new Wormhole API: separate send_data()/get_data() calls

docs/api.md

@@ -45,6 +45,16 @@ Transit class currently distinguishes "Sender" from "Receiver", so the
 programs on each side must have some way to decide (ahead of time) which is
 which.
 
+Each side gets to do one `send_data()` call and one `get_data()` call.
+`get_data` will wait until the other side has done `send_data`, so the
+application developer must be careful to avoid deadlocks (don't get before
+you send on both sides in the same protocol). When both sides are done, they
+must call `close()`, to let the library know that the connection is complete
+and it can deallocate the channel. If you forget to call `close()`, the
+server will not free the channel, and other users will suffer longer
+invitation codes as a result. To encourage `close()`, the library will log an
+error if a Wormhole object is destroyed before being closed.
+
 ## Examples
 
 The synchronous+blocking flow looks like this:
@@ -56,7 +66,9 @@ mydata = b"initiator's data"
 i = Wormhole(b"appid", RENDEZVOUS_RELAY)
 code = i.get_code()
 print("Invitation Code: %s" % code)
-theirdata = i.get_data(mydata)
+i.send_data(mydata)
+theirdata = i.get_data()
+i.close()
 print("Their data: %s" % theirdata.decode("ascii"))
 ```
 
@@ -68,7 +80,9 @@ mydata = b"receiver's data"
 code = sys.argv[1]
 r = Wormhole(b"appid", RENDEZVOUS_RELAY)
 r.set_code(code)
-theirdata = r.get_data(mydata)
+r.send_data(mydata)
+theirdata = r.get_data()
+r.close()
 print("Their data: %s" % theirdata.decode("ascii"))
 ```
 
@@ -85,11 +99,13 @@ w1 = Wormhole(b"appid", RENDEZVOUS_RELAY)
 d = w1.get_code()
 def _got_code(code):
     print "Invitation Code:", code
-    return w1.get_data(outbound_message)
+    return w1.send_data(outbound_message)
 d.addCallback(_got_code)
+d.addCallback(lambda _: w1.get_data())
 def _got_data(inbound_message):
     print "Inbound message:", inbound_message
 d.addCallback(_got_data)
+d.addCallback(w1.close)
 d.addBoth(lambda _: reactor.stop())
 reactor.run()
 ```
@@ -99,17 +115,26 @@ On the other side, you call `set_code()` instead of waiting for `get_code()`:
 ```python
 w2 = Wormhole(b"appid", RENDEZVOUS_RELAY)
 w2.set_code(code)
-d = w2.get_data(my_message)
+d = w2.send_data(my_message)
 ...
 ```
 
-You can call `d=w.get_verifier()` before `get_data()`: this will perform the
+Note that the Twisted-form `close()` accepts (and returns) an optional
-first half of the PAKE negotiation, then fire the Deferred with a verifier
+argument, so you can use `d.addCallback(w.close)` instead of
-object (bytes) which can be converted into a printable representation and
+`d.addCallback(lambda _: w.close())`.
-manually compared. When the users are convinced that `get_verifier()` from
+
-both sides are the same, call `d=get_data()` to continue the transfer. If you
+## Verifier
-call `get_data()` first, it will perform the complete transfer without
+
-pausing.
+You can call `w.get_verifier()` before `send_data()/get_data()`: this will
+perform the first half of the PAKE negotiation, then return a verifier object
+(bytes) which can be converted into a printable representation and manually
+compared. When the users are convinced that `get_verifier()` from both sides
+are the same, call `send_data()/get_data()` to continue the transfer. If you
+call `send_data()/get_data()` before `get_verifier()`, it will perform the
+complete transfer without pausing.
+
+The Twisted form of `get_verifier()` returns a Deferred that fires with the
+verifier bytes.
 
 ## Generating the Invitation Code
 
@@ -204,9 +229,10 @@ Wormhole.from_serialized(data)`).
 
 There is exactly one point at which you can serialize the wormhole: *after*
 establishing the invitation code, but before waiting for `get_verifier()` or
-`get_data()`. If you are creating a new code, the correct time is during the
+`get_data()`, or calling `send_data()`. If you are creating a new invitation
-callback fired by `get_code()`. If you are accepting a pre-generated code,
+code, the correct time is during the callback fired by `get_code()`. If you
-the time is just after calling `set_code()`.
