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/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
+        )))