pgstat: add WAL receiver status view & SRF

This new view provides insight into the state of a running WAL receiver
in a HOT standby node.
The information returned includes the PID of the WAL receiver process,
its status (stopped, starting, streaming, etc), start LSN and TLI, last
received LSN and TLI, timestamp of last message send and receipt, latest
end-of-WAL LSN and time, and the name of the slot (if any).

Access to the detailed data is only granted to superusers; others only
get the PID.

Author: Michael Paquier
Reviewer: Haribabu Kommi

7 files changed, 279 insertions, 1 deletions

diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index c503636a88..85459d04b4 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -301,6 +301,14 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
      </row>
 
      <row>
+      <entry><structname>pg_stat_wal_receiver</><indexterm><primary>pg_stat_wal_receiver</primary></indexterm></entry>
+      <entry>Only one row, showing statistics about the WAL receiver from
+       that receiver's connected server.
+       See <xref linkend="pg-stat-wal-receiver-view"> for details.
+      </entry>
+     </row>
+
+     <row>
       <entry><structname>pg_stat_ssl</><indexterm><primary>pg_stat_ssl</primary></indexterm></entry>
       <entry>One row per connection (regular and replication), showing information about
        SSL used on this connection.
@@ -833,6 +841,89 @@ postgres   27093  0.0  0.0  30096  2752 ?        Ss   11:34   0:00 postgres: ser
    listed; no information is available about downstream standby servers.
   </para>
 
+  <table id="pg-stat-wal-receiver-view" xreflabel="pg_stat_wal_receiver">
+   <title><structname>pg_stat_wal_receiver</structname> View</title>
+   <tgroup cols="3">
+    <thead>
+    <row>
+      <entry>Column</entry>
+      <entry>Type</entry>
+      <entry>Description</entry>
+     </row>
+    </thead>
+
+   <tbody>
+    <row>
+     <entry><structfield>pid</></entry>
+     <entry><type>integer</></entry>
+     <entry>Process ID of the WAL receiver process</entry>
+    </row>
+    <row>
+     <entry><structfield>status</></entry>
+     <entry><type>text</></entry>
+     <entry>Activity status of the WAL receiver process</entry>
+    </row>
+    <row>
+     <entry><structfield>receive_start_lsn</></entry>
+     <entry><type>pg_lsn</></entry>
+     <entry>First transaction log position used when WAL receiver is
+      started</entry>
+    </row>
+    <row>
+     <entry><structfield>receive_start_tli</></entry>
+     <entry><type>integer</></entry>
+     <entry>First timeline number used when WAL receiver is started</entry>
+    </row>
+    <row>
+     <entry><structfield>received_lsn</></entry>
+     <entry><type>pg_lsn</></entry>
+     <entry>Last transaction log position already received and flushed to
+      disk, the initial value of this field being the first log position used
+      when WAL receiver is started</entry>
+    </row>
+    <row>
+     <entry><structfield>received_tli</></entry>
+     <entry><type>integer</></entry>
+     <entry>Timeline number of last transaction log position received and
+      flushed to disk, the initial value of this field being the timeline
+      number of the first log position used when WAL receiver is started
+     </entry>
+    </row>
+    <row>
+     <entry><structfield>last_msg_send_time</></entry>
+     <entry><type>timestamp with time zone</></entry>
+     <entry>Send time of last message received from origin WAL sender</entry>
+    </row>
+    <row>
+     <entry><structfield>last_msg_receipt_time</></entry>
+     <entry><type>timestamp with time zone</></entry>
+     <entry>Receipt time of last message received from origin WAL sender</entry>
+    </row>
+    <row>
+     <entry><structfield>latest_end_lsn</></entry>
+     <entry><type>pg_lsn</></entry>
+     <entry>Last transaction log position reported to origin WAL sender</entry>
+    </row>
+    <row>
+     <entry><structfield>latest_end_time</></entry>
+     <entry><type>timestamp with time zone</></entry>
+     <entry>Time of last transaction log position reported to origin WAL sender</entry>
+    </row>
+    <row>
+     <entry><structfield>slot_name</></entry>
+     <entry><type>text</></entry>
+     <entry>Replication slot name used by this WAL receiver</entry>
+    </row>
+   </tbody>
+   </tgroup>
+  </table>
+
+  <para>
+   The <structname>pg_stat_wal_receiver</structname> view will contain only
+   one row, showing statistics about the WAL receiver from that receiver's
+   connected server.
+  </para>
+
   <table id="pg-stat-ssl-view" xreflabel="pg_stat_ssl">
    <title><structname>pg_stat_ssl</structname> View</title>
    <tgroup cols="3">
diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index 2052afdc8d..506a884e33 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -662,6 +662,22 @@ CREATE VIEW pg_stat_replication AS
     WHERE S.usesysid = U.oid AND
             S.pid = W.pid;
 
