server: give old 0.4.0 senders a "you must upgrade" error

Without this, old senders will throw a messy 404 traceback when talking
to a modern server.

Unfortunately 0.4.0 receivers don't make API calls in the right order,
so they throw a 404 before seeing our "you need to upgrade" message.

NEWS.md

@@ -1,6 +1,13 @@
 
 User-visible changes in "magic-wormhole":
 
+## Release ?? (??)
+
+* Arrange for 0.4.0 senders to print an error message when connecting to a
+  current (0.5.0) server, instead of an ugly stack trace. Unfortunately 0.4.0
+  receivers still display the traceback, since they don't check the welcome
+  message before using a missing API.
+
 ## Release 0.5.0 (07-Oct-2015)
 
 * Change the CLI to merge send-file with send-text, and receive-file with

src/wormhole/servers/relay_server.py

@@ -63,6 +63,9 @@ class ChannelLister(resource.Resource):
         self._relay = relay
 
     def render_GET(self, request):
+        if b"appid" not in request.args:
+            e = NeedToUpgradeErrorResource(self._relay.welcome)
+            return e.get_message()
         appid = request.args[b"appid"][0].decode("utf-8")
         #print("LIST", appid)
         app = self._relay.get_app(appid)
@@ -95,6 +98,29 @@ class Allocator(resource.Resource):
                 "channelid": channelid}
         return (json.dumps(data)+"\n").encode("utf-8")
 
+    def getChild(self, path, req):
+        # wormhole-0.4.0 "send" started with "POST /allocate/SIDE".
+        # wormhole-0.5.0 changed that to "POST /allocate". We catch the old
+        # URL here to deliver a nicer error message (with upgrade
+        # instructions) than an ugly 404.
+        return NeedToUpgradeErrorResource(self._relay.welcome)
+
+class NeedToUpgradeErrorResource(resource.Resource):
+    def __init__(self, welcome):
+        resource.Resource.__init__(self)
+        w = welcome.copy()
+        w["error"] = "Sorry, you must upgrade your client to use this server."
+        message = {"welcome": w}
+        self._message = (json.dumps(message)+"\n").encode("utf-8")
+    def get_message(self):
+        return self._message
+    def render_POST(self, request):
+        return self._message
+    def render_GET(self, request):
+        return self._message
+    def getChild(self, path, req):
+        return self
+
 class Adder(resource.Resource):
     def __init__(self, relay):
         resource.Resource.__init__(self)
@@ -311,6 +337,15 @@ class Relay(resource.Resource, service.MultiService):
         self.putChild(b"get", Getter(self))
         self.putChild(b"deallocate", Deallocator(self))
 
+    def getChild(self, path, req):
+        # 0.4.0 used "POST /CID/SIDE/post/MSGNUM"
+        # 0.5.0 replaced it with "POST /add (json body)"
+        # give a nicer error message to old clients
+        if (len(req.postpath) >= 2
+            and req.postpath[1] in (b"post", b"poll", b"deallocate")):
+            return NeedToUpgradeErrorResource(self.welcome)
+        return resource.NoResource("No such child resource.")
+
     def get_app(self, appid):
         assert isinstance(appid, type(u""))
         if not appid in self._apps:

src/wormhole/test/test_server.py

@@ -165,6 +165,35 @@ class API(ServerBase, unittest.TestCase):
 
         return d
 
+    UPGRADE_ERROR = "Sorry, you must upgrade your client to use this server."
+    def test_old_allocate(self):
+        # 0.4.0 used "POST /allocate/SIDE".
+        # 0.5.0 replaced it with "POST /allocate".
+        # test that an old client gets a useful error message, not a 404.
+        d = self.post("allocate/abc", {})
+        def _check(data):
+            self.failUnlessEqual(data["welcome"]["error"], self.UPGRADE_ERROR)
+        d.addCallback(_check)
+        return d
+
+    def test_old_list(self):
+        # 0.4.0 used "GET /list".
+        # 0.5.0 replaced it with "GET /list?appid="
+        d = self.get("list", {}) # no appid
+        def _check(data):
+            self.failUnlessEqual(data["welcome"]["error"], self.UPGRADE_ERROR)
+        d.addCallback(_check)
+        return d
+
+    def test_old_post(self):
+        # 0.4.0 used "POST /CID/SIDE/post/MSGNUM"
+        # 0.5.0 replaced it with "POST /add (json body)"
+        d = self.post("1/abc/post/pake", {})
+        def _check(data):
+            self.failUnlessEqual(data["welcome"]["error"], self.UPGRADE_ERROR)
+        d.addCallback(_check)
+        return d
+
     def add_message(self, message, side="abc", phase="1"):
         return self.post("add",
                          {"appid": "app1",