Add Message Capture Test

Add a functional test for CaptureMessage.  This connects and then
disconnects a peer so that the handshake can be used to check if capture
is being done correctly.

Included in a docstring in the test is the following:

From the data file we'll only check the structure.

We won't care about things like:
- Deserializing the payload of the message
    - This is managed by the deserialize methods in
      test_framework.messages
- The order of the messages
    - There's no reason why we can't, say, change the order of the
      messages in the handshake
- Message Type
    - We can add new message types

We're ignoring these because they're simply too brittle to test here.

test/functional/p2p_message_capture.py

@@ -0,0 +1,76 @@
+#!/usr/bin/env python3
+# Copyright (c) 2020 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Test per-peer message capture capability.
+
+Additionally, the output of contrib/message-capture/message-capture-parser.py should be verified manually.
+"""
+
+import glob
+from io import BytesIO
+import os
+
+from test_framework.p2p import P2PDataStore, MESSAGEMAP
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import assert_equal
+
+TIME_SIZE = 8
+LENGTH_SIZE = 4
+MSGTYPE_SIZE = 12
+
+def mini_parser(dat_file):
+    """Parse a data file created by CaptureMessage.
+
+    From the data file we'll only check the structure.
+
+    We won't care about things like:
+    - Deserializing the payload of the message
+        - This is managed by the deserialize methods in test_framework.messages
+    - The order of the messages
+        - There's no reason why we can't, say, change the order of the messages in the handshake
+    - Message Type
+        - We can add new message types
+
+    We're ignoring these because they're simply too brittle to test here.
+    """
+    with open(dat_file, 'rb') as f_in:
+        # This should have at least one message in it
+        assert(os.fstat(f_in.fileno()).st_size >= TIME_SIZE + LENGTH_SIZE + MSGTYPE_SIZE)
+        while True:
+            tmp_header_raw = f_in.read(TIME_SIZE + LENGTH_SIZE + MSGTYPE_SIZE)
+            if not tmp_header_raw:
+                break
+            tmp_header = BytesIO(tmp_header_raw)
+            int.from_bytes(tmp_header.read(TIME_SIZE), "little")      # type: int
+            raw_msgtype = tmp_header.read(MSGTYPE_SIZE)
+            msgtype = raw_msgtype.split(b'\x00', 1)[0]     # type: bytes
+            remainder =  raw_msgtype.split(b'\x00', 1)[1]
+            assert(len(msgtype) > 0)
+            assert(msgtype in MESSAGEMAP)
+            assert(len(remainder) == 0 or not remainder.decode().isprintable())
+            length = int.from_bytes(tmp_header.read(LENGTH_SIZE), "little")  # type: int
+            data = f_in.read(length)
+            assert_equal(len(data), length)
+
+
+
+class MessageCaptureTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.num_nodes = 1
+        self.extra_args = [["-capturemessages"]]
+        self.setup_clean_chain = True
+
+    def run_test(self):
+        capturedir = os.path.join(self.nodes[0].datadir, "regtest/message_capture")
+        # Connect a node so that the handshake occurs
+        self.nodes[0].add_p2p_connection(P2PDataStore())
+        self.nodes[0].disconnect_p2ps()
+        recv_file = glob.glob(os.path.join(capturedir, "*/msgs_recv.dat"))[0]
+        mini_parser(recv_file)
+        sent_file = glob.glob(os.path.join(capturedir, "*/msgs_sent.dat"))[0]
+        mini_parser(sent_file)
+
+
+if __name__ == '__main__':
+    MessageCaptureTest().main()

test/functional/test_runner.py

@@ -264,6 +264,7 @@ BASE_SCRIPTS = [
     'p2p_add_connections.py',
     'p2p_unrequested_blocks.py',
     'p2p_blockfilters.py',
+    'p2p_message_capture.py',
     'feature_includeconf.py',
     'feature_asmap.py',
     'mempool_unbroadcast.py',