+are accepting a pre-generated code, the time is just after calling
+`set_code()`.
 
 To properly checkpoint the process, you should store the first message
 (returned by `start()`) next to the serialized wormhole instance, so you can

src/wormhole/blocking/transcribe.py

@@ -139,6 +139,9 @@ class Wormhole:
         self.code = None
         self.key = None
         self.verifier = None
+        self._sent_data = False
+        self._got_data = False
+        self._closed = False
 
     def handle_welcome(self, welcome):
         if ("motd" in welcome and
@@ -232,32 +235,37 @@ class Wormhole:
         self._get_key()
         return self.verifier
 
-    def get_data(self, outbound_data):
+    def send_data(self, outbound_data):
-        # only call this once
+        if self._sent_data: raise UsageError # only call this once
         if not isinstance(outbound_data, type(b"")): raise UsageError
         if self.code is None: raise UsageError
         if self.channel is None: raise UsageError
-        try:
-            self._get_key()
-            return self._get_data2(outbound_data)
-        finally:
-            self.channel.deallocate()
-
-    def _get_data2(self, outbound_data):
         # Without predefined roles, we can't derive predictably unique keys
         # for each side, so we use the same key for both. We use random
-        # nonces to keep the messages distinct, and check for reflection.
+        # nonces to keep the messages distinct, and the Channel automatically
+        # ignores reflections.
+        self._get_key()
         data_key = self.derive_key(b"data-key")
-
         outbound_encrypted = self._encrypt_data(data_key, outbound_data)
         self.channel.send(u"data", outbound_encrypted)
 
+    def get_data(self):
+        if self._got_data: raise UsageError # only call this once
+        if self.code is None: raise UsageError
+        if self.channel is None: raise UsageError
+        self._get_key()
+        data_key = self.derive_key(b"data-key")
         inbound_encrypted = self.channel.get(u"data")
-        # _find_inbound_message() ignores any inbound message that matches
-        # something we previously sent out, so we don't need to explicitly
-        # check for reflection. A reflection attack will just not progress.
         try:
             inbound_data = self._decrypt_data(data_key, inbound_encrypted)
             return inbound_data
         except CryptoError:
             raise WrongPasswordError
+
+    def close(self):
+        self.channel.deallocate()
+        self._closed = True
+
+    def __del__(self):
+        if not self._closed:
+            print("Error: a Wormhole instance was not closed", file=sys.stderr)

src/wormhole/scripts/cmd_receive_file.py

@@ -33,12 +33,14 @@ def receive_file(args):
             "relay_connection_hints": transit_receiver.get_relay_hints(),
             },
         }).encode("utf-8")
+    w.send_data(mydata)
     try:
-        data = json.loads(w.get_data(mydata).decode("utf-8"))
+        data = json.loads(w.get_data().decode("utf-8"))
     except WrongPasswordError as e:
         print("ERROR: " + e.explain(), file=sys.stderr)
         return 1
     #print("their data: %r" % (data,))
+    w.close()
 
     if "error" in data:
         print("ERROR: " + data["error"], file=sys.stderr)

src/wormhole/scripts/cmd_receive_text.py

@@ -24,11 +24,13 @@ def receive_text(args):
         print("Verifier %s." % verifier)
 
     data = json.dumps({"message": "ok"}).encode("utf-8")
+    w.send_data(data)
     try:
-        them_bytes = w.get_data(data)
+        them_bytes = w.get_data()
     except WrongPasswordError as e:
         print("ERROR: " + e.explain(), file=sys.stderr)
         return 1
+    w.close()
     them_d = json.loads(them_bytes.decode("utf-8"))
     if "error" in them_d:
         print("ERROR: " + them_d["error"], file=sys.stderr)

src/wormhole/scripts/cmd_send_file.py

@@ -45,7 +45,8 @@ def send_file(args):
                       file=sys.stderr)
                 reject_data = json.dumps({"error": "verification rejected",
