[scripts] add live capture for sniffer.py on simulation traffic (#5327)

This commit enhances sniffer.py so that we can use it as a standalone
sniffer for non Virtual Time simulations.

tests/scripts/thread-cert/pcap.py

@@ -80,6 +80,7 @@ class PcapCodec(object):
         pkt = self.encode_frame(frame, *timestamp)
         self._pcap_file.write(pkt)
         self._pcap_file.flush()
+        return pkt
 
     def __del__(self):
         self._pcap_file.close()

tests/scripts/thread-cert/sniffer.py

@@ -31,6 +31,9 @@ import collections
 import io
 import logging
 import os
+import sys
+import time
+
 import pcap
 import threading
 import traceback
@@ -62,6 +65,9 @@ class Sniffer:
         self._message_factory = message_factory
 
         self._pcap = pcap.PcapCodec(os.getenv('TEST_NAME', 'current'))
+        if __name__ == '__main__':
+            sys.stdout.buffer.write(self._pcap.encode_header())
+            sys.stdout.buffer.flush()
 
         # Create transport
         transport_factory = sniffer_transport.SnifferTransportFactory()
@@ -81,19 +87,27 @@ class Sniffer:
         while self._thread_alive.is_set():
             data, nodeid = self._transport.recv(self.RECV_BUFFER_SIZE)
 
-            self._pcap.append(data)
+            pkt = self._pcap.append(data)
+            if __name__ == '__main__':
+                try:
+                    sys.stdout.buffer.write(pkt)
+                    sys.stdout.flush()
+                except BrokenPipeError:
+                    self._thread_alive.clear()
+                    break
 
             # Ignore any exceptions
-            try:
-                messages = self._message_factory.create(io.BytesIO(data))
-                self.logger.debug("Received messages: {}".format(messages))
-                for msg in messages:
-                    self._buckets[nodeid].put(msg)
+            if self._message_factory is not None:
+                try:
+                    messages = self._message_factory.create(io.BytesIO(data))
+                    self.logger.debug("Received messages: {}".format(messages))
+                    for msg in messages:
+                        self._buckets[nodeid].put(msg)
 
-            except Exception as e:
-                # Just print the exception to the console
-                print("EXCEPTION: %s" % e)
-                traceback.print_exc()
+                except Exception as e:
+                    # Just print the exception to the console
+                    self.logger.error("EXCEPTION: %s" % e)
+                    traceback.print_exc()
 
         self.logger.debug("Sniffer stopped.")
 
@@ -115,7 +129,7 @@ class Sniffer:
 
         self._transport.close()
 
-        self._thread.join()
+        self._thread.join(timeout=1)
         self._thread = None
 
     def set_lowpan_context(self, cid, prefix):
@@ -140,3 +154,19 @@ class Sniffer:
             messages.append(bucket.get_nowait())
 
         return message.MessagesSet(messages)
+
+
+def run_sniffer():
+    sniffer = Sniffer(None)
+    sniffer.start()
+    while sniffer._thread_alive.is_set():
+        try:
+            time.sleep(1)
+        except KeyboardInterrupt:
+            break
+
+    sniffer.stop()
+
+
+if __name__ == '__main__':
+    run_sniffer()