Re: Potential data loss due to race condition during logical replication slot creation

On Mon, Mar 18, 2024 at 6:08 PM Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> If so, one idea to
> achieve could be that we maintain the highest_running_xid while
> serailizing the snapshot and then during restore if that
> highest_running_xid is <= builder->initial_xmin_horizon, then we
> ignore restoring the snapshot. We already have few such cases handled
> in SnapBuildRestore().

I think that builder->initial_xmin_horizon could be older than
highest_running_xid, for example, when there is a logical replication
slot whose catalog_xmin is old. However, even in this case, we might
need to ignore restoring the snapshot. For example, a slightly
modified test case still can cause the same problem.

The test case in the Kuroda-san's v2 patch:
permutation "s0_init" "s0_begin" "s0_insert1" "s1_init"
"s2_checkpoint" "s2_get_changes_slot0" "s0_insert2" "s0_commit"
"s1_get_changes_slot0"\ "s1_get_changes_slot1"

Modified-version test case (add "s0_insert1" between "s0_init" and "s0_begin"):
permutation "s0_init" "s0_insert1" "s0_begin" "s0_insert1" "s1_init"
"s2_checkpoint" "s2_get_changes_slot0" "s0_insert2" "s0_commit"
"s1_get_changes_slot0\ " "s1_get_changes_slot1"

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com