From 50894b61245d546d133cb9c3553e37632cfafb99 Mon Sep 17 00:00:00 2001
From: Ian Barwick <ian@2ndquadrant.com>
Date: Fri, 2 Feb 2018 15:01:23 +0900
Subject: [PATCH] "standby follow": check for replication slot availability on
 target node

---
 dbutils.c               | 39 +++++++++++++++++++++++++++++++++++++++
 dbutils.h               |  1 +
 repmgr-action-standby.c | 30 +++++++++++++++++++++++++++++-
 3 files changed, 69 insertions(+), 1 deletion(-)

diff --git a/dbutils.c b/dbutils.c
index f5fbd1f2..afa06ca9 100644
--- a/dbutils.c
+++ b/dbutils.c
@@ -3550,6 +3550,45 @@ get_slot_record(PGconn *conn, char *slot_name, t_replication_slot *record)
 	return RECORD_FOUND;
 }
 
+
+int
+get_free_replication_slots(PGconn *conn)
+{
+	PQExpBufferData query;
+	PGresult   *res = NULL;
+	int			free_slots = 0;
+
+	initPQExpBuffer(&query);
+
+	appendPQExpBuffer(&query,
+					  " SELECT pg_catalog.current_setting('max_replication_slots')::INT - "
+					  "        COUNT(*) AS free_slots"
+					  "   FROM pg_catalog.pg_replication_slots");
+
+	res = PQexec(conn, query.data);
+	termPQExpBuffer(&query);
+
+	if (PQresultStatus(res) != PGRES_TUPLES_OK)
+	{
+		log_error(_("unable to execute replication slot query"));
+		log_detail("%s", PQerrorMessage(conn));
+		PQclear(res);
+		return -1;
+	}
+
+	if (PQntuples(res) == 0)
+	{
+		PQclear(res);
+		return -1;
+	}
+
+	free_slots = atoi(PQgetvalue(res, 0, 0));
+
+	PQclear(res);
+	return free_slots;
+}
+
+
 /* ==================== */
 /* tablespace functions */
 /* ==================== */
diff --git a/dbutils.h b/dbutils.h
index d10d1ec5..b59b6de8 100644
--- a/dbutils.h
+++ b/dbutils.h
@@ -445,6 +445,7 @@ void		create_slot_name(char *slot_name, int node_id);
 bool		create_replication_slot(PGconn *conn, char *slot_name, int server_version_num, PQExpBufferData *error_msg);
 bool		drop_replication_slot(PGconn *conn, char *slot_name);
 RecordStatus get_slot_record(PGconn *conn, char *slot_name, t_replication_slot *record);
+int			get_free_replication_slots(PGconn *conn);
 
 /* tablespace functions */
 bool		get_tablespace_name_by_location(PGconn *conn, const char *location, char *name);
diff --git a/repmgr-action-standby.c b/repmgr-action-standby.c
index 65bbe96f..de19735b 100644
--- a/repmgr-action-standby.c
+++ b/repmgr-action-standby.c
@@ -1679,6 +1679,33 @@ do_standby_follow(void)
 					primary_id);
 	}
 
+	/* if replication slots in use, check at least one free slot is available */
+
+	if (config_file_options.use_replication_slots)
+	{
+		int free_slots = get_free_replication_slots(primary_conn);
+		if (free_slots < 0)
+		{
+			log_error(_("unable to determine number of free replication slots on the primary"));
+			PQfinish(primary_conn);
+			exit(ERR_BAD_CONFIG);
+		}
+
+		if (free_slots == 0)
+		{
+			log_error(_("no free replication slots available on the primary"));
+			log_hint(_("consider increasing \"max_replication_slots\""));
+			PQfinish(primary_conn);
+			exit(ERR_BAD_CONFIG);
+		}
+		else if (runtime_options.dry_run == true)
+		{
+			log_info(_("replication slots in use, %i free slots on the primary"),
+					 free_slots);
+		}
+
+	}
+
 	/* XXX check this is not current upstream anyway */
 	/* check replication connection */
 	initialize_conninfo_params(&repl_conninfo, false);
@@ -1700,6 +1727,7 @@ do_standby_follow(void)
 		log_info(_("replication connection to primary node was successful"));
 	}
 
+
 	/* check system_identifiers match */
 	local_system_identifier = get_system_identifier(config_file_options.data_directory);
 	success = identify_system(repl_conn, &primary_identification);
@@ -1728,7 +1756,7 @@ do_standby_follow(void)
 		log_detail(_("system identifier is %lu"), local_system_identifier);
 	}
 
-	/* TODO: check timelines*/
+	/* TODO: check timelines */
 
 	PQfinish(repl_conn);
 	free_conninfo_params(&repl_conninfo);