Make numeric user_id checker start at @0, and don't ratelimit on checking

changelog.d/6338.bugfix

@@ -0,0 +1 @@
+Prevent the server taking a long time to start up when guest registration is enabled.

synapse/handlers/register.py

@@ -24,7 +24,6 @@
     AuthError,
     Codes,
     ConsentNotGivenError,
-    LimitExceededError,
     RegistrationError,
     SynapseError,
 )
@@ -168,6 +167,7 @@ def register_user(
         Raises:
             RegistrationError if there was a problem registering.
         """
+        yield self.check_registration_ratelimit(address)
 
         yield self.auth.check_auth_blocking(threepid=threepid)
         password_hash = None
@@ -217,8 +217,8 @@ def register_user(
 
         else:
             # autogen a sequential user ID
-            user = None
-            while not user:
+            # Fail after being unable to find a suitable ID a few times
+            for x in range(10):
                 localpart = yield self._generate_user_id()
                 user = UserID(localpart, self.hs.hostname)
                 user_id = user.to_string()
@@ -233,10 +233,12 @@ def register_user(
                         create_profile_with_displayname=default_display_name,
                         address=address,
                     )
+
+                    # Successfully registered
+                    break
                 except SynapseError:
                     # if user id is taken, just generate another
-                    user = None
-                    user_id = None
+                    pass
 
         if not self.hs.config.user_consent_at_registration:
             yield self._auto_join_rooms(user_id)
@@ -414,6 +416,29 @@ def _join_user_to_room(self, requester, room_identifier):
             ratelimit=False,
         )
 
+    def check_registration_ratelimit(self, address):
+        """A simple helper method to check whether the registration rate limit has been hit
+        for a given IP address
+
+        Args:
+            address (str|None): the IP address used to perform the registration. If this is
+                None, no ratelimiting will be performed.
+
+        Raises:
+            LimitExceededError: If the rate limit has been exceeded.
+        """
+        if not address:
+            return
+
+        time_now = self.clock.time()
+
+        self.ratelimiter.ratelimit(
+            address,
+            time_now_s=time_now,
+            rate_hz=self.hs.config.rc_registration.per_second,
+            burst_count=self.hs.config.rc_registration.burst_count,
+        )
+
     def register_with_store(
         self,
         user_id,
@@ -446,22 +471,6 @@ def register_with_store(
         Returns:
             Deferred
         """
-        # Don't rate limit for app services
-        if appservice_id is None and address is not None:
-            time_now = self.clock.time()
-
-            allowed, time_allowed = self.ratelimiter.can_do_action(
-                address,
-                time_now_s=time_now,
-                rate_hz=self.hs.config.rc_registration.per_second,
-                burst_count=self.hs.config.rc_registration.burst_count,
-            )
-
-            if not allowed:
-                raise LimitExceededError(
-                    retry_after_ms=int(1000 * (time_allowed - time_now))
-                )
-
         if self.hs.config.worker_app:
             return self._register_client(
                 user_id=user_id,

synapse/replication/http/register.py

@@ -75,6 +75,8 @@ def _serialize_payload(
     async def _handle_request(self, request, user_id):
         content = parse_json_object_from_request(request)
 
+        self.registration_handler.check_registration_ratelimit(content["address"])
+
         await self.registration_handler.register_with_store(
             user_id=user_id,
             password_hash=content["password_hash"],

synapse/storage/data_stores/main/registration.py

@@ -488,14 +488,14 @@ def find_next_generated_user_id_localpart(self):
         we can. Unfortunately, it's possible some of them are already taken by
         existing users, and there may be gaps in the already taken range. This
         function returns the start of the first allocatable gap. This is to
-        avoid the case of ID 10000000 being pre-allocated, so us wasting the
-        first (and shortest) many generated user IDs.
+        avoid the case of ID 1000 being pre-allocated and starting at 1001 while
+        0-999 are available.
         """
 
         def _find_next_generated_user_id(txn):
-            # We bound between '@1' and '@a' to avoid pulling the entire table
+            # We bound between '@0' and '@a' to avoid pulling the entire table
             # out.
-            txn.execute("SELECT name FROM users WHERE '@1' <= name AND name < '@a'")
+            txn.execute("SELECT name FROM users WHERE '@0' <= name AND name < '@a'")
 
             regex = re.compile(r"^@(\d+):")