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Avoid repeated decoding of prepared transactions after a restart.

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In commit a271a1b50e, we allowed decoding at prepare time and the prepare
was decoded again if there is a restart after decoding it. It was done
that way because we can't distinguish between the cases where we have not
decoded the prepare because it was prior to consistent snapshot or we have
decoded it earlier but restarted. To distinguish between these two cases,
we have introduced an initial_consistent_point at the slot level which is
an LSN at which we found a consistent point at the time of slot creation.
This is also the point where we have exported a snapshot for the initial
copy. So, prepare transaction prior to this point are sent along with
commit prepared.

This commit bumps SNAPBUILD_VERSION because of change in SnapBuild. It
will break existing slots which is fine in a major release.

Author: Ajin Cherian, based on idea by Andres Freund
Reviewed-by: Amit Kapila and Vignesh C
Discussion: https://postgr.es/m/d0f60d60-133d-bf8d-bd70-47784d8fabf3@enterprisedb.com
This commit is contained in:
Amit Kapila
2021-03-01 09:11:18 +05:30
parent 6230912f23
commit 8bdb1332eb
10 changed files with 61 additions and 67 deletions
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10
src/backend/replication/logical/reorderbuffer.c
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@ -2672,6 +2672,7 @@ ReorderBufferPrepare(ReorderBuffer *rb, TransactionId xid,
void
ReorderBufferFinishPrepared(ReorderBuffer *rb, TransactionId xid,
XLogRecPtr commit_lsn, XLogRecPtr end_lsn,
XLogRecPtr initial_consistent_point,
TimestampTz commit_time, RepOriginId origin_id,
XLogRecPtr origin_lsn, char *gid, bool is_commit)
{
@ -2698,12 +2699,11 @@ ReorderBufferFinishPrepared(ReorderBuffer *rb, TransactionId xid,
/*
* It is possible that this transaction is not decoded at prepare time
* either because by that time we didn't have a consistent snapshot or it
* was decoded earlier but we have restarted. We can't distinguish between
* those two cases so we send the prepare in both the cases and let
* downstream decide whether to process or skip it. We don't need to
* decode the xact for aborts if it is not done already.
* was decoded earlier but we have restarted. We only need to send the
* prepare if it was not decoded earlier. We don't need to decode the xact
* for aborts if it is not done already.
*/
if (!rbtxn_prepared(txn) && is_commit)
if ((txn->final_lsn < initial_consistent_point) && is_commit)
{
txn->txn_flags |= RBTXN_PREPARE;