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.

doc/src/sgml/protocol.sgml

@@ -1617,7 +1617,10 @@ The commands accepted in walsender mode are:
       </term>
       <listitem>
       <para>
-          The standby's current xmin.
+          The standby's current xmin. This may be 0, if the standby is
+          sending notification that Hot Standby feedback will no
+          longer be sent on this connection. Later non-zero messages may
+          reinitiate the feedback mechanism.
       </para>
       </listitem>
       </varlistentry>

src/backend/replication/walreceiver.c

@@ -38,6 +38,7 @@
 #include <signal.h>
 #include <unistd.h>
 
+#include "access/transam.h"
 #include "access/xlog_internal.h"
 #include "libpq/pqsignal.h"
 #include "miscadmin.h"
@@ -123,7 +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);
 static void ProcessWalSndrMessage(XLogRecPtr walEnd, TimestampTz sendTime);
 
 /* Signal handlers */
@@ -312,6 +313,7 @@ WalReceiverMain(void)
 		{
 			got_SIGHUP = false;
 			ProcessConfigFile(PGC_SIGHUP);
+			XLogWalRcvSendHSFeedback(true);
 		}
 
 		/* Wait a while for data to arrive */
@@ -340,7 +342,7 @@ WalReceiverMain(void)
 			 * master anyway, to report any progress in applying WAL.
 			 */
 			XLogWalRcvSendReply();
-			XLogWalRcvSendHSFeedback();
+			XLogWalRcvSendHSFeedback(false);
 		}
 	}
 }
@@ -621,7 +623,7 @@ XLogWalRcvFlush(bool dying)
 		if (!dying)
 		{
 			XLogWalRcvSendReply();
-			XLogWalRcvSendHSFeedback();
+			XLogWalRcvSendHSFeedback(false);
 		}
 	}
 }
@@ -681,45 +683,62 @@ 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
@@ -744,6 +763,10 @@ 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

@@ -656,9 +656,12 @@ 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))
+	{
+		MyPgXact->xmin = InvalidTransactionId;
 		return;
+	}
 
 	/*
 	 * Check that the provided xmin/epoch are sane, that is, not in the future