                                           }).encode("utf-8")
-                w.get_data(reject_data)
+                w.send_data(reject_data)
+                w.close()
                 return 1
 
     filesize = os.stat(filename).st_size
@@ -59,12 +60,13 @@ def send_file(args):
             "relay_connection_hints": transit_sender.get_relay_hints(),
             },
         }).encode("utf-8")
-
+    w.send_data(data)
     try:
-        them_bytes = w.get_data(data)
+        them_bytes = w.get_data()
     except WrongPasswordError as e:
         print("ERROR: " + e.explain(), file=sys.stderr)
         return 1
+    w.close()
     them_d = json.loads(them_bytes.decode("utf-8"))
     #print("them: %r" % (them_d,))
 

src/wormhole/scripts/cmd_send_text.py

@@ -39,17 +39,20 @@ def send_text(args):
                       file=sys.stderr)
                 reject_data = json.dumps({"error": "verification rejected",
                                           }).encode("utf-8")
-                w.get_data(reject_data)
+                w.send_data(reject_data)
+                w.close()
                 return 1
 
     message = args.text
     data = json.dumps({"message": message,
                        }).encode("utf-8")
+    w.send_data(data)
     try:
-        them_bytes = w.get_data(data)
+        them_bytes = w.get_data()
     except WrongPasswordError as e:
         print("ERROR: " + e.explain(), file=sys.stderr)
         return 1
+    w.close()
     them_d = json.loads(them_bytes.decode("utf-8"))
     if them_d["message"] == "ok":
         print("text sent")

src/wormhole/test/test_blocking.py

@@ -9,6 +9,14 @@ class Blocking(ServerBase, unittest.TestCase):
     # we need Twisted to run the server, but we run the sender and receiver
     # with deferToThread()
 
+    def doBoth(self, call1, call2):
+        f1 = call1[0]
+        f1args = call1[1:]
+        f2 = call2[0]
+        f2args = call2[1:]
+        return gatherResults([deferToThread(f1, *f1args),
+                              deferToThread(f2, *f2args)], True)
+
     def test_basic(self):
         appid = b"appid"
         w1 = BlockingWormhole(appid, self.relayurl)
@@ -16,13 +24,39 @@ class Blocking(ServerBase, unittest.TestCase):
         d = deferToThread(w1.get_code)
         def _got_code(code):
             w2.set_code(code)
-            return gatherResults([deferToThread(w1.get_data, b"data1"),
+            return self.doBoth([w1.send_data, b"data1"],
-                                  deferToThread(w2.get_data, b"data2")], True)
+                               [w2.send_data, b"data2"])
         d.addCallback(_got_code)
+        def _sent(res):
+            return self.doBoth([w1.get_data], [w2.get_data])
+        d.addCallback(_sent)
         def _done(dl):
             (dataX, dataY) = dl
             self.assertEqual(dataX, b"data2")
             self.assertEqual(dataY, b"data1")
+            return self.doBoth([w1.close], [w2.close])
+        d.addCallback(_done)
+        return d
+
+    def test_interleaved(self):
+        appid = b"appid"
+        w1 = BlockingWormhole(appid, self.relayurl)
+        w2 = BlockingWormhole(appid, self.relayurl)
+        d = deferToThread(w1.get_code)
+        def _got_code(code):
+            w2.set_code(code)
+            return self.doBoth([w1.send_data, b"data1"],
+                               [w2.get_data])
+        d.addCallback(_got_code)
+        def _sent(res):
+            (_, dataY) = res
+            self.assertEqual(dataY, b"data1")
+            return self.doBoth([w1.get_data], [w2.send_data, b"data2"])
+        d.addCallback(_sent)
+        def _done(dl):
+            (dataX, _) = dl
+            self.assertEqual(dataX, b"data2")
+            return self.doBoth([w1.close], [w2.close])
         d.addCallback(_done)
         return d
 
@@ -32,12 +66,15 @@ class Blocking(ServerBase, unittest.TestCase):
         w2 = BlockingWormhole(appid, self.relayurl)
         w1.set_code("123-purple-elephant")
         w2.set_code("123-purple-elephant")
