Need to lock when accessing global prepared transaction data during checkpoint. (#10094)

Those data are stored as extended checkpoint so that prepared transactions are
not forgotten after checkpoint. If we access them without lock, it is possible
that we access some inconsistent data. That will lead to some unknown behaviors.
Reviewed-by: NHao Wu <hawu@pivotal.io>
Reviewed-by: NAshwin Agrawal <aagrawal@pivotal.io>

Cherry-picked from 3c284a3b

src/backend/access/transam/twophase.c

@@ -2298,8 +2298,8 @@ TwoPhaseRecoverMirror(void)
 void
 getTwoPhasePreparedTransactionData(prepared_transaction_agg_state **ptas, char *caller)
 {
-	int			numberOfPrepareXacts     = TwoPhaseState->numPrepXacts;
-	GlobalTransaction *globalTransactionArray   = TwoPhaseState->prepXacts;
+	int			numberOfPrepareXacts;
+	GlobalTransaction *globalTransactionArray;
 	TransactionId xid;
 	XLogRecPtr *recordPtr = NULL;
 	int			maxCount;
@@ -2312,6 +2312,11 @@ getTwoPhasePreparedTransactionData(prepared_transaction_agg_state **ptas, char *
 
 	TwoPhaseAddPreparedTransactionInit(ptas, &maxCount);
 
+	LWLockAcquire(TwoPhaseStateLock, LW_SHARED);
+
+	numberOfPrepareXacts = TwoPhaseState->numPrepXacts;
+	globalTransactionArray = TwoPhaseState->prepXacts;
+
 	elog(PersistentRecovery_DebugPrintLevel(),
 		 "getTwoPhasePreparedTransactionData: numberOfPrepareXacts = %d",
 		 numberOfPrepareXacts);
@@ -2336,6 +2341,8 @@ getTwoPhasePreparedTransactionData(prepared_transaction_agg_state **ptas, char *
 									   recordPtr,
 									   caller);
     }
+
+	LWLockRelease(TwoPhaseStateLock);
 }  /* end getTwoPhasePreparedTransactionData */