Allocate guest user IDs numericcally

The current random IDs are ugly and confusing when presented in UIs.
This makes them prettier and easier to read.

Also, disable non-automated registration of numeric IDs so that we don't
need to worry so much about people carving out our automated address
space and us needing to keep retrying ID registration.

synapse/handlers/register.py

@@ -21,7 +21,6 @@ from synapse.api.errors import (
     AuthError, Codes, SynapseError, RegistrationError, InvalidCaptchaError
 )
 from ._base import BaseHandler
-import synapse.util.stringutils as stringutils
 from synapse.util.async import run_on_reactor
 from synapse.http.client import CaptchaServerHttpClient
 
@@ -45,6 +44,8 @@ class RegistrationHandler(BaseHandler):
         self.distributor.declare("registered_user")
         self.captcha_client = CaptchaServerHttpClient(hs)
 
+        self._next_generated_user_id = None
+
     @defer.inlineCallbacks
     def check_username(self, localpart, guest_access_token=None):
         yield run_on_reactor()
@@ -91,7 +92,7 @@ class RegistrationHandler(BaseHandler):
 
         Args:
             localpart : The local part of the user ID to register. If None,
-              one will be randomly generated.
+              one will be generated.
             password (str) : The password to assign to this user so they can
             login again. This can be None which means they cannot login again
             via a password (e.g. the user is an application service user).
@@ -108,6 +109,18 @@ class RegistrationHandler(BaseHandler):
         if localpart:
             yield self.check_username(localpart, guest_access_token=guest_access_token)
 
+            was_guest = guest_access_token is not None
+
+            if not was_guest:
+                try:
+                    int(localpart)
+                    raise RegistrationError(
+                        400,
+                        "Numeric user IDs are reserved for guest users."
+                    )
+                except ValueError:
+                    pass
+
             user = UserID(localpart, self.hs.hostname)
             user_id = user.to_string()
 
@@ -118,40 +131,36 @@ class RegistrationHandler(BaseHandler):
                 user_id=user_id,
                 token=token,
                 password_hash=password_hash,
-                was_guest=guest_access_token is not None,
+                was_guest=was_guest,
                 make_guest=make_guest,
             )
 
             yield registered_user(self.distributor, user)
         else:
-            # autogen a random user ID
+            # autogen a sequential user ID
             attempts = 0
-            user_id = None
             token = None
-            while not user_id:
+            user = None
+            while not user:
+                localpart = yield self._generate_user_id(attempts > 0)
+                user = UserID(localpart, self.hs.hostname)
+                user_id = user.to_string()
+                yield self.check_user_id_is_valid(user_id)
+                if generate_token:
+                    token = self.auth_handler().generate_access_token(user_id)
                 try:
-                    localpart = self._generate_user_id()
-                    user = UserID(localpart, self.hs.hostname)
-                    user_id = user.to_string()
-                    yield self.check_user_id_is_valid(user_id)
-                    if generate_token:
-                        token = self.auth_handler().generate_access_token(user_id)
                     yield self.store.register(
                         user_id=user_id,
                         token=token,
                         password_hash=password_hash,
                         make_guest=make_guest
                     )
-
-                    yield registered_user(self.distributor, user)
                 except SynapseError:
                     # if user id is taken, just generate another
                     user_id = None
                     token = None
                     attempts += 1
-                    if attempts > 5:
-                        raise RegistrationError(
-                            500, "Cannot generate user ID.")
+            yield registered_user(self.distributor, user)
 
         # We used to generate default identicons here, but nowadays
         # we want clients to generate their own as part of their branding
@@ -283,8 +292,16 @@ class RegistrationHandler(BaseHandler):
                     errcode=Codes.EXCLUSIVE
                 )
 
-    def _generate_user_id(self):
-        return "-" + stringutils.random_string(18)
+    @defer.inlineCallbacks
+    def _generate_user_id(self, reseed=False):
+        if reseed or self._next_generated_user_id is None:
+            self._next_generated_user_id = (
+                yield self.store.find_next_generated_user_id_localpart()
+            )
+
+        id = self._next_generated_user_id
+        self._next_generated_user_id += 1
+        defer.returnValue(str(id))
 
     @defer.inlineCallbacks
     def _validate_captcha(self, ip_addr, private_key, challenge, response):