Reorder actions in ProcArrayApplyRecoveryInfo()

Since 5a1dfde, 2PC filenames use FullTransactionId.  Thus, it needs to
convert TransactionId to FullTransactionId in StandbyTransactionIdIsPrepared()
using TransamVariables->nextXid.  However, ProcArrayApplyRecoveryInfo()
first releases locks with usage StandbyTransactionIdIsPrepared(), then advances
TransamVariables->nextXid.  This sequence of actions could cause errors.
This commit makes ProcArrayApplyRecoveryInfo() advance
TransamVariables->nextXid before releasing locks.

Reported-by: Thomas Munro, Michael Paquier
Discussion: https://postgr.es/m/CA%2BhUKGLj_ve1_pNAnxwYU9rDcv7GOhsYXJt7jMKSA%3D5-6ss-Cw%40mail.gmail.com
Discussion: https://postgr.es/m/Zadp9f4E1MYvMJqe%40paquier.xyz

Author: akorotkov

src/backend/storage/ipc/procarray.c

@@ -1053,6 +1053,7 @@ void
 ProcArrayApplyRecoveryInfo(RunningTransactions running)
 {
 	TransactionId *xids;
+	TransactionId xid;
 	int			nxids;
 	int			i;
 
@@ -1066,6 +1067,16 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running)
 	 */
 	ExpireOldKnownAssignedTransactionIds(running->oldestRunningXid);
 
+	/*
+	 * Adjust TransamVariables->nextXid before StandbyReleaseOldLocks(),
+	 * because we will need it up to date for accessing two-phase transactions
+	 * in StandbyReleaseOldLocks().
+	 */
+	xid = running->nextXid;
+	TransactionIdRetreat(xid);
+	AdvanceNextFullTransactionIdPastXid(xid);
+	Assert(FullTransactionIdIsValid(TransamVariables->nextXid));
+
 	/*
 	 * Remove stale locks, if any.
 	 */
@@ -1275,11 +1286,6 @@ ProcArrayApplyRecoveryInfo(RunningTransactions running)
 
 	LWLockRelease(ProcArrayLock);
 
-	/* TransamVariables->nextXid must be beyond any observed xid. */
-	AdvanceNextFullTransactionIdPastXid(latestObservedXid);
-
-	Assert(FullTransactionIdIsValid(TransamVariables->nextXid));
-
 	KnownAssignedXidsDisplay(DEBUG3);
 	if (standbyState == STANDBY_SNAPSHOT_READY)
 		elog(DEBUG1, "recovery snapshots are now enabled");