Use parse_{int,str} and assert from http.servlet

parse_integer and parse_string can take a request and raise errors
in case we have wrong or missing params.
This PR tries to use them more to deduplicate some code and make it
better readable

synapse/streams/config.py

@@ -16,6 +16,7 @@
 import logging
 
 from synapse.api.errors import SynapseError
+from synapse.http.servlet import parse_integer, parse_string
 from synapse.types import StreamToken
 
 logger = logging.getLogger(__name__)
@@ -56,23 +57,10 @@ class PaginationConfig(object):
     @classmethod
     def from_request(cls, request, raise_invalid_params=True,
                      default_limit=None):
-        def get_param(name, default=None):
-            lst = request.args.get(name, [])
-            if len(lst) > 1:
-                raise SynapseError(
-                    400, "%s must be specified only once" % (name,)
-                )
-            elif len(lst) == 1:
-                return lst[0]
-            else:
-                return default
+        direction = parse_string(request, "dir", default='f', allowed_values=['f', 'b'])
 
-        direction = get_param("dir", 'f')
-        if direction not in ['f', 'b']:
-            raise SynapseError(400, "'dir' parameter is invalid.")
-
-        from_tok = get_param("from")
-        to_tok = get_param("to")
+        from_tok = parse_string(request, "from")
+        to_tok = parse_string(request, "to")
 
         try:
             if from_tok == "END":
@@ -88,12 +76,7 @@ class PaginationConfig(object):
         except Exception:
             raise SynapseError(400, "'to' paramater is invalid")
 
-        limit = get_param("limit", None)
-        if limit is not None and not limit.isdigit():
-            raise SynapseError(400, "'limit' parameter must be an integer.")
-
-        if limit is None:
-            limit = default_limit
+        limit = parse_integer(request, "limit", default=default_limit)
 
         try:
             return PaginationConfig(from_tok, to_tok, direction, limit)