Track conflict_reason in pg_replication_slots.

This patch changes the existing 'conflicting' field to 'conflict_reason'
in pg_replication_slots. This new field indicates the reason for the
logical slot's conflict with recovery. It is always NULL for physical
slots, as well as for logical slots which are not invalidated. The
non-NULL values indicate that the slot is marked as invalidated. Possible
values are:

wal_removed = required WAL has been removed.
rows_removed = required rows have been removed.
wal_level_insufficient = the primary doesn't have a wal_level sufficient
to perform logical decoding.

The existing users of 'conflicting' column can get the same answer by
using 'conflict_reason' IS NOT NULL.

Author: Shveta Malik
Reviewed-by: Amit Kapila, Bertrand Drouvot, Michael Paquier
Discussion: https://postgr.es/m/ZYOE8IguqTbp-seF@paquier.xyz

Author: Amit Kapila

doc/src/sgml/system-views.sgml

@@ -2525,11 +2525,34 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
 
      <row>
       <entry role="catalog_table_entry"><para role="column_definition">
-       <structfield>conflicting</structfield> <type>bool</type>
+       <structfield>conflict_reason</structfield> <type>text</type>
       </para>
       <para>
-       True if this logical slot conflicted with recovery (and so is now
-       invalidated). Always NULL for physical slots.
+       The reason for the logical slot's conflict with recovery. It is always
+       NULL for physical slots, as well as for logical slots which are not
+       invalidated. The non-NULL values indicate that the slot is marked
+       as invalidated. Possible values are:
+       <itemizedlist spacing="compact">
+        <listitem>
+         <para>
+          <literal>wal_removed</literal> means that the required WAL has been
+          removed.
+         </para>
+        </listitem>
+        <listitem>
+         <para>
+          <literal>rows_removed</literal> means that the required rows have
+          been removed.
+         </para>
+        </listitem>
+        <listitem>
+         <para>
+          <literal>wal_level_insufficient</literal> means that the
+          primary doesn't have a <xref linkend="guc-wal-level"/> sufficient to
+          perform logical decoding.
+         </para>
+        </listitem>
+       </itemizedlist>
       </para></entry>
      </row>
     </tbody>

src/backend/catalog/system_views.sql

@@ -1023,7 +1023,7 @@ CREATE VIEW pg_replication_slots AS
             L.wal_status,
             L.safe_wal_size,
             L.two_phase,
-            L.conflicting
+            L.conflict_reason
     FROM pg_get_replication_slots() AS L
             LEFT JOIN pg_database D ON (L.datoid = D.oid);
 

src/backend/replication/slotfuncs.c

@@ -406,10 +406,24 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 			nulls[i++] = true;
 		else
 		{
-			if (slot_contents.data.invalidated != RS_INVAL_NONE)
-				values[i++] = BoolGetDatum(true);
-			else
-				values[i++] = BoolGetDatum(false);
+			switch (slot_contents.data.invalidated)
+			{
+				case RS_INVAL_NONE:
+					nulls[i++] = true;
+					break;
+
+				case RS_INVAL_WAL_REMOVED:
+					values[i++] = CStringGetTextDatum("wal_removed");
+					break;
+
+				case RS_INVAL_HORIZON:
+					values[i++] = CStringGetTextDatum("rows_removed");
+					break;
+
+				case RS_INVAL_WAL_LEVEL:
+					values[i++] = CStringGetTextDatum("wal_level_insufficient");
+					break;
+			}
 		}
 
 		Assert(i == PG_GET_REPLICATION_SLOTS_COLS);

src/bin/pg_upgrade/info.c

@@ -667,13 +667,13 @@ get_old_cluster_logical_slot_infos(DbInfo *dbinfo, bool live_check)
 	 * removed.
 	 */
 	res = executeQueryOrDie(conn, "SELECT slot_name, plugin, two_phase, "
-							"%s as caught_up, conflicting as invalid "
+							"%s as caught_up, conflict_reason IS NOT NULL as invalid "
 							"FROM pg_catalog.pg_replication_slots "
 							"WHERE slot_type = 'logical' AND "
 							"database = current_database() AND "
 							"temporary IS FALSE;",
 							live_check ? "FALSE" :
-							"(CASE WHEN conflicting THEN FALSE "
+							"(CASE WHEN conflict_reason IS NOT NULL THEN FALSE "
 							"ELSE (SELECT pg_catalog.binary_upgrade_logical_slot_has_caught_up(slot_name)) "
 							"END)");
 

src/include/catalog/catversion.h

@@ -57,6 +57,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	202401021
+#define CATALOG_VERSION_NO	202401041
 
