Validate the server name for the /publicRooms endpoint. (#9161)

If a remote server name is provided, ensure it is something reasonable
before making remote connections to it.

changelog.d/9161.bugfix

@@ -0,0 +1 @@
+Fix a long-standing bug "ValueError: invalid literal for int() with base 10" when `/publicRooms` is requested with an invalid `server` parameter.

synapse/rest/client/v1/room.py

@@ -32,6 +32,7 @@
 )
 from synapse.api.filtering import Filter
 from synapse.events.utils import format_event_for_client_v2
+from synapse.http.endpoint import parse_and_validate_server_name
 from synapse.http.servlet import (
     RestServlet,
     assert_params_in_dict,
@@ -347,8 +348,6 @@ async def on_GET(self, request):
             # provided.
             if server:
                 raise e
-            else:
-                pass
 
         limit = parse_integer(request, "limit", 0)
         since_token = parse_string(request, "since", None)
@@ -359,6 +358,14 @@ async def on_GET(self, request):
 
         handler = self.hs.get_room_list_handler()
         if server and server != self.hs.config.server_name:
+            # Ensure the server is valid.
+            try:
+                parse_and_validate_server_name(server)
+            except ValueError:
+                raise SynapseError(
+                    400, "Invalid server name: %s" % (server,), Codes.INVALID_PARAM,
+                )
+
             try:
                 data = await handler.get_remote_public_room_list(
                     server, limit=limit, since_token=since_token
@@ -402,6 +409,14 @@ async def on_POST(self, request):
 
         handler = self.hs.get_room_list_handler()
         if server and server != self.hs.config.server_name:
+            # Ensure the server is valid.
+            try:
+                parse_and_validate_server_name(server)
+            except ValueError:
+                raise SynapseError(
+                    400, "Invalid server name: %s" % (server,), Codes.INVALID_PARAM,
+                )
+
             try:
                 data = await handler.get_remote_public_room_list(
                     server,