-        d = gatherResults([deferToThread(w1.get_data, b"data1"),
+        d = self.doBoth([w1.send_data, b"data1"], [w2.send_data, b"data2"])
-                           deferToThread(w2.get_data, b"data2")], True)
+        def _sent(res):
+            return self.doBoth([w1.get_data], [w2.get_data])
+        d.addCallback(_sent)
         def _done(dl):
             (dataX, dataY) = dl
             self.assertEqual(dataX, b"data2")
             self.assertEqual(dataY, b"data1")
+            return self.doBoth([w1.close], [w2.close])
         d.addCallback(_done)
         return d
 
@@ -48,20 +85,23 @@ class Blocking(ServerBase, unittest.TestCase):
         d = deferToThread(w1.get_code)
         def _got_code(code):
             w2.set_code(code)
-            return gatherResults([deferToThread(w1.get_verifier),
+            return self.doBoth([w1.get_verifier], [w2.get_verifier])
-                                  deferToThread(w2.get_verifier)], True)
         d.addCallback(_got_code)
         def _check_verifier(res):
             v1, v2 = res
             self.failUnlessEqual(type(v1), type(b""))
             self.failUnlessEqual(v1, v2)
-            return gatherResults([deferToThread(w1.get_data, b"data1"),
+            return self.doBoth([w1.send_data, b"data1"],
-                                  deferToThread(w2.get_data, b"data2")], True)
+                               [w2.send_data, b"data2"])
         d.addCallback(_check_verifier)
+        def _sent(res):
+            return self.doBoth([w1.get_data], [w2.get_data])
+        d.addCallback(_sent)
         def _done(dl):
             (dataX, dataY) = dl
             self.assertEqual(dataX, b"data2")
             self.assertEqual(dataY, b"data1")
+            return self.doBoth([w1.close], [w2.close])
         d.addCallback(_done)
         return d
 
@@ -72,13 +112,13 @@ class Blocking(ServerBase, unittest.TestCase):
         d = deferToThread(w1.get_code)
         def _got_code(code):
             w2.set_code(code+"not")
-            return gatherResults([deferToThread(w1.get_verifier),
+            return self.doBoth([w1.get_verifier], [w2.get_verifier])
-                                  deferToThread(w2.get_verifier)], True)
         d.addCallback(_got_code)
         def _check_verifier(res):
             v1, v2 = res
             self.failUnlessEqual(type(v1), type(b""))
             self.failIfEqual(v1, v2)
+            return self.doBoth([w1.close], [w2.close])
         d.addCallback(_check_verifier)
         return d
 
@@ -86,7 +126,8 @@ class Blocking(ServerBase, unittest.TestCase):
         appid = b"appid"
         w1 = BlockingWormhole(appid, self.relayurl)
         self.assertRaises(UsageError, w1.get_verifier)
-        self.assertRaises(UsageError, w1.get_data, b"data")
+        self.assertRaises(UsageError, w1.get_data)
+        self.assertRaises(UsageError, w1.send_data, b"data")
         w1.set_code("123-purple-elephant")
         self.assertRaises(UsageError, w1.set_code, "123-nope")
         self.assertRaises(UsageError, w1.get_code)
@@ -94,6 +135,7 @@ class Blocking(ServerBase, unittest.TestCase):
         d = deferToThread(w2.get_code)
         def _done(code):
             self.assertRaises(UsageError, w2.get_code)
+            return self.doBoth([w1.close], [w2.close])
         d.addCallback(_done)
         return d
 
@@ -111,15 +153,19 @@ class Blocking(ServerBase, unittest.TestCase):
             self.assertEqual(type(s), type(""))
             unpacked = json.loads(s) # this is supposed to be JSON
             self.assertEqual(type(unpacked), dict)
-            new_w1 = BlockingWormhole.from_serialized(s)
+            self.new_w1 = BlockingWormhole.from_serialized(s)
-            return gatherResults([deferToThread(new_w1.get_data, b"data1"),
+            return self.doBoth([self.new_w1.send_data, b"data1"],
-                                  deferToThread(w2.get_data, b"data2")], True)
+                               [w2.send_data, b"data2"])
         d.addCallback(_got_code)
+        def _sent(res):
