Reset hot standby xmin on master after restart

Hot_standby_feedback could be reset by reload and worked correctly, but if
the server was restarted rather than reloaded the xmin was not reset.
Force reset always if hot_standby_feedback is enabled at startup.

Ants Aasma, Craig Ringer

Reported-by: Ants Aasma

src/backend/replication/walreceiver.c

@@ -1167,7 +1167,10 @@ XLogWalRcvSendReply(bool force, bool requestReply)
  * 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.
+ * to forget about the xmin on this standby. We also send this message
+ * on first connect because a previous connection might have set xmin
+ * on a replication slot. (If we're not using a slot it's harmless to
+ * send a feedback message explicitly setting InvalidTransactionId).
  */
 static void
 XLogWalRcvSendHSFeedback(bool immed)
@@ -1177,7 +1180,8 @@ XLogWalRcvSendHSFeedback(bool immed)
 	uint32		nextEpoch;
 	TransactionId xmin;
 	static TimestampTz sendTime = 0;
-	static bool master_has_standby_xmin = false;
+	/* initially true so we always send at least one feedback message */
+	static bool master_has_standby_xmin = true;
 
 	/*
 	 * If the user doesn't want status to be reported to the master, be sure
@@ -1202,14 +1206,17 @@ XLogWalRcvSendHSFeedback(bool immed)
 	}
 
 	/*
-	 * If Hot Standby is not yet active there is nothing to send. Check this
+	 * If Hot Standby is not yet accepting connections there is nothing to
-	 * after the interval has expired to reduce number of calls.
+	 * send. Check this after the interval has expired to reduce number of
+	 * calls.
+	 *
+	 * Bailing out here also ensures that we don't send feedback until we've
+	 * read our own replication slot state, so we don't tell the master to
+	 * discard needed xmin or catalog_xmin from any slots that may exist
+	 * on this replica.
 	 */
 	if (!HotStandbyActive())
-	{
-		Assert(!master_has_standby_xmin);
 		return;
-	}
 
 	/*
 	 * Make the expensive call to get the oldest xmin once we are certain

src/test/recovery/t/001_stream_rep.pl

@@ -3,7 +3,7 @@ use strict;
 use warnings;
 use PostgresNode;
 use TestLib;
-use Test::More tests => 22;
+use Test::More tests => 24;
 
 # Initialize master node
 my $node_master = get_new_node('master');
@@ -161,3 +161,22 @@ is($catalog_xmin, '', 'non-cascaded slot xmin still null with hs_feedback reset'
 ($xmin, $catalog_xmin) = get_slot_xmins($node_standby_1, $slotname_2);
 is($xmin, '', 'cascaded slot xmin null with hs feedback reset');
 is($catalog_xmin, '', 'cascaded slot xmin still null with hs_feedback reset');
+
+diag "re-enabling hot_standby_feedback and disabling while stopped";
+$node_standby_2->safe_psql('postgres', 'ALTER SYSTEM SET hot_standby_feedback = on;');
+$node_standby_2->reload;
+
+$node_master->safe_psql('postgres', qq[INSERT INTO tab_int VALUES (11000);]);
+replay_check();
+
+$node_standby_2->safe_psql('postgres', 'ALTER SYSTEM SET hot_standby_feedback = off;');
+$node_standby_2->stop;
+
+($xmin, $catalog_xmin) = get_slot_xmins($node_standby_1, $slotname_2);
+isnt($xmin, '', 'cascaded slot xmin non-null with postgres shut down');
+
+# Xmin from a previous run should be cleared on startup.
+$node_standby_2->start;
+
+($xmin, $catalog_xmin) = get_slot_xmins($node_standby_1, $slotname_2);
+is($xmin, '', 'cascaded slot xmin reset after startup with hs feedback reset');