Allow guests to register and call /events?room_id=

This follows the same flows-based flow as regular registration, but as
the only implemented flow has no requirements, it auto-succeeds. In the
future, other flows (e.g. captcha) may be required, so clients should
treat this like the regular registration flow choices.

synapse/rest/client/v2_alpha/register.py

@@ -16,7 +16,7 @@
 from twisted.internet import defer
 
 from synapse.api.constants import LoginType
-from synapse.api.errors import SynapseError, Codes
+from synapse.api.errors import SynapseError, Codes, UnrecognizedRequestError
 from synapse.http.servlet import RestServlet
 
 from ._base import client_v2_pattern, parse_json_dict_from_request
@@ -55,6 +55,19 @@ class RegisterRestServlet(RestServlet):
     def on_POST(self, request):
         yield run_on_reactor()
 
+        kind = "user"
+        if "kind" in request.args:
+            kind = request.args["kind"][0]
+
+        if kind == "guest":
+            ret = yield self._do_guest_registration()
+            defer.returnValue(ret)
+            return
+        elif kind != "user":
+            raise UnrecognizedRequestError(
+                "Do not understand membership kind: %s" % (kind,)
+            )
+
         if '/register/email/requestToken' in request.path:
             ret = yield self.onEmailTokenRequest(request)
             defer.returnValue(ret)
@@ -236,6 +249,18 @@ class RegisterRestServlet(RestServlet):
         ret = yield self.identity_handler.requestEmailToken(**body)
         defer.returnValue((200, ret))
 
+    @defer.inlineCallbacks
+    def _do_guest_registration(self):
+        if not self.hs.config.allow_guest_access:
+            defer.returnValue((403, "Guest access is disabled"))
+        user_id, _ = yield self.registration_handler.register(generate_token=False)
+        access_token = self.auth_handler.generate_access_token(user_id, ["guest = true"])
+        defer.returnValue((200, {
+            "user_id": user_id,
+            "access_token": access_token,
+            "home_server": self.hs.hostname,
+        }))
+
 
 def register_servlets(hs, http_server):
     RegisterRestServlet(hs).register(http_server)