+            return self.doBoth(self.new_w1.get_data(), w2.get_data())
+        d.addCallback(_sent)
         def _done(dl):
             (dataX, dataY) = dl
             self.assertEqual(dataX, b"data2")
             self.assertEqual(dataY, b"data1")
             self.assertRaises(UsageError, w2.serialize) # too late
+            return self.doBoth([w1.close], [w2.close])
         d.addCallback(_done)
         return d
     test_serialize.skip = "not yet implemented for the blocking flavor"

src/wormhole/test/test_twisted.py

@@ -5,6 +5,10 @@ from ..twisted.transcribe import Wormhole, UsageError
 from .common import ServerBase
 
 class Basic(ServerBase, unittest.TestCase):
+
+    def doBoth(self, d1, d2):
+        return gatherResults([d1, d2], True)
+
     def test_basic(self):
         appid = b"appid"
         w1 = Wormhole(appid, self.relayurl)
@@ -12,13 +16,37 @@ class Basic(ServerBase, unittest.TestCase):
         d = w1.get_code()
         def _got_code(code):
             w2.set_code(code)
-            return gatherResults([w1.get_data(b"data1"),
+            return self.doBoth(w1.send_data(b"data1"), w2.send_data(b"data2"))
-                                  w2.get_data(b"data2")], True)
         d.addCallback(_got_code)
+        def _sent(res):
+            return self.doBoth(w1.get_data(), w2.get_data())
+        d.addCallback(_sent)
         def _done(dl):
             (dataX, dataY) = dl
             self.assertEqual(dataX, b"data2")
             self.assertEqual(dataY, b"data1")
+            return self.doBoth(w1.close(), w2.close())
+        d.addCallback(_done)
+        return d
+
+    def test_interleaved(self):
+        appid = b"appid"
+        w1 = Wormhole(appid, self.relayurl)
+        w2 = Wormhole(appid, self.relayurl)
+        d = w1.get_code()
+        def _got_code(code):
+            w2.set_code(code)
+            return self.doBoth(w1.send_data(b"data1"), w2.get_data())
+        d.addCallback(_got_code)
+        def _sent(res):
+            (_, dataY) = res
+            self.assertEqual(dataY, b"data1")
+            return self.doBoth(w1.get_data(), w2.send_data(b"data2"))
+        d.addCallback(_sent)
+        def _done(dl):
+            (dataX, _) = dl
+            self.assertEqual(dataX, b"data2")
+            return self.doBoth(w1.close(), w2.close())
         d.addCallback(_done)
         return d
 
@@ -28,12 +56,15 @@ class Basic(ServerBase, unittest.TestCase):
         w2 = Wormhole(appid, self.relayurl)
         w1.set_code("123-purple-elephant")
         w2.set_code("123-purple-elephant")
-        d = gatherResults([w1.get_data(b"data1"),
+        d = self.doBoth(w1.send_data(b"data1"), w2.send_data(b"data2"))
-                           w2.get_data(b"data2")], True)
+        def _sent(res):
+            return self.doBoth(w1.get_data(), w2.get_data())
+        d.addCallback(_sent)
         def _done(dl):
             (dataX, dataY) = dl
             self.assertEqual(dataX, b"data2")
             self.assertEqual(dataY, b"data1")
+            return self.doBoth(w1.close(), w2.close())
         d.addCallback(_done)
         return d
 
@@ -44,19 +75,22 @@ class Basic(ServerBase, unittest.TestCase):
         d = w1.get_code()
         def _got_code(code):
             w2.set_code(code)
-            return gatherResults([w1.get_verifier(), w2.get_verifier()], True)
+            return self.doBoth(w1.get_verifier(), w2.get_verifier())
         d.addCallback(_got_code)
         def _check_verifier(res):
             v1, v2 = res
             self.failUnlessEqual(type(v1), type(b""))
             self.failUnlessEqual(v1, v2)
-            return gatherResults([w1.get_data(b"data1"),
+            return self.doBoth(w1.send_data(b"data1"), w2.send_data(b"data2"))
-                                  w2.get_data(b"data2")], True)
         d.addCallback(_check_verifier)
+        def _sent(res):
+            return self.doBoth(w1.get_data(), w2.get_data())
+        d.addCallback(_sent)
         def _done(dl):
             (dataX, dataY) = dl
             self.assertEqual(dataX, b"data2")
