From 8bb7b15894462c6a0ba81f7198f23a140100331d Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Tue, 26 Nov 2019 15:50:17 +0000
Subject: [PATCH 1/3] Fix find_next_generated_user_id_localpart

---
 .../storage/data_stores/main/registration.py    | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/synapse/storage/data_stores/main/registration.py b/synapse/storage/data_stores/main/registration.py
index 6a594c160c08..c124bbb88bd9 100644
--- a/synapse/storage/data_stores/main/registration.py
+++ b/synapse/storage/data_stores/main/registration.py
@@ -19,7 +19,6 @@
 import re
 
 from six import iterkeys
-from six.moves import range
 
 from twisted.internet import defer
 from twisted.internet.defer import Deferred
@@ -482,12 +481,8 @@ 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 1000 being pre-allocated and starting at 1001 while
-        0-999 are available.
+        Generated user IDs are integers, so we find the largest integer user ID
+        already taken and return that plus one.
         """
 
         def _find_next_generated_user_id(txn):
@@ -503,9 +498,11 @@ def _find_next_generated_user_id(txn):
                 match = regex.search(user_id)
                 if match:
                     found.add(int(match.group(1)))
-            for i in range(len(found) + 1):
-                if i not in found:
-                    return i
+
+            if not found:
+                return 1
+
+            return max(found) + 1
 
         return (
             (

From ba110a2030274dc785b37b1d5ad20acc4de9c612 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Tue, 26 Nov 2019 15:54:10 +0000
Subject: [PATCH 2/3] Newsfile

---
 changelog.d/6420.bugfix | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 changelog.d/6420.bugfix

diff --git a/changelog.d/6420.bugfix b/changelog.d/6420.bugfix
new file mode 100644
index 000000000000..aef47cccaaa8
--- /dev/null
+++ b/changelog.d/6420.bugfix
@@ -0,0 +1 @@
+Fix broken guest registration when there are existing blocks of numeric user IDs.

From f8f14ba466a18ee38827629b8e9ab2d5931e5713 Mon Sep 17 00:00:00 2001
From: Erik Johnston <erik@matrix.org>
Date: Tue, 26 Nov 2019 16:06:41 +0000
Subject: [PATCH 3/3] Don't construct a set

---
 synapse/storage/data_stores/main/registration.py | 9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)

diff --git a/synapse/storage/data_stores/main/registration.py b/synapse/storage/data_stores/main/registration.py
index c124bbb88bd9..0a3c1f0510a3 100644
--- a/synapse/storage/data_stores/main/registration.py
+++ b/synapse/storage/data_stores/main/registration.py
@@ -492,17 +492,14 @@ def _find_next_generated_user_id(txn):
 
             regex = re.compile(r"^@(\d+):")
 
-            found = set()
+            max_found = 0
 
             for (user_id,) in txn:
                 match = regex.search(user_id)
                 if match:
-                    found.add(int(match.group(1)))
+                    max_found = max(int(match.group(1)), max_found)
 
-            if not found:
-                return 1
-
-            return max(found) + 1
+            return max_found + 1
 
         return (
             (