repmgrd: during failover, check if other nodes have seen the primary

In a situation where only some standbys are cut off from the primary,
a failover would result in a split brain/split cluster situation,
as it's likely one of the cut-off standbys will promote itself, and
other cut-off standbys (but not all standbys) will follow it.

To prevent this happening, interrogate the other sibiling nodes to
check whether they've seen the primary within a reasonably short interval;
if this is the case, do not take any failover action.

This feature is experimental.

dbutils.c

@@ -4781,6 +4781,7 @@ init_replication_info(ReplInfo *replication_info)
 	replication_info->replication_lag_time = 0;
 	replication_info->receiving_streamed_wal = true;
 	replication_info->wal_replay_paused = false;
+	replication_info->primary_last_seen = -1;
 }
 
 
@@ -4807,7 +4808,8 @@ get_replication_info(PGconn *conn, ReplInfo *replication_info)
 						 "          END "
 						 "        END AS replication_lag_time, "
 						 "        last_wal_receive_lsn >= last_wal_replay_lsn AS receiving_streamed_wal, "
-						 "        wal_replay_paused "
+						 "        wal_replay_paused, "
+						 "        primary_last_seen "
 						 "   FROM ( "
 						 " SELECT CURRENT_TIMESTAMP AS ts, "
 						 "        pg_catalog.pg_last_xact_replay_timestamp() AS last_xact_replay_timestamp, ");
@@ -4821,7 +4823,7 @@ get_replication_info(PGconn *conn, ReplInfo *replication_info)
 							 "        CASE WHEN pg_catalog.pg_is_in_recovery() IS FALSE "
 							 "          THEN FALSE "
 							 "          ELSE pg_catalog.pg_is_wal_replay_paused() "
-							 "        END AS wal_replay_paused ");
+							 "        END AS wal_replay_paused, ");
 	}
 	else
 	{
@@ -4843,10 +4845,14 @@ get_replication_info(PGconn *conn, ReplInfo *replication_info)
 							 "        CASE WHEN pg_catalog.pg_is_in_recovery() IS FALSE "
 							 "          THEN FALSE "
 							 "          ELSE pg_catalog.pg_is_xlog_replay_paused() "
-							 "        END AS wal_replay_paused ");
+							 "        END AS wal_replay_paused, ");
 	}
 
 	appendPQExpBufferStr(&query,
+						 "        CASE WHEN pg_catalog.pg_is_in_recovery() IS FALSE "
+						 "          THEN -1 "
+						 "          ELSE repmgr.get_primary_last_seen() "
+						 "        END AS primary_last_seen "
 						 "          ) q ");
 
 	log_verbose(LOG_DEBUG, "get_replication_info():\n%s", query.data);
@@ -4868,6 +4874,7 @@ get_replication_info(PGconn *conn, ReplInfo *replication_info)
 		replication_info->replication_lag_time = atoi(PQgetvalue(res, 0, 4));
 		replication_info->receiving_streamed_wal = atobool(PQgetvalue(res, 0, 5));
 		replication_info->wal_replay_paused = atobool(PQgetvalue(res, 0, 6));
+		replication_info->primary_last_seen = atoi(PQgetvalue(res, 0, 7));
 	}
 
 	termPQExpBuffer(&query);

dbutils.h

@@ -308,6 +308,7 @@ typedef struct
 	int			replication_lag_time;
 	bool		receiving_streamed_wal;
 	bool		wal_replay_paused;
+	int			primary_last_seen;
 } ReplInfo;
 
 typedef struct

repmgrd-physical.c

@@ -3103,6 +3103,9 @@ do_election(void)
 	 */
 	bool		primary_location_seen = false;
 
+
+	int			nodes_with_primary_still_visible = 0;
+
 	electoral_term = get_current_term(local_conn);
 
 	if (electoral_term == -1)
@@ -3294,7 +3297,7 @@ do_election(void)
 		{
 			/*
 			 * Theoretically the repmgrd on the node should have resumed WAL play
-			 * at this point
+			 * at this point.
 			 */
 			if (sibling_replication_info.last_wal_receive_lsn > sibling_replication_info.last_wal_replay_lsn)
 			{
@@ -3303,6 +3306,25 @@ do_election(void)
 			}
 		}
 
+		/*
+		 * Check if node has seen primary "recently" - if so, we may have "partial primary visibility".
+		 * For now we'll assume the primary is visible if it's been seen less than
+		 * monitor_interval_secs * 2 seconds ago. We may need to adjust this, and/or make the value
+		 * configurable.
+		 */
+
+
+		if (sibling_replication_info.primary_last_seen < (config_file_options.monitor_interval_secs * 2))
+		{
+			nodes_with_primary_still_visible++;
+			log_notice(_("node %i last saw primary node %i second(s) ago, considering primary still visible"),
+					   cell->node_info->node_id, sibling_replication_info.primary_last_seen);
+		}
+		else
+		{
+			log_info(_("node %i last saw primary node %i second(s) ago"),
+					 cell->node_info->node_id, sibling_replication_info.primary_last_seen);
+		}
 		/* get node's last receive LSN - if "higher" than current winner, current node is candidate */
 		cell->node_info->last_wal_receive_lsn = sibling_replication_info.last_wal_receive_lsn;
 
@@ -3370,9 +3392,23 @@ do_election(void)
 		return ELECTION_CANCELLED;
 	}
 
-	log_debug("visible nodes: %i; total nodes: %i",
+	if (nodes_with_primary_still_visible > 0)
+	{
+		log_notice(_("%i nodes can seen the primary"), nodes_with_primary_still_visible);
+		// XXX list nodes as detail
+
+		monitoring_state = MS_DEGRADED;
+		INSTR_TIME_SET_CURRENT(degraded_monitoring_start);
+
+		reset_node_voting_status();
+
+		return ELECTION_CANCELLED;
+	}
+
+	log_info(_("visible nodes: %i; total nodes: %i; no nodes have seen the primary within the last %i seconds"),
 			  visible_nodes,
-			  total_nodes);
+			 total_nodes,
+			 (config_file_options.monitor_interval_secs * 2));
 
 	if (visible_nodes <= (total_nodes / 2.0))
 	{