+ * * RecoverPreparedTransactions(), StandbyRecoverPreparedTransactions() + * and PrescanPreparedTransactions() have been modified to go throug + * gxact->inredo entries that have not made to disk yet.
It seems to me that there should be an initial scan of pg_twophase at the beginning of recovery, discarding on the way with a WARNING entries that are older than the checkpoint redo horizon. This should fill in shmem entries using something close to PrepareRedoAdd(), and mark those entries as inredo. Then, at the end of recovery, PrescanPreparedTransactions does not need to look at the entries in pg_twophase. And that's the case as well of RecoverPreparedTransaction(). I think that you could get the patch much simplified this way, as any 2PC data can be fetched directly from WAL segments and there is no need to rely on scans of pg_twophase, this is replaced by scans of entries in TwoPhaseState.
I don't think this will work. We cannot replace pg_twophase with shmem entries + WAL pointers. This is because we cannot expect to have WAL entries around for long running prepared queries which survive across checkpoints.