Make numeric user_id checker start at @0, and don't ratelimit on checking (#6338)

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 @@ from synapse.api.errors import (
     AuthError,
     Codes,
     ConsentNotGivenError,
-    LimitExceededError,
     RegistrationError,
     SynapseError,
 )
@@ -168,6 +167,7 @@ class RegistrationHandler(BaseHandler):
         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 +217,13 @@ class RegistrationHandler(BaseHandler):
 
         else:
             # autogen a sequential user ID
+            fail_count = 0
             user = None
             while not user:
+                # Fail after being unable to find a suitable ID a few times
+                if fail_count > 10:
+                    raise SynapseError(500, "Unable to find a suitable guest user ID")
+
                 localpart = yield self._generate_user_id()
                 user = UserID(localpart, self.hs.hostname)
                 user_id = user.to_string()
@@ -233,10 +238,14 @@ class RegistrationHandler(BaseHandler):
                         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
+                    fail_count += 1
 
         if not self.hs.config.user_consent_at_registration:
             yield self._auto_join_rooms(user_id)
@@ -414,6 +423,29 @@ class RegistrationHandler(BaseHandler):
             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 +478,6 @@ class RegistrationHandler(BaseHandler):
         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 @@ class ReplicationRegisterServlet(ReplicationEndpoint):
     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 @@ class RegistrationWorkerStore(SQLBaseStore):
         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
+        avoid the case of ID 1000 being pre-allocated and starting at 1001 while
-        first (and shortest) many generated user IDs.
+        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+):")