repmgrd: return predictable default values for get_primary_last_seen()

Return 0 if the node is not in recovery. In which case it's probably
rather pointless calling this function anyway.

Return -1 if the "last_seen" field has never been set (i.e. repmgrd
hasn't started yet).

dbutils.c

@@ -3168,6 +3168,7 @@ is_downstream_node_attached(PGconn *conn, char *node_name)
 	return true;
 }
 
+
 void
 set_primary_last_seen(PGconn *conn)
 {

repmgr.c

@@ -225,8 +225,8 @@ repmgr_shmem_startup(void)
 		memset(shared_state->repmgrd_pidfile, 0, MAXPGPATH);
 		shared_state->repmgrd_paused = false;
 		shared_state->current_electoral_term = 0;
-		/* we should probably set this to an earlier value */
-		shared_state->primary_last_seen = GetCurrentTimestamp();
+		/* arbitrary "magic" date to indicate this field hasn't been updated */
+		shared_state->primary_last_seen = POSTGRES_EPOCH_JDATE;
 		shared_state->voting_status = VS_NO_VOTE;
 		shared_state->candidate_node_id = UNKNOWN_NODE_ID;
 		shared_state->follow_new_primary = false;
@@ -391,12 +391,26 @@ get_primary_last_seen(PG_FUNCTION_ARGS)
 	if (!shared_state)
 		PG_RETURN_INT32(-1);
 
+	/* A primary is always visible */
+	if (!RecoveryInProgress())
+		PG_RETURN_INT32(0);
+
 	LWLockAcquire(shared_state->lock, LW_SHARED);
 
 	last_seen = shared_state->primary_last_seen;
 
 	LWLockRelease(shared_state->lock);
 
+	/*
+	 * "last_seen" is initialised with the PostgreSQL epoch as a
+	 * "magic" value to indicate the field hasn't ever been updated
+	 * by repmgrd. We return -1 instead, rather than imply that the
+	 * primary was last seen at the turn of the century.
+	 */
+	if (last_seen == POSTGRES_EPOCH_JDATE)
+		PG_RETURN_INT32(-1);
+
+
 	TimestampDifference(last_seen, GetCurrentTimestamp(),
 						&secs, &microsecs);