Add confirmed_flush column to pg_replication_slots.

There's no reason not to expose both restart_lsn and confirmed_flush
since they have rather distinct meanings. The former is the oldest WAL
still required and valid for both physical and logical slots, whereas
the latter is the location up to which a logical slot's consumer has
confirmed receiving data. Most of the time a slot will require older
WAL (i.e. restart_lsn) than the confirmed
position (i.e. confirmed_flush_lsn).

Author: Marko Tiikkaja, editorialized by me
Discussion: 559D110B.1020109@joh.to

contrib/test_decoding/expected/ddl.out

@@ -673,7 +673,7 @@ SELECT pg_drop_replication_slot('regression_slot');
 
 /* check that the slot is gone */
 SELECT * FROM pg_replication_slots;
- slot_name | plugin | slot_type | datoid | database | active | active_pid | xmin | catalog_xmin | restart_lsn 
------------+--------+-----------+--------+----------+--------+------------+------+--------------+-------------
+ slot_name | plugin | slot_type | datoid | database | active | active_pid | xmin | catalog_xmin | restart_lsn | confirmed_flush_lsn 
+-----------+--------+-----------+--------+----------+--------+------------+------+--------------+-------------+---------------------
 (0 rows)
 

doc/src/sgml/catalogs.sgml

@@ -5577,6 +5577,17 @@
       automatically removed during checkpoints.
       </entry>
      </row>
+
+     <row>
+      <entry><structfield>confirmed_flush</structfield></entry>
+      <entry><type>pg_lsn</type></entry>
+      <entry></entry>
+      <entry>The address (<literal>LSN</literal>) up to which the logical
+      slot's consumer has confirmed receiving data. Data older than this is
+      not available anymore. <literal>NULL</> for physical slots.
+      </entry>
+     </row>
+
     </tbody>
    </tgroup>
   </table>

doc/src/sgml/high-availability.sgml

@@ -934,9 +934,9 @@ postgres=# SELECT * FROM pg_create_physical_replication_slot('node_a_slot');
  node_a_slot |
 
 postgres=# SELECT * FROM pg_replication_slots;
-  slot_name  | slot_type | datoid | database | active | xmin | restart_lsn
--------------+-----------+--------+----------+--------+------+-------------
- node_a_slot | physical  |        |          | f      |      |
+  slot_name  | slot_type | datoid | database | active | xmin | restart_lsn | confirmed_flush_lsn
+-------------+-----------+--------+----------+--------+------+-------------+---------------------
+ node_a_slot | physical  |        |          | f      |      |             |
 (1 row)
 </programlisting>
      To configure the standby to use this slot, <varname>primary_slot_name</>

doc/src/sgml/logicaldecoding.sgml

@@ -62,10 +62,10 @@ postgres=# SELECT * FROM pg_create_logical_replication_slot('regression_slot', '
  regression_slot | 0/16B1970
 (1 row)
 
-postgres=# SELECT slot_name, plugin, slot_type, database, active, restart_lsn FROM pg_replication_slots;
-    slot_name    |    plugin     | slot_type | database | active | restart_lsn
------------------+---------------+-----------+----------+--------+-------------
- regression_slot | test_decoding | logical   | postgres | f      | 0/16A4408
+postgres=# SELECT slot_name, plugin, slot_type, database, active, restart_lsn, confirmed_flush_lsn FROM pg_replication_slots;
+    slot_name    |    plugin     | slot_type | database | active | restart_lsn | confirmed_flush_lsn
+-----------------+---------------+-----------+----------+--------+-------------+-----------------
+ regression_slot | test_decoding | logical   | postgres | f      | 0/16A4408   | 0/16A4440
 (1 row)
 
 postgres=# -- There are no changes to see yet

src/backend/catalog/system_views.sql

@@ -676,7 +676,8 @@ CREATE VIEW pg_replication_slots AS
             L.active_pid,
             L.xmin,
             L.catalog_xmin,
-            L.restart_lsn
+            L.restart_lsn,
+            L.confirmed_flush_lsn
     FROM pg_get_replication_slots() AS L
             LEFT JOIN pg_database D ON (L.datoid = D.oid);
 