 #endif

src/include/catalog/pg_proc.dat

@@ -11115,9 +11115,9 @@
   proname => 'pg_get_replication_slots', prorows => '10', proisstrict => 'f',
   proretset => 't', provolatile => 's', prorettype => 'record',
   proargtypes => '',
-  proallargtypes => '{name,name,text,oid,bool,bool,int4,xid,xid,pg_lsn,pg_lsn,text,int8,bool,bool}',
+  proallargtypes => '{name,name,text,oid,bool,bool,int4,xid,xid,pg_lsn,pg_lsn,text,int8,bool,text}',
   proargmodes => '{o,o,o,o,o,o,o,o,o,o,o,o,o,o,o}',
-  proargnames => '{slot_name,plugin,slot_type,datoid,temporary,active,active_pid,xmin,catalog_xmin,restart_lsn,confirmed_flush_lsn,wal_status,safe_wal_size,two_phase,conflicting}',
+  proargnames => '{slot_name,plugin,slot_type,datoid,temporary,active,active_pid,xmin,catalog_xmin,restart_lsn,confirmed_flush_lsn,wal_status,safe_wal_size,two_phase,conflict_reason}',
   prosrc => 'pg_get_replication_slots' },
 { oid => '3786', descr => 'set up a logical replication slot',
   proname => 'pg_create_logical_replication_slot', provolatile => 'v',

src/test/recovery/t/035_standby_logical_decoding.pl

@@ -168,27 +168,25 @@ sub change_hot_standby_feedback_and_wait_for_xmins
 	}
 }
 
-# Check conflicting status in pg_replication_slots.
-sub check_slots_conflicting_status
+# Check conflict_reason in pg_replication_slots.
+sub check_slots_conflict_reason
 {
-	my ($conflicting) = @_;
+	my ($slot_prefix, $reason) = @_;
 
-	if ($conflicting)
-	{
-		$res = $node_standby->safe_psql(
-			'postgres', qq(
-				 select bool_and(conflicting) from pg_replication_slots;));
+	my $active_slot = $slot_prefix . 'activeslot';
+	my $inactive_slot = $slot_prefix . 'inactiveslot';
 
-		is($res, 't', "Logical slots are reported as conflicting");
-	}
-	else
-	{
-		$res = $node_standby->safe_psql(
-			'postgres', qq(
-				select bool_or(conflicting) from pg_replication_slots;));
+	$res = $node_standby->safe_psql(
+		'postgres', qq(
+			 select conflict_reason from pg_replication_slots where slot_name = '$active_slot';));
 
-		is($res, 'f', "Logical slots are reported as non conflicting");
-	}
+	is($res, "$reason", "$active_slot conflict_reason is $reason");
+
+	$res = $node_standby->safe_psql(
+		'postgres', qq(
+			 select conflict_reason from pg_replication_slots where slot_name = '$inactive_slot';));
+
+	is($res, "$reason", "$inactive_slot conflict_reason is $reason");
 }
 
 # Drop the slots, re-create them, change hot_standby_feedback,
@@ -260,13 +258,13 @@ sub check_for_invalidation
 	qq[SELECT * FROM pg_create_physical_replication_slot('$primary_slotname');]
 );
 
-# Check conflicting is NULL for physical slot
+# Check conflict_reason is NULL for physical slot
 $res = $node_primary->safe_psql(
 	'postgres', qq[
-		 SELECT conflicting is null FROM pg_replication_slots where slot_name = '$primary_slotname';]
+		 SELECT conflict_reason is null FROM pg_replication_slots where slot_name = '$primary_slotname';]
 );
 
-is($res, 't', "Physical slot reports conflicting as NULL");
+is($res, 't', "Physical slot reports conflict_reason as NULL");
 
 my $backup_name = 'b1';
 $node_primary->backup($backup_name);
@@ -483,8 +481,8 @@ sub check_for_invalidation
 # Check invalidation in the logfile and in pg_stat_database_conflicts
 check_for_invalidation('vacuum_full_', 1, 'with vacuum FULL on pg_class');
 
-# Verify slots are reported as conflicting in pg_replication_slots
-check_slots_conflicting_status(1);
+# Verify conflict_reason is 'rows_removed' in pg_replication_slots
+check_slots_conflict_reason('vacuum_full_', 'rows_removed');
 
 $handle =
   make_slot_active($node_standby, 'vacuum_full_', 0, \$stdout, \$stderr);
@@ -502,16 +500,16 @@ sub check_for_invalidation
 ##################################################
 $node_standby->restart;
 
