Fix recovery of 2PC transaction during crash recovery

A crash in the middle of a checkpoint with some two-phase state data
already flushed to disk by this checkpoint could cause a follow-up crash
recovery to recover twice the same transaction, once from what has been
found in pg_twophase/ at the beginning of recovery and a second time
when replaying its corresponding record.

This would lead to FATAL failures in the startup process during
recovery, where the same transaction would have a state recovered twice
instead of once:
LOG:  recovering prepared transaction 731 from shared memory
LOG:  recovering prepared transaction 731 from shared memory
FATAL:  lock ExclusiveLock on object 731/0/0 is already held

This issue is fixed by skipping the addition of any 2PC state coming
from a record whose equivalent 2PC state file has already been loaded in
TwoPhaseState at the beginning of recovery by restoreTwoPhaseData(),
which is OK as long as the system has not reached a consistent state.

The timing to get a messed up recovery processing is very racy, and
would very unlikely happen.  The thread that has reported the issue has
demonstrated the bug using injection points to force a PANIC in the
middle of a checkpoint.

Issue introduced in 728bd99, so backpatch all the way down.

Reported-by: "suyu.cmj" <mengjuan.cmj@alibaba-inc.com>
Author: "suyu.cmj" <mengjuan.cmj@alibaba-inc.com>
Author: Michael Paquier
Discussion: https://postgr.es/m/109e6994-b971-48cb-84f6-829646f18b4c.mengjuan.cmj@alibaba-inc.com
Backpatch-through: 11

Author: michaelpq

src/backend/access/transam/twophase.c

@@ -86,6 +86,7 @@
 #include "access/xlog.h"
 #include "access/xloginsert.h"
 #include "access/xlogreader.h"
+#include "access/xlogrecovery.h"
 #include "access/xlogutils.h"
 #include "catalog/pg_type.h"
 #include "catalog/storage.h"
@@ -2477,6 +2478,38 @@ PrepareRedoAdd(char *buf, XLogRecPtr start_lsn,
 	 * that it got added in the redo phase
 	 */
 
+	/*
+	 * In the event of a crash while a checkpoint was running, it may be
+	 * possible that some two-phase data found its way to disk while its
+	 * corresponding record needs to be replayed in the follow-up recovery.
+	 * As the 2PC data was on disk, it has already been restored at the
+	 * beginning of recovery with restoreTwoPhaseData(), so skip this record
+	 * to avoid duplicates in TwoPhaseState.  If a consistent state has been
+	 * reached, the record is added to TwoPhaseState and it should have no
+	 * corresponding file in pg_twophase.
+	 */
+	if (!XLogRecPtrIsInvalid(start_lsn))
+	{
+		char		path[MAXPGPATH];
+
+		TwoPhaseFilePath(path, hdr->xid);
+
+		if (access(path, F_OK) == 0)
+		{
+			ereport(reachedConsistency ? ERROR : WARNING,
+					(errmsg("could not recover two-phase state file for transaction %u",
+							hdr->xid),
+					 errdetail("Two-phase state file has been found in WAL record %X/%X, but this transaction has already been restored from disk.",
+							   LSN_FORMAT_ARGS(start_lsn))));
+			return;
+		}
+
+		if (errno != ENOENT)
+			ereport(ERROR,
+					(errcode_for_file_access(),
+					 errmsg("could not access file \"%s\": %m", path)));
+	}
+
 	/* Get a free gxact from the freelist */
 	if (TwoPhaseState->freeGXacts == NULL)
 		ereport(ERROR,