src/backend/replication/slotfuncs.c

@@ -158,7 +158,7 @@ pg_drop_replication_slot(PG_FUNCTION_ARGS)
 Datum
 pg_get_replication_slots(PG_FUNCTION_ARGS)
 {
-#define PG_GET_REPLICATION_SLOTS_COLS 9
+#define PG_GET_REPLICATION_SLOTS_COLS 10
 	ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
 	TupleDesc	tupdesc;
 	Tuplestorestate *tupstore;
@@ -206,6 +206,7 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 		TransactionId xmin;
 		TransactionId catalog_xmin;
 		XLogRecPtr	restart_lsn;
+		XLogRecPtr	confirmed_flush_lsn;
 		pid_t		active_pid;
 		Oid			database;
 		NameData	slot_name;
@@ -224,6 +225,7 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 			catalog_xmin = slot->data.catalog_xmin;
 			database = slot->data.database;
 			restart_lsn = slot->data.restart_lsn;
+			confirmed_flush_lsn = slot->data.confirmed_flush;
 			namecpy(&slot_name, &slot->data.name);
 			namecpy(&plugin, &slot->data.plugin);
 
@@ -273,6 +275,11 @@ pg_get_replication_slots(PG_FUNCTION_ARGS)
 		else
 			nulls[i++] = true;
 
+		if (confirmed_flush_lsn != InvalidXLogRecPtr)
+			values[i++] = LSNGetDatum(confirmed_flush_lsn);
+		else
+			nulls[i++] = true;
+
 		tuplestore_putvalues(tupstore, tupdesc, values, nulls);
 	}
 

src/include/catalog/catversion.h

@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201508062
+#define CATALOG_VERSION_NO	201508101
 
 #endif

src/include/catalog/pg_proc.h

@@ -5197,7 +5197,7 @@ DATA(insert OID = 3779 (  pg_create_physical_replication_slot PGNSP PGUID 12 1 0
 DESCR("create a physical replication slot");
 DATA(insert OID = 3780 (  pg_drop_replication_slot PGNSP PGUID 12 1 0 0 0 f f f f f f v 1 0 2278 "19" _null_ _null_ _null_ _null_ _null_ pg_drop_replication_slot _null_ _null_ _null_ ));
 DESCR("drop a replication slot");
-DATA(insert OID = 3781 (  pg_get_replication_slots	PGNSP PGUID 12 1 10 0 0 f f f f f t s 0 0 2249 "" "{19,19,25,26,16,23,28,28,3220}" "{o,o,o,o,o,o,o,o,o}" "{slot_name,plugin,slot_type,datoid,active,active_pid,xmin,catalog_xmin,restart_lsn}" _null_ _null_ pg_get_replication_slots _null_ _null_ _null_ ));
+DATA(insert OID = 3781 (  pg_get_replication_slots	PGNSP PGUID 12 1 10 0 0 f f f f f t s 0 0 2249 "" "{19,19,25,26,16,23,28,28,3220,3220}" "{o,o,o,o,o,o,o,o,o,o}" "{slot_name,plugin,slot_type,datoid,active,active_pid,xmin,catalog_xmin,restart_lsn,confirmed_flush_lsn}" _null_ _null_ pg_get_replication_slots _null_ _null_ _null_ ));
 DESCR("information about replication slots currently in use");
 DATA(insert OID = 3786 (  pg_create_logical_replication_slot PGNSP PGUID 12 1 0 0 0 f f f f f f v 2 0 2249 "19 19" "{19,19,25,3220}" "{i,i,o,o}" "{slot_name,plugin,slot_name,xlog_position}" _null_ _null_ pg_create_logical_replication_slot _null_ _null_ _null_ ));
 DESCR("set up a logical replication slot");

src/test/regress/expected/rules.out

@@ -1416,8 +1416,9 @@ pg_replication_slots| SELECT l.slot_name,
     l.active_pid,
     l.xmin,
     l.catalog_xmin,
-    l.restart_lsn
-   FROM (pg_get_replication_slots() l(slot_name, plugin, slot_type, datoid, active, active_pid, xmin, catalog_xmin, restart_lsn)
+    l.restart_lsn,
+    l.confirmed_flush_lsn
+   FROM (pg_get_replication_slots() l(slot_name, plugin, slot_type, datoid, active, active_pid, xmin, catalog_xmin, restart_lsn, confirmed_flush_lsn)
      LEFT JOIN pg_database d ON ((l.datoid = d.oid)));
 pg_roles| SELECT pg_authid.rolname,
     pg_authid.rolsuper,