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diff --git a/synapse/rest/client/_base.py b/synapse/rest/client/_base.py
new file mode 100644
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+++ b/synapse/rest/client/_base.py
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+# Copyright 2014-2016 OpenMarket Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""This module contains base REST classes for constructing client v1 servlets.
+"""
+import logging
+import re
+from typing import Iterable, Pattern
+
+from synapse.api.errors import InteractiveAuthIncompleteError
+from synapse.api.urls import CLIENT_API_PREFIX
+from synapse.types import JsonDict
+
+logger = logging.getLogger(__name__)
+
+
+def client_patterns(
+    path_regex: str,
+    releases: Iterable[int] = (0,),
+    unstable: bool = True,
+    v1: bool = False,
+) -> Iterable[Pattern]:
+    """Creates a regex compiled client path with the correct client path
+    prefix.
+
+    Args:
+        path_regex: The regex string to match. This should NOT have a ^
+            as this will be prefixed.
+        releases: An iterable of releases to include this endpoint under.
+        unstable: If true, include this endpoint under the "unstable" prefix.
+        v1: If true, include this endpoint under the "api/v1" prefix.
+    Returns:
+        An iterable of patterns.
+    """
+    patterns = []
+
+    if unstable:
+        unstable_prefix = CLIENT_API_PREFIX + "/unstable"
+        patterns.append(re.compile("^" + unstable_prefix + path_regex))
+    if v1:
+        v1_prefix = CLIENT_API_PREFIX + "/api/v1"
+        patterns.append(re.compile("^" + v1_prefix + path_regex))
+    for release in releases:
+        new_prefix = CLIENT_API_PREFIX + "/r%d" % (release,)
+        patterns.append(re.compile("^" + new_prefix + path_regex))
+
+    return patterns
+
+
+def set_timeline_upper_limit(filter_json: JsonDict, filter_timeline_limit: int) -> None:
+    """
+    Enforces a maximum limit of a timeline query.
+
+    Params:
+        filter_json: The timeline query to modify.
+        filter_timeline_limit: The maximum limit to allow, passing -1 will
+            disable enforcing a maximum limit.
+    """
+    if filter_timeline_limit < 0:
+        return  # no upper limits
+    timeline = filter_json.get("room", {}).get("timeline", {})
+    if "limit" in timeline:
+        filter_json["room"]["timeline"]["limit"] = min(
+            filter_json["room"]["timeline"]["limit"], filter_timeline_limit
+        )
+
+
+def interactive_auth_handler(orig):
+    """Wraps an on_POST method to handle InteractiveAuthIncompleteErrors
+
+    Takes a on_POST method which returns an Awaitable (errcode, body) response
+    and adds exception handling to turn a InteractiveAuthIncompleteError into
+    a 401 response.
+
+    Normal usage is:
+
+    @interactive_auth_handler
+    async def on_POST(self, request):
+        # ...
+        await self.auth_handler.check_auth
+    """
+
+    async def wrapped(*args, **kwargs):
+        try:
+            return await orig(*args, **kwargs)
+        except InteractiveAuthIncompleteError as e:
+            return 401, e.result
+
+    return wrapped