isotpsoft: optimize for slow (slcan) interfaces: don't call select when the internal state will do

Author: BenGardiner

scapy/contrib/isotp/isotp_soft_socket.py

@@ -611,7 +611,14 @@ def can_recv(self):
                 log_isotp.warning("Error in can_recv: %s",
                                   traceback.format_exc())
         if not self.closed and not self.can_socket.closed:
-            if self.can_socket.select([self.can_socket], 0):
+            # Determine poll_time from ISOTP state only.
+            # Avoid calling select() here — on slow serial interfaces
+            # (slcan), each select() triggers a mux() call that reads
+            # N frames at ~2.5ms each, wasting time that could be spent
+            # processing frames already in the rx_queue.
+            if self.rx_state == ISOTP_WAIT_DATA or \
+                    self.tx_state == ISOTP_WAIT_FC or \
+                    self.tx_state == ISOTP_WAIT_FIRST_FC:
                 poll_time = 0.0
             else:
                 poll_time = self.rx_tx_poll_rate