Fix a multithreading issue in writing pcap files (#204)

When there are separate threads that are sending and receiving packets,
both threads may try to write the packet data to the pcap file. This
causes the packet to get interleaved, thus basically making it corrupted
and unable to be read by tcpdump, wireshark, etc.

This is happening due to a missing lock on `self.cvar`. Fix it by
wrapping the write call in the `send` method and the initial assignment
in `start_pcap` method with a lock on `self.cvar`.

Signed-off-by: Saikrishna Arcot <[email protected]>

src/ptf/dataplane.py

@@ -727,8 +727,9 @@ def send(self, device_number, port_number, packet):
  self.logger.warn(
  "The %s kernel may not send packets smaller than 15 bytes", sys.platform
  )
- if self.pcap_writer:
- self.pcap_writer.write(packet, time.time(), device_number, port_number)
+ with self.cvar:
+ if self.pcap_writer:
+ self.pcap_writer.write(packet, time.time(), device_number, port_number)
  bytes = self.ports[(device_number, port_number)].send(packet)
  self.tx_counters[(device_number, port_number)] += 1
  if bytes != len(packet):
@@ -1020,8 +1021,9 @@ def flush(self):
  self.packet_queues[port_id] = []
 
  def start_pcap(self, filename):
- assert self.pcap_writer == None
- self.pcap_writer = PcapWriter(filename)
+ with self.cvar:
+ assert self.pcap_writer == None
+ self.pcap_writer = PcapWriter(filename)
 
  def stop_pcap(self):
  if self.pcap_writer: