>> Having CheckPointTwoPhase() do the flush would mean shifting the data
>> from KnownPreparedList into TwoPhaseState shmem.
> Er, no. For CheckPointTwoPhase(), at recovery what needs to be done is
> to have all the entries in KnownPreparedList() flushed to disk and
> have those entries removed while holding a shared memory lock.

The KnownPreparedList is constructed by the recovery process.
CheckPointTwoPhase() gets called by the checkpointer process. The
checkpointer does not have access to this valid KnownPreparedList.

>And for
> the rest we need to be careful to have PrescanPreparedTransactions,
> RecoverPreparedTransactions and StandbyRecoverPreparedTransactions
> aware of entries that are in KnownPreparedList().

Yeah, that part is straightforward. It does involve duplication of the
earlier while loops to work against KnownPreparedList. A smart single
while loop which loops over the 2PC files followed by the list would
help here :-)

> Let's leave the
> business of putting the information from KnownPreparedList to
> TwoPhaseState in RecoverPreparedTransactions, which need to be aware
> of entries in KnownPreparedList() anyway. The only thing that differs
> is how the 2PC information is fetched: from the segments or from the
> files in pg_twophase.

Yeah.  This part is also ok. We also got to be careful to mark the
shmem gxact entry with "ondisk = false" and need to set
prepare_start_lsn/prepare_end_lsn properly as well.

>> I wonder what's the best location for this in the common case when we
>> do shutdown of standby.  We could add code in XLOG_CHECKPOINT_SHUTDOWN
>> and XLOG_CHECKPOINT_ONLINE xlog_redo code path.
> ShutdownXLOG() calls CreateRestartPoint() when a standby shuts down,
> so doing all the durability work in CheckPointTwoPhase() would take
> care of any problems.

ShutdownXLOG() gets called from the checkpointer process. See comments
above about the checkpointer not having access to the proper

The following test sequence will trigger the issue:

1) start master
2) start replica
3) prepare a transaction on master
4) shutdown master
5) shutdown replica

CheckPointTwoPhase() in (5) does not sync this prepared transaction
because the checkpointer's KnownPreparedList is empty.

 Nikhil Sontakke                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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