+CREATE VIEW pg_stat_wal_receiver AS
+    SELECT
+            s.pid,
+            s.status,
+            s.receive_start_lsn,
+            s.receive_start_tli,
+            s.received_lsn,
+            s.received_tli,
+            s.last_msg_send_time,
+            s.last_msg_receipt_time,
+            s.latest_end_lsn,
+            s.latest_end_time,
+            s.slot_name
+    FROM pg_stat_get_wal_receiver() s
+    WHERE s.pid IS NOT NULL;
+
 CREATE VIEW pg_stat_ssl AS
     SELECT
             S.pid,
diff --git a/src/backend/replication/walreceiver.c b/src/backend/replication/walreceiver.c
index 81f152940d..7b36e02faa 100644
--- a/src/backend/replication/walreceiver.c
+++ b/src/backend/replication/walreceiver.c
@@ -46,9 +46,12 @@
 #include <signal.h>
 #include <unistd.h>
 
+#include "access/htup_details.h"
 #include "access/timeline.h"
 #include "access/transam.h"
 #include "access/xlog_internal.h"
+#include "catalog/pg_type.h"
+#include "funcapi.h"
 #include "libpq/pqformat.h"
 #include "libpq/pqsignal.h"
 #include "miscadmin.h"
@@ -57,7 +60,9 @@
 #include "storage/ipc.h"
 #include "storage/pmsignal.h"
 #include "storage/procarray.h"
+#include "utils/builtins.h"
 #include "utils/guc.h"
+#include "utils/pg_lsn.h"
 #include "utils/ps_status.h"
 #include "utils/resowner.h"
 #include "utils/timestamp.h"
@@ -1215,3 +1220,152 @@ ProcessWalSndrMessage(XLogRecPtr walEnd, TimestampTz sendTime)
 		pfree(receipttime);
 	}
 }
+
+/*
+ * Return a string constant representing the state. This is used
+ * in system functions and views, and should *not* be translated.
+ */
+static const char *
+WalRcvGetStateString(WalRcvState state)
+{
+	switch (state)
+	{
+		case WALRCV_STOPPED:
+			return "stopped";
+		case WALRCV_STARTING:
+			return "starting";
+		case WALRCV_STREAMING:
+			return "streaming";
+		case WALRCV_WAITING:
+			return "waiting";
+		case WALRCV_RESTARTING:
+			return "restarting";
+		case WALRCV_STOPPING:
+			return "stopping";
+	}
+	return "UNKNOWN";
+}
+
+/*
+ * Returns activity of WAL receiver, including pid, state and xlog locations
+ * received from the WAL sender of another server.
+ */
+Datum
+pg_stat_get_wal_receiver(PG_FUNCTION_ARGS)
+{
+#define PG_STAT_GET_WAL_RECEIVER_COLS	11
+	TupleDesc	tupdesc;
+	Datum		values[PG_STAT_GET_WAL_RECEIVER_COLS];
+	bool		nulls[PG_STAT_GET_WAL_RECEIVER_COLS];
+	WalRcvData *walrcv = WalRcv;
+	WalRcvState state;
+	XLogRecPtr	receive_start_lsn;
+	TimeLineID	receive_start_tli;
+	XLogRecPtr	received_lsn;
+	TimeLineID	received_tli;
+	TimestampTz	last_send_time;
+	TimestampTz	last_receipt_time;
+	XLogRecPtr	latest_end_lsn;
+	TimestampTz	latest_end_time;
+	char	   *slotname;
+
+	/* No WAL receiver, just return a tuple with NULL values */
+	if (walrcv->pid == 0)
+		PG_RETURN_NULL();
+
+	/* Initialise values and NULL flags arrays */
+	MemSet(values, 0, sizeof(values));
+	MemSet(nulls, 0, sizeof(nulls));
+
+	/* Initialise attributes information in the tuple descriptor */
+	tupdesc = CreateTemplateTupleDesc(PG_STAT_GET_WAL_RECEIVER_COLS, false);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 1, "pid",
+					   INT4OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 2, "status",
