Add HOLD_INTERRUPTS section into FinishPreparedTransaction.

If an interrupt arrives in the middle of FinishPreparedTransaction
and any callback decide to call CHECK_FOR_INTERRUPTS (e.g.
RemoveTwoPhaseFile can write a warning with ereport, which checks for
interrupts) then it's possible to leave current GXact undeleted.

Backpatch to all supported branches

Stas Kelvich

Discussion: ihttps://www.postgresql.org/message-id/3AD85097-A3F3-4EBA-99BD-C38EDF8D2949@postgrespro.ru

src/backend/access/transam/twophase.c

@@ -1487,6 +1487,9 @@ FinishPreparedTransaction(const char *gid, bool isCommit)
 	/* compute latestXid among all children */
 	latestXid = TransactionIdLatest(xid, hdr->nsubxacts, children);
 
+	/* Prevent cancel/die interrupt while cleaning up */
+	HOLD_INTERRUPTS();
+
 	/*
 	 * The order of operations here is critical: make the XLOG entry for
 	 * commit or abort, then mark the transaction committed or aborted in
@@ -1578,6 +1581,8 @@ FinishPreparedTransaction(const char *gid, bool isCommit)
 	LWLockRelease(TwoPhaseStateLock);
 	MyLockedGxact = NULL;
 
+	RESUME_INTERRUPTS();
+
 	pfree(buf);
 }