Add GET account data routes (#4303)

As per https://github.com/matrix-org/matrix-doc/issues/1339

changelog.d/4303.feature

@@ -0,0 +1 @@
+Add routes for reading account data.

synapse/rest/client/v2_alpha/account_data.py

@@ -17,7 +17,7 @@ import logging
 
 from twisted.internet import defer
 
-from synapse.api.errors import AuthError, SynapseError
+from synapse.api.errors import AuthError, NotFoundError, SynapseError
 from synapse.http.servlet import RestServlet, parse_json_object_from_request
 
 from ._base import client_v2_patterns
@@ -28,6 +28,7 @@ logger = logging.getLogger(__name__)
 class AccountDataServlet(RestServlet):
     """
     PUT /user/{user_id}/account_data/{account_dataType} HTTP/1.1
+    GET /user/{user_id}/account_data/{account_dataType} HTTP/1.1
     """
     PATTERNS = client_v2_patterns(
         "/user/(?P<user_id>[^/]*)/account_data/(?P<account_data_type>[^/]*)"
@@ -57,10 +58,26 @@ class AccountDataServlet(RestServlet):
 
         defer.returnValue((200, {}))
 
+    @defer.inlineCallbacks
+    def on_GET(self, request, user_id, account_data_type):
+        requester = yield self.auth.get_user_by_req(request)
+        if user_id != requester.user.to_string():
+            raise AuthError(403, "Cannot get account data for other users.")
+
+        event = yield self.store.get_global_account_data_by_type_for_user(
+            account_data_type, user_id,
+        )
+
+        if event is None:
+            raise NotFoundError("Account data not found")
+
+        defer.returnValue((200, event))
+
 
 class RoomAccountDataServlet(RestServlet):
     """
     PUT /user/{user_id}/rooms/{room_id}/account_data/{account_dataType} HTTP/1.1
+    GET /user/{user_id}/rooms/{room_id}/account_data/{account_dataType} HTTP/1.1
     """
     PATTERNS = client_v2_patterns(
         "/user/(?P<user_id>[^/]*)"
@@ -99,6 +116,21 @@ class RoomAccountDataServlet(RestServlet):
 
         defer.returnValue((200, {}))
 
+    @defer.inlineCallbacks
+    def on_GET(self, request, user_id, room_id, account_data_type):
+        requester = yield self.auth.get_user_by_req(request)
+        if user_id != requester.user.to_string():
+            raise AuthError(403, "Cannot get account data for other users.")
+
+        event = yield self.store.get_account_data_for_room_and_type(
+            user_id, room_id, account_data_type,
+        )
+
+        if event is None:
+            raise NotFoundError("Room account data not found")
+
+        defer.returnValue((200, event))
+
 
 def register_servlets(hs, http_server):
     AccountDataServlet(hs).register(http_server)