register_new_matrix_user: read server url from config

Fixes #3672:
`https://localhost:8448` is virtually never right.

changelog.d/13616.bugfix

@@ -0,0 +1 @@
+Fix a longstanding bug in `register_new_matrix_user` which meant it was always necessary to explicitly give a server URL.

synapse/_scripts/register_new_matrix_user.py

@@ -20,11 +20,13 @@
 import hmac
 import logging
 import sys
-from typing import Callable, Optional
+from typing import Any, Callable, Dict, Optional
 
 import requests
 import yaml
 
+_DEFAULT_SERVER_URL = "http://localhost:8008"
+
 
 def request_registration(
     user: str,
@@ -203,31 +205,76 @@ def main() -> None:
 
     parser.add_argument(
         "server_url",
-        default="https://localhost:8448",
         nargs="?",
-        help="URL to use to talk to the homeserver. Defaults to "
-        " 'https://localhost:8448'.",
+        help="URL to use to talk to the homeserver. By default, tries to find a "
+        "suitable URL from the configuration file. Otherwise, defaults to "
+        f"'{_DEFAULT_SERVER_URL}'.",
     )
 
     args = parser.parse_args()
 
     if "config" in args and args.config:
         config = yaml.safe_load(args.config)
+
+    if args.shared_secret:
+        secret = args.shared_secret
+    else:
+        # argparse should check that we have either config or shared secret
+        assert config
+
         secret = config.get("registration_shared_secret", None)
         if not secret:
             print("No 'registration_shared_secret' defined in config.")
             sys.exit(1)
+
+    if args.server_url:
+        server_url = args.server_url
+    elif config:
+        listening_port = _find_client_listener(config)
+        if listening_port:
+            server_url = f"http://{listening_port}"
+        else:
+            server_url = _DEFAULT_SERVER_URL
+            print(
+                "Unable to find a suitable HTTP listener in the configuration file. "
+                f"Trying {server_url} as a last resort.",
+                file=sys.stderr,
+            )
     else:
-        secret = args.shared_secret
+        server_url = _DEFAULT_SERVER_URL
+        print(
+            f"No server url or configuration file given. Defaulting to {server_url}.",
+            file=sys.stderr,
+        )
 
     admin = None
     if args.admin or args.no_admin:
         admin = args.admin
 
     register_new_user(
-        args.user, args.password, args.server_url, secret, admin, args.user_type
+        args.user, args.password, server_url, secret, admin, args.user_type
     )
 
 
+def _find_client_listener(config: Dict[str, Any]) -> Optional[str]:
+    # try to find a listener in the config. Returns a host:port pair
+    for listener in config.get("listeners", []):
+        if listener.get("type") != "http" or listener.get("tls", False):
+            continue
+
+        if not any(
+            name == "client"
+            for resource in listener.get("resources", [])
+            for name in resource.get("names", [])
+        ):
+            continue
+
+        # TODO: consider bind_addresses
+        return f"localhost:{listener['port']}"
+
+    # no suitable listeners?
+    return None
+
+
 if __name__ == "__main__":
     main()