repmgrd: don't start if node is inactive and failover=automatic

If failover=automatic, it would be reasonable to expect repmgrd to
consider this node as a promotion candidate, however this will not
happen if it is marked inactive. This often happens when a failed
primary is recloned as a standby but not re-registered, and if
repmgrd would run it would give the incorrect impression that
failover capability is available.

Addresses GitHub #153.

FAQ.md

@@ -151,6 +151,9 @@ General
 
   In `repmgr.conf`, set its priority to a value of 0 or less.
 
+  Additionally, if `failover` is set to `manual`, the node will never
+  be considered as a promotion candidate.
+
 - Does `repmgrd` support delayed standbys?
 
   `repmgrd` can monitor delayed standbys - those set up with
@@ -169,3 +172,11 @@ General
 
   Configure your system's `logrotate` service to do this; see example
   in README.md
+
+- I've recloned a failed master as a standby, but `repmgrd` refuses to start?
+
+  Check you registered the standby after recloning. If unregistered the standby
+  cannot be considered as a promotion candidate even if `failover` is set to
+  `automatic`, which is probably not what you want. `repmgrd` will start if
+  `failover` is set to `manual` so the node's replication status can still
+  be monitored, if desired.

repmgrd.c

@@ -311,6 +311,41 @@ main(int argc, char **argv)
 
 	log_debug("node id is %i, upstream is %i\n", node_info.node_id, node_info.upstream_node_id);
 
+    /*
+     * Check if node record is active - if not, and `failover=automatic`, the node
+     * won't be considered as a promotion candidate; this often happens when
+     * a failed primary is recloned and the node was not re-registered, giving
+     * the impression failover capability is there when it's not. In this case
+     * abort with an error and a hint about registering.
+     *
+     * If `failover=manual`, repmgrd can continue to passively monitor the node, but
+     * we should nevertheless issue a warning and the same hint.
+     */
+
+    if (node_info.active == false)
+    {
+        char *hint = "Check that 'repmgr (master|standby) register' was executed for this node";
+
+        switch (local_options.failover)
+        {
+            case AUTOMATIC_FAILOVER:
+                log_err(_("This node is marked as inactive and cannot be used for failover\n"));
+                log_hint(_("%s\n"), hint);
+                terminate(ERR_BAD_CONFIG);
+
+            case MANUAL_FAILOVER:
+                log_warning(_("This node is marked as inactive and will be passively monitored only\n"));
+                log_hint(_("%s\n"), hint);
+                break;
+
+            default:
+                /* This should never happen */
+                log_err(_("Unknown failover mode %i\n"), local_options.failover);
+                terminate(ERR_BAD_CONFIG);
+        }
+
+    }
+
 	/*
 	 * MAIN LOOP This loops cycles at startup and once per failover and
 	 * Requisites: