transit: fix race, file-xfer basically works, but noisy

The failed connections are throwing exceptions that should be caught and
ignored.

bin/receive_file.py

@@ -1,5 +1,5 @@
 from __future__ import print_function
-import os, json
+import sys, os, json
 from binascii import unhexlify
 from nacl.secret import SecretBox
 from wormhole.blocking.transcribe import Receiver
@@ -26,6 +26,7 @@ file_data = data["file"]
 xfer_key = unhexlify(file_data["key"].encode("ascii"))
 filename = os.path.basename(file_data["filename"]) # unicode
 filesize = file_data["filesize"]
+encrypted_filesize = filesize + SecretBox.NONCE_SIZE+16
 
 # now receive the rest of the owl
 tdata = data["transit"]
@@ -34,7 +35,10 @@ transit_receiver.set_transit_key(tdata["key"])
 transit_receiver.add_sender_direct_hints(tdata["direct_connection_hints"])
 transit_receiver.add_sender_relay_hints(tdata["relay_connection_hints"])
 skt = transit_receiver.establish_connection()
-encrypted = skt.recv(filesize)
+encrypted = skt.recv(encrypted_filesize)
+if len(encrypted) != encrypted_filesize:
+    print("Connection dropped before file received")
+    sys.exit(1)
 
 decrypted = SecretBox(xfer_key).decrypt(encrypted)
 

bin/send_file.py

@@ -47,7 +47,7 @@ encrypted = box.encrypt(plaintext, nonce)
 tdata = them_d["transit"]
 transit_sender.add_receiver_hints(tdata["direct_connection_hints"])
 skt = transit_sender.establish_connection()
-skt.write(encrypted)
+skt.send(encrypted)
 skt.close()
 
 print("file sent")

src/wormhole/blocking/transit.py

@@ -100,17 +100,17 @@ class TransitSender:
 
         with self._negotiation_check_lock:
             if self.winning_skt:
-                winner = False
+                is_winner = False
             else:
-                winner = True
+                is_winner = True
                 self.winning_skt = skt
 
-        if winner:
-            self.winning_skt.send("go\n")
+        if is_winner:
+            skt.send("go\n")
             self.winning.set()
         else:
-            winner.send("nevermind\n")
-            winner.close()
+            skt.send("nevermind\n")
+            skt.close()
 
 class BadHandshake(Exception):
     pass
@@ -168,9 +168,19 @@ def handle(skt, client_address, owner, send_handshake, expected_handshake):
 
 class MyTCPServer(SocketServer.TCPServer):
     allow_reuse_address = True
+
     def process_request(self, request, client_address):
-        if not self.owner.key:
-            raise BadHandshake("connection received before set_transit_key()")
+        kc = self.owner._have_transit_key
+        kc.acquire()
+        while not self.owner._transit_key:
+            kc.wait()
+        # owner._transit_key is either None or set to a value. We don't
+        # modify it from here, so we can release the condition lock before
+        # grabbing the key.
+        kc.release()
+
+        # Once it is set, we can get handler_(send|receive)_handshake, which
+        # is what we actually care about.
         t = threading.Thread(target=handle,
                              args=(request, client_address,
                                    self.owner,
@@ -183,7 +193,8 @@ class TransitReceiver:
     def __init__(self):
         self.winning = threading.Event()
         self._negotiation_check_lock = threading.Lock()
-        self.key = None
+        self._have_transit_key = threading.Condition()
+        self._transit_key = None
         server = MyTCPServer(("",9999), None)
         _, port = server.server_address
         self.my_direct_hints = ["%s,%d" % (addr, port)
@@ -196,12 +207,18 @@ class TransitReceiver:
 
     def get_direct_hints(self):
         return self.my_direct_hints
+
     def set_transit_key(self, key):
-        # TODO consider race: sender knows the hints and the key, connects to
-        # transit before receiver gets relay message (with key)
-        self.key = key
+        # This _have_transit_key condition/lock protects us against the race
+        # where the sender knows the hints and the key, and connects to the
+        # receiver's transit socket before the receiver gets relay message
+        # (and thus the key).
+        self._have_transit_key.acquire()
+        self._transit_key = key
         self.handler_send_handshake = build_receiver_handshake(key)
         self.handler_expected_handshake = build_sender_handshake(key) + "go\n"
+        self._have_transit_key.notify_all()
+        self._have_transit_key.release()
 
     def add_sender_direct_hints(self, hints):
         self.sender_direct_hints = hints # TODO ignored