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.

2 files changed, 72 insertions, 19 deletions

diff --git a/synapse/handlers/register.py b/synapse/handlers/register.py
index b8fbcf9233..2660fd21a2 100644
--- a/synapse/handlers/register.py
+++ b/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):
diff --git a/synapse/storage/registration.py b/synapse/storage/registration.py
index bd35e19be6..967c732bda 100644
--- a/synapse/storage/registration.py
+++ b/synapse/storage/registration.py
@@ -13,6 +13,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import re
+
 from twisted.internet import defer
 
 from synapse.api.errors import StoreError, Codes
@@ -351,3 +353,37 @@ class RegistrationStore(SQLBaseStore):
 
         ret = yield self.runInteraction("count_users", _count_users)
         defer.returnValue(ret)
+
+    @defer.inlineCallbacks
+    def find_next_generated_user_id_localpart(self):
+        """
+        Gets the localpart of the next generated user ID.
+
+        Generated user IDs are integers, and we aim for them to be as small as
+        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.
+        """
+        def _find_next_generated_user_id(txn):
+            txn.execute("SELECT name FROM users")
+            rows = self.cursor_to_dict(txn)
+
+            regex = re.compile("^@(\d+):")
+
+            found = set()
+
+            for r in rows:
+                user_id = r["name"]
+                match = regex.search(user_id)
+                if match:
+                    found.add(int(match.group(1)))
+            for i in xrange(len(found) + 1):
+                if i not in found:
+                    return i
+
+        defer.returnValue((yield self.runInteraction(
+            "find_next_generated_user_id",
+            _find_next_generated_user_id
+        )))