+					   TEXTOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 3, "receive_start_lsn",
+					   LSNOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 4, "receive_start_tli",
+					   INT4OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 5, "received_lsn",
+					   LSNOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 6, "received_tli",
+					   INT4OID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 7, "last_msg_send_time",
+					   TIMESTAMPTZOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 8, "last_msg_receipt_time",
+					   TIMESTAMPTZOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 9, "latest_end_lsn",
+					   LSNOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 10, "latest_end_time",
+					   TIMESTAMPTZOID, -1, 0);
+	TupleDescInitEntry(tupdesc, (AttrNumber) 11, "slot_name",
+					   TEXTOID, -1, 0);
+
+	BlessTupleDesc(tupdesc);
+
+	/* Take a lock to ensure value consistency */
+	SpinLockAcquire(&walrcv->mutex);
+	state = walrcv->walRcvState;
+	receive_start_lsn = walrcv->receiveStart;
+	receive_start_tli = walrcv->receiveStartTLI;
+	received_lsn = walrcv->receivedUpto;
+	received_tli = walrcv->receivedTLI;
+	last_send_time = walrcv->lastMsgSendTime;
+	last_receipt_time = walrcv->lastMsgReceiptTime;
+	latest_end_lsn = walrcv->latestWalEnd;
+	latest_end_time = walrcv->latestWalEndTime;
+	slotname = pstrdup(walrcv->slotname);
+	SpinLockRelease(&walrcv->mutex);
+
+	/* Fetch values */
+	values[0] = Int32GetDatum(walrcv->pid);
+
+	if (!superuser())
+	{
+		/*
+		 * Only superusers can see details. Other users only get the pid
+		 * value to know whether it is a WAL receiver, but no details.
+		 */
+		MemSet(&nulls[1], true, PG_STAT_GET_WAL_RECEIVER_COLS - 1);
+	}
+	else
+	{
+		values[1] = CStringGetTextDatum(WalRcvGetStateString(state));
+
+		if (XLogRecPtrIsInvalid(receive_start_lsn))
+			nulls[2] = true;
+		else
+			values[2] = LSNGetDatum(receive_start_lsn);
+		values[3] = Int32GetDatum(receive_start_tli);
+		if (XLogRecPtrIsInvalid(received_lsn))
+			nulls[4] = true;
+		else
+			values[4] = LSNGetDatum(received_lsn);
+		values[5] = Int32GetDatum(received_tli);
+		if (last_send_time == 0)
+			nulls[6] = true;
+		else
+			values[6] = TimestampTzGetDatum(last_send_time);
+		if (last_receipt_time == 0)
+			nulls[7] = true;
+		else
+			values[7] = TimestampTzGetDatum(last_receipt_time);
+		if (XLogRecPtrIsInvalid(latest_end_lsn))
+			nulls[8] = true;
+		else
+			values[8] = LSNGetDatum(latest_end_lsn);
+		if (latest_end_time == 0)
+			nulls[9] = true;
+		else
+			values[9] = TimestampTzGetDatum(latest_end_time);
+		if (*slotname == '\0')
+			nulls[10] = true;
+		else
+			values[10] = CStringGetTextDatum(slotname);
+	}
+
+	/* Returns the record as Datum */
+	PG_RETURN_DATUM(HeapTupleGetDatum(
+						  heap_form_tuple(tupdesc, values, nulls)));
+}
diff --git a/src/include/catalog/catversion.h b/src/include/catalog/catversion.h
index 31a132ea98..eaa7a51891 100644
--- a/src/include/catalog/catversion.h
+++ b/src/include/catalog/catversion.h
@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	201601052
+#define CATALOG_VERSION_NO	201601071
 