             self.assertEqual(dataY, b"data1")
+            return self.doBoth(w1.close(), w2.close())
         d.addCallback(_done)
         return d
 
@@ -67,12 +101,13 @@ class Basic(ServerBase, unittest.TestCase):
         d = w1.get_code()
         def _got_code(code):
             w2.set_code(code+"not")
-            return gatherResults([w1.get_verifier(), w2.get_verifier()], True)
+            return self.doBoth(w1.get_verifier(), w2.get_verifier())
         d.addCallback(_got_code)
         def _check_verifier(res):
             v1, v2 = res
             self.failUnlessEqual(type(v1), type(b""))
             self.failIfEqual(v1, v2)
+            return self.doBoth(w1.close(), w2.close())
         d.addCallback(_check_verifier)
         return d
 
@@ -80,13 +115,17 @@ class Basic(ServerBase, unittest.TestCase):
         appid = b"appid"
         w1 = Wormhole(appid, self.relayurl)
         self.assertRaises(UsageError, w1.get_verifier)
-        self.assertRaises(UsageError, w1.get_data, b"data")
+        self.assertRaises(UsageError, w1.send_data, b"data")
+        self.assertRaises(UsageError, w1.get_data)
         w1.set_code("123-purple-elephant")
         self.assertRaises(UsageError, w1.set_code, "123-nope")
         self.assertRaises(UsageError, w1.get_code)
         w2 = Wormhole(appid, self.relayurl)
         d = w2.get_code()
         self.assertRaises(UsageError, w2.get_code)
+        def _got_code(code):
+            return self.doBoth(w1.close(), w2.close())
+        d.addCallback(_got_code)
         return d
 
     def test_serialize(self):
@@ -103,15 +142,19 @@ class Basic(ServerBase, unittest.TestCase):
             self.assertEqual(type(s), type(""))
             unpacked = json.loads(s) # this is supposed to be JSON
             self.assertEqual(type(unpacked), dict)
-            new_w1 = Wormhole.from_serialized(s)
+            self.new_w1 = Wormhole.from_serialized(s)
-            return gatherResults([new_w1.get_data(b"data1"),
+            return self.doBoth(self.new_w1.send_data(b"data1"),
-                                  w2.get_data(b"data2")], True)
+                               w2.send_data(b"data2"))
         d.addCallback(_got_code)
+        def _sent(res):
+            return self.doBoth(self.new_w1.get_data(), w2.get_data())
+        d.addCallback(_sent)
         def _done(dl):
             (dataX, dataY) = dl
-            self.assertEqual(dataX, b"data2")
+            self.assertEqual((dataX, dataY), (b"data2", b"data1"))
-            self.assertEqual(dataY, b"data1")
             self.assertRaises(UsageError, w2.serialize) # too late
+            return gatherResults([w1.close(), w2.close(), self.new_w1.close()],
+                                 True)
         d.addCallback(_done)
         return d
 

src/wormhole/twisted/demo.py

@@ -1,5 +1,6 @@
 from __future__ import print_function
 import sys, json
+from twisted.python import log
 from twisted.internet import reactor
 from .transcribe import Wormhole
 from .. import public_relay
@@ -14,8 +15,11 @@ if sys.argv[1] == "send-text":
     d = w.get_code()
     def _got_code(code):
         print("code is:", code)
-        return w.get_data(data)
+        return w.send_data(data)
     d.addCallback(_got_code)
+    def _sent(_):
+        return w.get_data()
+    d.addCallback(_sent)
     def _got_data(them_bytes):
         them_d = json.loads(them_bytes.decode("utf-8"))
         if them_d["message"] == "ok":
@@ -26,16 +30,19 @@ if sys.argv[1] == "send-text":
 elif sys.argv[1] == "receive-text":
     code = sys.argv[2]
     w.set_code(code)
-    data = json.dumps({"message": "ok"}).encode("utf-8")
+    d = w.get_data()
-    d = w.get_data(data)
     def _got_data(them_bytes):
         them_d = json.loads(them_bytes.decode("utf-8"))
         if "error" in them_d:
             print("ERROR: " + them_d["error"], file=sys.stderr)
             return 1
         print(them_d["message"])
+        data = json.dumps({"message": "ok"}).encode("utf-8")
+        return w.send_data(data)
     d.addCallback(_got_data)
 else:
