contrib: Add a mitmproxy script to block well-known lookups

This is useful if a test server sends incorrect homeserver URLs in their
well-known data, for example the Synapse instance Complement starts up
does so.

contrib/mitmproxy/well-known-block.py

@@ -0,0 +1,37 @@
+"""
+A mitmproxy script that blocks and removes well known Matrix server
+information.
+
+There are two ways a Matrix server can trigger the client to reconfigure the
+homeserver URL:
+
+    1. By responding to a `./well-known/matrix/client` request with a new
+    homeserver URL.
+
+    2. By including a new homeserver URL inside the `/login` response.
+
+To run execute it with mitmproxy:
+
+    >>> mitmproxy -s well-known-block.py`
+
+"""
+import json
+
+from mitmproxy import http
+
+
+def request(flow):
+    if flow.request.path == "/.well-known/matrix/client":
+        flow.response = http.HTTPResponse.make(
+            404,  # (optional) status code
+            b"Not found",  # (optional) content
+            {"Content-Type": "text/html"}  # (optional) headers
+        )
+
+
+def response(flow: http.HTTPFlow):
+    if flow.request.path == "/_matrix/client/r0/login":
+        if flow.response.status_code == 200:
+            body = json.loads(flow.response.content)
+            body.pop("well_known", None)
+            flow.response.text = json.dumps(body)