Track unconverted device list outbound pokes using a position instead (#14516)

When a local device list change is added to
`device_lists_changes_in_room`, the `converted_to_destinations` flag is
set to `FALSE` and the `_handle_new_device_update_async` background
process is started. This background process looks for unconverted rows
in `device_lists_changes_in_room`, copies them to
`device_lists_outbound_pokes` and updates the flag.

To update the `converted_to_destinations` flag, the database performs a
`DELETE` and `INSERT` internally, which fragments the table. To avoid
this, track unconverted rows using a `(stream ID, room ID)` position
instead of the flag.

From now on, the `converted_to_destinations` column indicates rows that
need converting to outbound pokes, but does not indicate whether the
conversion has already taken place.

Closes #14037.

Signed-off-by: Sean Quah <seanq@matrix.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/changelog.d/14516.misc b/changelog.d/14516.misc
new file mode 100644
index 0000000000..51666c6ffc
--- /dev/null
+++ b/changelog.d/14516.misc
@@ -0,0 +1 @@
+Refactor conversion of device list changes in room to outbound pokes to track unconverted rows using a `(stream ID, room ID)` position instead of updating the `converted_to_destinations` flag on every row.