BusABC.recv: keep calling _recv_internal until it returns None

Even if recv() is called with timeout=0, the caller's intention is
probably for recv() to check all of the messages that have already
arrived at the interface until one of them matches the filters.

This is already the way recv() behaves for interface drivers that take
advantage of hardware or OS-level filtering, but those that use BusABC's
default software-based filtering might return None even if a matching
message has already arrived.

can/bus.py

@@ -120,19 +120,17 @@ def recv(self, timeout: Optional[float] = None) -> Optional[Message]:
             # try to get a message
             msg, already_filtered = self._recv_internal(timeout=time_left)
 
-            # return it, if it matches
-            if msg and (already_filtered or self._matches_filters(msg)):
-                LOG.log(self.RECV_LOGGING_LEVEL, "Received: %s", msg)
-                return msg
-
-            # if not, and timeout is None, try indefinitely
-            elif timeout is None:
+            if msg:
+                # return it, if it matches
+                if already_filtered or self._matches_filters(msg):
+                    LOG.log(self.RECV_LOGGING_LEVEL, "Received: %s", msg)
+                    return msg
+                # if not, keep trying
                 continue
 
-            # try next one only if there still is time, and with
-            # reduced timeout
-            else:
-                time_left = timeout - (time() - start)
+            # try again only if there still is time, and with reduced timeout
+            if timeout is not None:
+                time_left = max(0, timeout - (time() - start))
 
                 if time_left > 0:
                     continue

test/test_message_filtering.py

@@ -46,6 +46,18 @@ def test_match_example_message(self):
         self.assertFalse(self.bus._matches_filters(EXAMPLE_MSG))
         self.assertTrue(self.bus._matches_filters(HIGHEST_MSG))
 
+    def test_empty_queue_up_to_match(self):
+        self.bus.set_filters(MATCH_EXAMPLE)
+        bus2 = Bus(interface="virtual", channel="testy")
+        bus2.send(HIGHEST_MSG)
+        bus2.send(EXAMPLE_MSG)
+        actual = self.bus.recv(timeout=0)
+        self.assertTrue(
+            EXAMPLE_MSG.equals(
+                actual, timestamp_delta=None, check_direction=False, check_channel=False
+            )
+        )
+
 
 if __name__ == "__main__":
     unittest.main()