make test: work around scapy truncated packets

Under stress, it's possible to hit a race condition, when the packet
header is fully written to pcap, but not all packet data - yet.
Scapy is stupid enough to:
1. not detect and report this error, truncating the packet instead
2. continue munching more data from wrong offset
The work around is to scan the file ahead, parse the packet header,
figure out how much data we need, wait for the file to be big
enough, then restore the file position back to where it was
and finally let scapy parse the packet.

Change-Id: I9fc71d3ebdc62ecab6c90b90f177d0eaeb09b8bb
Signed-off-by: Klement Sekera <ksekera@cisco.com>

test/framework.py

@@ -8,6 +8,7 @@ import resource
 from collections import deque
 from threading import Thread
 from inspect import getdoc
+from traceback import format_exception
 from hook import StepHook, PollHook
 from vpp_pg_interface import VppPGInterface
 from vpp_sub_interface import VppSubInterface
@@ -287,6 +288,9 @@ class VppTestCase(unittest.TestCase):
 
     def tearDown(self):
         """ Show various debug prints after each test """
+        self.logger.debug("--- tearDown() for %s.%s(%s) called ---" %
+                          (self.__class__.__name__, self._testMethodName,
+                           self._testMethodDoc))
         if not self.vpp_dead:
             self.logger.debug(self.vapi.cli("show trace"))
             self.logger.info(self.vapi.ppcli("show int"))
@@ -297,6 +301,9 @@ class VppTestCase(unittest.TestCase):
 
     def setUp(self):
         """ Clear trace before running each test"""
+        self.logger.debug("--- setUp() for %s.%s(%s) called ---" %
+                          (self.__class__.__name__, self._testMethodName,
+                           self._testMethodDoc))
         if self.vpp_dead:
             raise Exception("VPP is dead when setting up the test")
         time.sleep(.1)
@@ -586,6 +593,11 @@ class VppTestResult(unittest.TestResult):
         :param test:
 
         """
+        if hasattr(test, 'logger'):
+            test.logger.debug("--- addSuccess() %s.%s(%s) called"
+                              % (test.__class__.__name__,
+                                 test._testMethodName,
+                                 test._testMethodDoc))
         unittest.TestResult.addSuccess(self, test)
         self.result_string = colorize("OK", GREEN)
 
@@ -597,6 +609,12 @@ class VppTestResult(unittest.TestResult):
         :param reason:
 
         """
+        if hasattr(test, 'logger'):
+            test.logger.debug("--- addSkip() %s.%s(%s) called, reason is %s"
+                              % (test.__class__.__name__,
+                                 test._testMethodName,
+                                 test._testMethodDoc,
+                                 reason))
         unittest.TestResult.addSkip(self, test, reason)
         self.result_string = colorize("SKIP", YELLOW)
 
@@ -608,6 +626,13 @@ class VppTestResult(unittest.TestResult):
         :param err: error message
 
         """
+        if hasattr(test, 'logger'):
+            test.logger.debug("--- addFailure() %s.%s(%s) called, err is %s"
+                              % (test.__class__.__name__,
+                                 test._testMethodName,
+                                 test._testMethodDoc, err))
+            test.logger.debug("formatted exception is:\n%s" %
+                              "".join(format_exception(*err)))
         unittest.TestResult.addFailure(self, test, err)
         if hasattr(test, 'tempdir'):
             self.result_string = colorize("FAIL", RED) + \
@@ -623,6 +648,13 @@ class VppTestResult(unittest.TestResult):
         :param err: error message
 
         """
+        if hasattr(test, 'logger'):
+            test.logger.debug("--- addError() %s.%s(%s) called, err is %s"
+                              % (test.__class__.__name__,
+                                 test._testMethodName,
+                                 test._testMethodDoc, err))
+            test.logger.debug("formatted exception is:\n%s" %
+                              "".join(format_exception(*err)))
         unittest.TestResult.addError(self, test, err)
         if hasattr(test, 'tempdir'):
             self.result_string = colorize("ERROR", RED) + \

test/vpp_pg_interface.py

@@ -1,6 +1,7 @@
 import os
 import time
 import socket
+import struct
 from traceback import format_exc, format_stack
 from scapy.utils import wrpcap, rdpcap, PcapReader
 from scapy.plist import PacketList
@@ -283,6 +284,37 @@ class VppPGInterface(VppInterface):
             return False
         return True
 
+    def wait_for_packet_data(self, deadline):
+        """
+        Wait until enough data is available in the file handled by internal
+        pcap reader so that a whole packet can be read.
+
+        :param deadline: timestamp by which the data must arrive
+        :raises Exception: if not enough data by deadline
+        """
+        orig_pos = self._pcap_reader.f.tell()  # save file position
+        enough_data = False
+        while time.time() < deadline:
+            # read packet header from pcap
+            hdr = self._pcap_reader.f.read(16)
+            if len(hdr) < 16:
+                time.sleep(0)  # yield
+                continue  # cannot read full header, continue looping
+            # find the capture length - caplen
+            sec, usec, caplen, wirelen = struct.unpack(
+                self._pcap_reader.endian + "IIII", hdr)
+            self._pcap_reader.f.seek(0, 2)  # seek to end of file
+            end_pos = self._pcap_reader.f.tell()  # get position at end
+            if end_pos >= orig_pos + len(hdr) + caplen:
+                enough_data = True  # yay, we have enough data
+                break
+            self.test.logger.debug("Partial packet data in pcap")
+            time.sleep(0)  # yield
+        self._pcap_reader.f.seek(orig_pos, 0)  # restore original position
+        if not enough_data:
+            raise Exception(
+                "Not enough data to read a full packet within deadline")
+
     def wait_for_packet(self, timeout, filter_out_fn=is_ipv6_misc):
         """
         Wait for next packet captured with a timeout
@@ -311,6 +343,7 @@ class VppPGInterface(VppInterface):
 
         self.test.logger.debug("Waiting for packet")
         while time.time() < deadline:
+            self.wait_for_packet_data(deadline)
             p = self._pcap_reader.recv()
             if p is not None:
                 if filter_out_fn is not None and filter_out_fn(p):