 #endif
diff --git a/src/include/catalog/pg_proc.h b/src/include/catalog/pg_proc.h
index 9250545d3b..86b09a15ea 100644
--- a/src/include/catalog/pg_proc.h
+++ b/src/include/catalog/pg_proc.h
@@ -2787,6 +2787,8 @@ DATA(insert OID = 2022 (  pg_stat_get_activity			PGNSP PGUID 12 1 100 0 0 f f f
 DESCR("statistics: information about currently active backends");
 DATA(insert OID = 3099 (  pg_stat_get_wal_senders	PGNSP PGUID 12 1 10 0 0 f f f f f t s r 0 0 2249 "" "{23,25,3220,3220,3220,3220,23,25}" "{o,o,o,o,o,o,o,o}" "{pid,state,sent_location,write_location,flush_location,replay_location,sync_priority,sync_state}" _null_ _null_ pg_stat_get_wal_senders _null_ _null_ _null_ ));
 DESCR("statistics: information about currently active replication");
+DATA(insert OID = 3317 (  pg_stat_get_wal_receiver	PGNSP PGUID 12 1 0 0 0 f f f f f f s r 0 0 2249 "" "{23,25,3220,23,3220,23,1184,1184,3220,1184,25}" "{o,o,o,o,o,o,o,o,o,o,o}" "{pid,status,receive_start_lsn,receive_start_tli,received_lsn,received_tli,last_msg_send_time,last_msg_receipt_time,latest_end_lsn,latest_end_time,slot_name}" _null_ _null_ pg_stat_get_wal_receiver _null_ _null_ _null_ ));
+DESCR("statistics: information about WAL receiver");
 DATA(insert OID = 2026 (  pg_backend_pid				PGNSP PGUID 12 1 0 0 0 f f f f t f s r 0 0 23 "" _null_ _null_ _null_ _null_ _null_ pg_backend_pid _null_ _null_ _null_ ));
 DESCR("statistics: current backend PID");
 DATA(insert OID = 1937 (  pg_stat_get_backend_pid		PGNSP PGUID 12 1 0 0 0 f f f f t f s r 1 0 23 "23" _null_ _null_ _null_ _null_ _null_ pg_stat_get_backend_pid _null_ _null_ _null_ ));
diff --git a/src/include/replication/walreceiver.h b/src/include/replication/walreceiver.h
index db40d9d760..6eacb095d1 100644
--- a/src/include/replication/walreceiver.h
+++ b/src/include/replication/walreceiver.h
@@ -14,6 +14,7 @@
 
 #include "access/xlog.h"
 #include "access/xlogdefs.h"
+#include "fmgr.h"
 #include "storage/latch.h"
 #include "storage/spin.h"
 #include "pgtime.h"
@@ -148,6 +149,7 @@ extern PGDLLIMPORT walrcv_disconnect_type walrcv_disconnect;
 
 /* prototypes for functions in walreceiver.c */
 extern void WalReceiverMain(void) pg_attribute_noreturn();
+extern Datum pg_stat_get_wal_receiver(PG_FUNCTION_ARGS);
 
 /* prototypes for functions in walreceiverfuncs.c */
 extern Size WalRcvShmemSize(void);
diff --git a/src/test/regress/expected/rules.out b/src/test/regress/expected/rules.out
index 80374e4d50..28b061f9d5 100644
--- a/src/test/regress/expected/rules.out
+++ b/src/test/regress/expected/rules.out
@@ -1848,6 +1848,19 @@ pg_stat_user_tables| SELECT pg_stat_all_tables.relid,
     pg_stat_all_tables.autoanalyze_count
    FROM pg_stat_all_tables
   WHERE ((pg_stat_all_tables.schemaname <> ALL (ARRAY['pg_catalog'::name, 'information_schema'::name])) AND (pg_stat_all_tables.schemaname !~ '^pg_toast'::text));
+pg_stat_wal_receiver| SELECT s.pid,
+    s.status,
+    s.receive_start_lsn,
+    s.receive_start_tli,
+    s.received_lsn,
+    s.received_tli,
+    s.last_msg_send_time,
+    s.last_msg_receipt_time,
+    s.latest_end_lsn,
+    s.latest_end_time,
+    s.slot_name
+   FROM pg_stat_get_wal_receiver() s(pid, status, receive_start_lsn, receive_start_tli, received_lsn, received_tli, last_msg_send_time, last_msg_receipt_time, latest_end_lsn, latest_end_time, slot_name)
+  WHERE (s.pid IS NOT NULL);
 pg_stat_xact_all_tables| SELECT c.oid AS relid,
     n.nspname AS schemaname,
     c.relname,