3 files changed, 37 insertions, 12 deletions
diff --git a/synapse/config/registration.py b/synapse/config/registration.py
index 7a059c6dec..ea9b50fe97 100644
--- a/synapse/config/registration.py
+++ b/synapse/config/registration.py
@@ -190,6 +190,8 @@ class RegistrationConfig(Config):
         # The success template used during fallback auth.
         self.fallback_success_template = self.read_template("auth_success.html")
 
+        self.inhibit_user_in_use_error = config.get("inhibit_user_in_use_error", False)
+
     def generate_config_section(self, generate_secrets=False, **kwargs):
         if generate_secrets:
             registration_shared_secret = 'registration_shared_secret: "%s"' % (
@@ -446,6 +448,16 @@ class RegistrationConfig(Config):
         # Defaults to true.
         #
         #auto_join_rooms_for_guests: false
+
+        # Whether to inhibit errors raised when registering a new account if the user ID
+        # already exists. If turned on, that requests to /register/available will always
+        # show a user ID as available, and Synapse won't raise an error when starting
+        # a registration with a user ID that already exists. However, Synapse will still
+        # raise an error if the registration completes and the username conflicts.
+        #
+        # Defaults to false.
+        #
+        #inhibit_user_in_use_error: true
         """
             % locals()
         )
diff --git a/synapse/handlers/register.py b/synapse/handlers/register.py
index f08a516a75..a719d5eef3 100644
--- a/synapse/handlers/register.py
+++ b/synapse/handlers/register.py
@@ -132,6 +132,7 @@ class RegistrationHandler:
         localpart: str,
         guest_access_token: Optional[str] = None,
         assigned_user_id: Optional[str] = None,
+        inhibit_user_in_use_error: bool = False,
     ) -> None:
         if types.contains_invalid_mxid_characters(localpart):
             raise SynapseError(
@@ -171,21 +172,22 @@ class RegistrationHandler:
 
         users = await self.store.get_users_by_id_case_insensitive(user_id)
         if users:
-            if not guest_access_token:
+            if not inhibit_user_in_use_error and not guest_access_token:
                 raise SynapseError(
                     400, "User ID already taken.", errcode=Codes.USER_IN_USE
                 )
-            user_data = await self.auth.get_user_by_access_token(guest_access_token)
-            if (
-                not user_data.is_guest
-                or UserID.from_string(user_data.user_id).localpart != localpart
-            ):
-                raise AuthError(
-                    403,
-                    "Cannot register taken user ID without valid guest "
-                    "credentials for that user.",
-                    errcode=Codes.FORBIDDEN,
-                )
+            if guest_access_token:
+                user_data = await self.auth.get_user_by_access_token(guest_access_token)
+                if (
+                    not user_data.is_guest
+                    or UserID.from_string(user_data.user_id).localpart != localpart
+                ):
+                    raise AuthError(
+                        403,
+                        "Cannot register taken user ID without valid guest "
+                        "credentials for that user.",
+                        errcode=Codes.FORBIDDEN,
+                    )
 
         if guest_access_token is None:
             try:
diff --git a/synapse/rest/client/register.py b/synapse/rest/client/register.py
index 8b56c76aed..c59dae7c03 100644
--- a/synapse/rest/client/register.py
+++ b/synapse/rest/client/register.py
@@ -339,12 +339,19 @@ class UsernameAvailabilityRestServlet(RestServlet):
             ),
         )
 
+        self.inhibit_user_in_use_error = (
+            hs.config.registration.inhibit_user_in_use_error
+        )
+
     async def on_GET(self, request: Request) -> Tuple[int, JsonDict]:
         if not self.hs.config.registration.enable_registration:
             raise SynapseError(
                 403, "Registration has been disabled", errcode=Codes.FORBIDDEN
             )
 
+        if self.inhibit_user_in_use_error:
+            return 200, {"available": True}
+
         ip = request.getClientIP()
         with self.ratelimiter.ratelimit(ip) as wait_deferred:
             await wait_deferred
@@ -422,6 +429,9 @@ class RegisterRestServlet(RestServlet):
         self._refresh_tokens_enabled = (
             hs.config.registration.refreshable_access_token_lifetime is not None
         )
+        self._inhibit_user_in_use_error = (
+            hs.config.registration.inhibit_user_in_use_error
+        )
 
         self._registration_flows = _calculate_registration_flows(
             hs.config, self.auth_handler
@@ -564,6 +574,7 @@ class RegisterRestServlet(RestServlet):
                 desired_username,
                 guest_access_token=guest_access_token,
                 assigned_user_id=registered_user_id,
+                inhibit_user_in_use_error=self._inhibit_user_in_use_error,
             )
 
         # Check if the user-interactive authentication flows are complete, if