Reset master xmin when hot_standby_feedback disabled.

If walsender has xmin of standby then ensure we
reset the value to 0 when we change from hot_standby_feedback=on
to hot_standby_feedback=off.

src/backend/replication/walreceiver.c

@@ -124,7 +124,7 @@ static void XLogWalRcvProcessMsg(unsigned char type, char *buf, Size len);
 static void XLogWalRcvWrite(char *buf, Size nbytes, XLogRecPtr recptr);
 static void XLogWalRcvFlush(bool dying);
 static void XLogWalRcvSendReply(void);
-static void XLogWalRcvSendHSFeedback(void);
+static void XLogWalRcvSendHSFeedback(bool immed);
 
 /* Signal handlers */
 static void WalRcvSigHupHandler(SIGNAL_ARGS);
@@ -310,6 +310,7 @@ WalReceiverMain(void)
 		{
 			got_SIGHUP = false;
 			ProcessConfigFile(PGC_SIGHUP);
+			XLogWalRcvSendHSFeedback(true);
 		}
 
 		/* Wait a while for data to arrive */
@@ -338,7 +339,7 @@ WalReceiverMain(void)
 			 * master anyway, to report any progress in applying WAL.
 			 */
 			XLogWalRcvSendReply();
-			XLogWalRcvSendHSFeedback();
+			XLogWalRcvSendHSFeedback(false);
 		}
 	}
 }
@@ -591,7 +592,7 @@ XLogWalRcvFlush(bool dying)
 		if (!dying)
 		{
 			XLogWalRcvSendReply();
-			XLogWalRcvSendHSFeedback();
+			XLogWalRcvSendHSFeedback(false);
 		}
 	}
 }
@@ -651,45 +652,62 @@ XLogWalRcvSendReply(void)
 /*
  * Send hot standby feedback message to primary, plus the current time,
  * in case they don't have a watch.
+ *
+ * If the user disables feedback, send one final message to tell sender
+ * to forget about the xmin on this standby.
  */
 static void
-XLogWalRcvSendHSFeedback(void)
+XLogWalRcvSendHSFeedback(bool immed)
 {
 	char		buf[sizeof(StandbyHSFeedbackMessage) + 1];
 	TimestampTz now;
 	TransactionId nextXid;
 	uint32		nextEpoch;
 	TransactionId xmin;
+	static TimestampTz sendTime = 0;
+	static bool master_has_standby_xmin = false;
 
 	/*
 	 * If the user doesn't want status to be reported to the master, be sure
 	 * to exit before doing anything at all.
 	 */
-	if (wal_receiver_status_interval <= 0 || !hot_standby_feedback)
+	if ((wal_receiver_status_interval <= 0 || !hot_standby_feedback) &&
+		!master_has_standby_xmin)
 		return;
 
 	/* Get current timestamp. */
 	now = GetCurrentTimestamp();
 
-	/*
-	 * Send feedback at most once per wal_receiver_status_interval.
-	 */
-	if (!TimestampDifferenceExceeds(feedback_message.sendTime, now,
+	if (!immed)
+	{
+		/*
+		 * Send feedback at most once per wal_receiver_status_interval.
+		 */
+		if (!TimestampDifferenceExceeds(sendTime, now,
 									wal_receiver_status_interval * 1000))
-		return;
+			return;
+	}
+
+	sendTime = now;
 
 	/*
 	 * If Hot Standby is not yet active there is nothing to send. Check this
 	 * after the interval has expired to reduce number of calls.
 	 */
 	if (!HotStandbyActive())
+	{
+		Assert(!master_has_standby_xmin);
 		return;
+	}
 
 	/*
 	 * Make the expensive call to get the oldest xmin once we are certain
 	 * everything else has been checked.
 	 */
-	xmin = GetOldestXmin(true, false);
+	if (hot_standby_feedback)
+		xmin = GetOldestXmin(true, false);
+	else
+		xmin = InvalidTransactionId;
 
 	/*
 	 * Get epoch and adjust if nextXid and oldestXmin are different sides of
@@ -714,4 +732,8 @@ XLogWalRcvSendHSFeedback(void)
 	buf[0] = 'h';
 	memcpy(&buf[1], &feedback_message, sizeof(StandbyHSFeedbackMessage));
 	walrcv_send(buf, sizeof(StandbyHSFeedbackMessage) + 1);
+	if (TransactionIdIsValid(xmin))
+		master_has_standby_xmin = true;
+	else
+		master_has_standby_xmin = false;
 }

src/backend/replication/walsender.c

@@ -623,9 +623,12 @@ ProcessStandbyHSFeedbackMessage(void)
 		 reply.xmin,
 		 reply.epoch);
 
-	/* Ignore invalid xmin (can't actually happen with current walreceiver) */
+	/* Unset WalSender's xmin if the feedback message value is invalid */
 	if (!TransactionIdIsNormal(reply.xmin))
+	{
+		MyProc->xmin = InvalidTransactionId;
 		return;
+	}
 
 	/*
 	 * Check that the provided xmin/epoch are sane, that is, not in the future