Load `/register/available` endpoint on workers (#15268)

changelog.d/15268.feature

@@ -0,0 +1 @@
+Allow loading `/register/available` endpoint on workers.

docker/configure_workers_and_start.py

@@ -163,6 +163,7 @@ WORKERS_CONFIG: Dict[str, Dict[str, Any]] = {
             "^/_matrix/client/versions$",
             "^/_matrix/client/(api/v1|r0|v3|unstable)/voip/turnServer$",
             "^/_matrix/client/(r0|v3|unstable)/register$",
+            "^/_matrix/client/(r0|v3|unstable)/register/available$",
             "^/_matrix/client/(r0|v3|unstable)/auth/.*/fallback/web$",
             "^/_matrix/client/(api/v1|r0|v3|unstable)/rooms/.*/messages$",
             "^/_matrix/client/(api/v1|r0|v3|unstable)/rooms/.*/event",

docs/workers.md

@@ -245,6 +245,7 @@ information.
     # Registration/login requests
     ^/_matrix/client/(api/v1|r0|v3|unstable)/login$
     ^/_matrix/client/(r0|v3|unstable)/register$
+    ^/_matrix/client/(r0|v3|unstable)/register/available$
     ^/_matrix/client/v1/register/m.login.registration_token/validity$
 
     # Event sending requests

synapse/rest/client/register.py

@@ -956,7 +956,7 @@ def register_servlets(hs: "HomeServer", http_server: HttpServer) -> None:
     if hs.config.worker.worker_app is None:
         EmailRegisterRequestTokenRestServlet(hs).register(http_server)
         MsisdnRegisterRequestTokenRestServlet(hs).register(http_server)
-        UsernameAvailabilityRestServlet(hs).register(http_server)
         RegistrationSubmitTokenServlet(hs).register(http_server)
+    UsernameAvailabilityRestServlet(hs).register(http_server)
     RegistrationTokenValidityRestServlet(hs).register(http_server)
     RegisterRestServlet(hs).register(http_server)