-# Verify slots are reported as conflicting in pg_replication_slots
-check_slots_conflicting_status(1);
+# Verify conflict_reason is retained across a restart.
+check_slots_conflict_reason('vacuum_full_', 'rows_removed');
 
 ##################################################
 # Verify that invalidated logical slots do not lead to retaining WAL.
 ##################################################
 
 # Get the restart_lsn from an invalidated slot
 my $restart_lsn = $node_standby->safe_psql('postgres',
-	"SELECT restart_lsn from pg_replication_slots WHERE slot_name = 'vacuum_full_activeslot' and conflicting is true;"
+	"SELECT restart_lsn from pg_replication_slots WHERE slot_name = 'vacuum_full_activeslot' and conflict_reason is not null;"
 );
 
 chomp($restart_lsn);
@@ -565,8 +563,8 @@ sub check_for_invalidation
 # Check invalidation in the logfile and in pg_stat_database_conflicts
 check_for_invalidation('row_removal_', $logstart, 'with vacuum on pg_class');
 
-# Verify slots are reported as conflicting in pg_replication_slots
-check_slots_conflicting_status(1);
+# Verify conflict_reason is 'rows_removed' in pg_replication_slots
+check_slots_conflict_reason('row_removal_', 'rows_removed');
 
 $handle =
   make_slot_active($node_standby, 'row_removal_', 0, \$stdout, \$stderr);
@@ -604,8 +602,8 @@ sub check_for_invalidation
 check_for_invalidation('shared_row_removal_', $logstart,
 	'with vacuum on pg_authid');
 
-# Verify slots are reported as conflicting in pg_replication_slots
-check_slots_conflicting_status(1);
+# Verify conflict_reason is 'rows_removed' in pg_replication_slots
+check_slots_conflict_reason('shared_row_removal_', 'rows_removed');
 
 $handle = make_slot_active($node_standby, 'shared_row_removal_', 0, \$stdout,
 	\$stderr);
@@ -657,7 +655,13 @@ sub check_for_invalidation
 ) or die "Timed out waiting confl_active_logicalslot to be updated";
 
 # Verify slots are reported as non conflicting in pg_replication_slots
-check_slots_conflicting_status(0);
+is( $node_standby->safe_psql(
+		'postgres',
+		q[select bool_or(conflicting) from
+		  (select conflict_reason is not NULL as conflicting
+		   from pg_replication_slots WHERE slot_type = 'logical')]),
+	'f',
+	'Logical slots are reported as non conflicting');
 
 # Turn hot_standby_feedback back on
 change_hot_standby_feedback_and_wait_for_xmins(1, 0);
@@ -693,8 +697,8 @@ sub check_for_invalidation
 # Check invalidation in the logfile and in pg_stat_database_conflicts
 check_for_invalidation('pruning_', $logstart, 'with on-access pruning');
 
-# Verify slots are reported as conflicting in pg_replication_slots
-check_slots_conflicting_status(1);
+# Verify conflict_reason is 'rows_removed' in pg_replication_slots
+check_slots_conflict_reason('pruning_', 'rows_removed');
 
 $handle = make_slot_active($node_standby, 'pruning_', 0, \$stdout, \$stderr);
 
@@ -737,8 +741,8 @@ sub check_for_invalidation
 # Check invalidation in the logfile and in pg_stat_database_conflicts
 check_for_invalidation('wal_level_', $logstart, 'due to wal_level');
 
-# Verify slots are reported as conflicting in pg_replication_slots
-check_slots_conflicting_status(1);
+# Verify conflict_reason is 'wal_level_insufficient' in pg_replication_slots
+check_slots_conflict_reason('wal_level_', 'wal_level_insufficient');
 
 $handle =
   make_slot_active($node_standby, 'wal_level_', 0, \$stdout, \$stderr);

src/test/regress/expected/rules.out

@@ -1473,8 +1473,8 @@ pg_replication_slots| SELECT l.slot_name,
     l.wal_status,
     l.safe_wal_size,
     l.two_phase,
-    l.conflicting
-   FROM (pg_get_replication_slots() l(slot_name, plugin, slot_type, datoid, temporary, active, active_pid, xmin, catalog_xmin, restart_lsn, confirmed_flush_lsn, wal_status, safe_wal_size, two_phase, conflicting)
+    l.conflict_reason
+   FROM (pg_get_replication_slots() l(slot_name, plugin, slot_type, datoid, temporary, active, active_pid, xmin, catalog_xmin, restart_lsn, confirmed_flush_lsn, wal_status, safe_wal_size, two_phase, conflict_reason)
      LEFT JOIN pg_database d ON ((l.datoid = d.oid)));
 pg_roles| SELECT pg_authid.rolname,
     pg_authid.rolsuper,