     raise ValueError("bad command")
+d.addCallback(w.close)
 d.addCallback(lambda _: reactor.stop())
+d.addErrback(log.err)
 reactor.run()

src/wormhole/twisted/transcribe.py

@@ -107,11 +107,11 @@ class Channel:
         d.addCallback(lambda _: msgs[0])
         return d
 
-    def deallocate(self, res):
+    def deallocate(self):
         # only try once, no retries
         d = post_json(self._agent, self._channel_url+"/deallocate",
                       {"side": self._side})
-        d.addBoth(lambda _: res) # ignore POST failure, pass-through result
+        d.addBoth(lambda _: None) # ignore POST failure
         return d
 
 class ChannelManager:
@@ -150,6 +150,9 @@ class Wormhole:
         self.code = None
         self.key = None
         self._started_get_code = False
+        self._sent_data = False
+        self._got_data = False
+        self._closed = False
 
     def _set_side(self, side):
         self._side = side
@@ -218,6 +221,8 @@ class Wormhole:
         # get_verifier/get_data
         if self.code is None: raise UsageError
         if self.key is not None: raise UsageError
+        if self._sent_data: raise UsageError
+        if self._got_data: raise UsageError
         data = {
             "appid": self.appid,
             "relay": self.relay,
@@ -282,32 +287,51 @@ class Wormhole:
         d.addCallback(lambda _: self.verifier)
         return d
 
-    def get_data(self, outbound_data):
+    def send_data(self, outbound_data):
-        # only call this once
+        if self._sent_data: raise UsageError # only call this once
         if not isinstance(outbound_data, type(b"")): raise UsageError
         if self.code is None: raise UsageError
-        d = self._get_key()
+        if self.channel is None: raise UsageError
-        d.addCallback(self._get_data2, outbound_data)
-        d.addBoth(self.channel.deallocate)
-        return d
-
-    def _get_data2(self, key, outbound_data):
         # Without predefined roles, we can't derive predictably unique keys
         # for each side, so we use the same key for both. We use random
-        # nonces to keep the messages distinct, and check for reflection.
+        # nonces to keep the messages distinct, and the Channel automatically
+        # ignores reflections.
+        d = self._get_key()
+        def _send(key):
             data_key = self.derive_key(b"data-key")
-
             outbound_encrypted = self._encrypt_data(data_key, outbound_data)
-        d = self.channel.send(u"data", outbound_encrypted)
+            return self.channel.send(u"data", outbound_encrypted)
+        d.addCallback(_send)
+        return d
 
-        d.addCallback(lambda _: self.channel.get(u"data"))
+    def get_data(self):
-        def _got_data(inbound_encrypted):
+        if self._got_data: raise UsageError # only call this once
-            #if inbound_encrypted == outbound_encrypted:
+        if self.code is None: raise UsageError
-            #    raise ReflectionAttack
+        if self.channel is None: raise UsageError
+        d = self._get_key()
+        def _get(key):
+            data_key = self.derive_key(b"data-key")
+            d1 = self.channel.get(u"data")
+            def _decrypt(inbound_encrypted):
                 try:
-                inbound_data = self._decrypt_data(data_key, inbound_encrypted)
+                    inbound_data = self._decrypt_data(data_key,
+                                                      inbound_encrypted)
                     return inbound_data
                 except CryptoError:
                     raise WrongPasswordError
-        d.addCallback(_got_data)
+            d1.addCallback(_decrypt)
+            return d1
+        d.addCallback(_get)
         return d
+
+    def close(self, res=None):
+        d = self.channel.deallocate()
+        def _closed(_):
+            self._closed = True
+            return res
+        d.addCallback(_closed)
+        return d
+
+    def __del__(self):
+        if not self._closed:
+            print("Error: a Wormhole instance was not closed", file=sys.stderr)