2 files changed, 5 insertions, 1 deletions
diff --git a/changelog.d/13226.bugfix b/changelog.d/13226.bugfix
new file mode 100644
index 0000000000..df96d41f37
--- /dev/null
+++ b/changelog.d/13226.bugfix
@@ -0,0 +1 @@
+Fix a long-standing bug where the `synapse_port_db` script could fail to copy rows with negative row ids.
diff --git a/synapse/_scripts/synapse_port_db.py b/synapse/_scripts/synapse_port_db.py
index 642fd41629..26834a437e 100755
--- a/synapse/_scripts/synapse_port_db.py
+++ b/synapse/_scripts/synapse_port_db.py
@@ -418,12 +418,15 @@ class Porter:
             self.progress.update(table, table_size)  # Mark table as done
             return
 
+        # We sweep over rowids in two directions: one forwards (rowids 1, 2, 3, ...)
+        # and another backwards (rowids 0, -1, -2, ...).
         forward_select = (
             "SELECT rowid, * FROM %s WHERE rowid >= ? ORDER BY rowid LIMIT ?" % (table,)
         )
 
         backward_select = (
-            "SELECT rowid, * FROM %s WHERE rowid <= ? ORDER BY rowid LIMIT ?" % (table,)
+            "SELECT rowid, * FROM %s WHERE rowid <= ? ORDER BY rowid DESC LIMIT ?"
+            % (table,)
         )
 
         do_forward = [True]