, wait until all fragments are received,
         reassemble them, and return the whole message.

tests/sync/test_messages.py

@@ -198,12 +198,32 @@ def test_get_timeout_after_first_frame(self):
         message = self.assembler.get()
         self.assertEqual(message, "café")
 
-    def test_get_if_received(self):
-        """get returns a text message if it's already received."""
+    def test_get_timeout_0_message_already_received(self):
+        """get(timeout=0) returns a message that is already received."""
         self.assembler.put(Frame(OP_TEXT, b"caf\xc3\xa9"))
         message = self.assembler.get(timeout=0)
         self.assertEqual(message, "café")
 
+    def test_get_timeout_0_message_not_received_yet(self):
+        """get(timeout=0) times out when no message is already received."""
+        with self.assertRaises(TimeoutError):
+            self.assembler.get(timeout=0)
+
+    def test_get_timeout_0_fragmented_message_already_received(self):
+        """get(timeout=0) returns a fragmented message that is already received."""
+        self.assembler.put(Frame(OP_TEXT, b"ca", fin=False))
+        self.assembler.put(Frame(OP_CONT, b"f\xc3", fin=False))
+        self.assembler.put(Frame(OP_CONT, b"\xa9"))
+        message = self.assembler.get(timeout=0)
+        self.assertEqual(message, "café")
+
+    def test_get_timeout_0_fragmented_message_partially_received(self):
+        """get(timeout=0) times out when a fragmented message is partially received."""
+        self.assembler.put(Frame(OP_TEXT, b"ca", fin=False))
+        self.assembler.put(Frame(OP_CONT, b"f\xc3", fin=False))
+        with self.assertRaises(TimeoutError):
+            self.assembler.get(timeout=0)
+
     # Test get_iter
 
     def test_get_iter_text_message_already_received(self):