repmgr: enable "primary unregister --force" to unregister an active primary

The primary must have no registered standby nodes.

Also document usage when unregistering a primary node which is actually
running as a standby.

HISTORY

@@ -2,6 +2,8 @@
         general: add support for PostgreSQL 13 (Ian)
         general: remove support for PostgreSQL 9.3 (Ian)
         config: add support for file inclusion directives (Ian)
+        repmgr: "primary unregister --force" will unregister an active primary
+          with no registered standby nodes (Ian)
         repmgr: add option --verify-backup to "standby clone" (Ian)
         repmgr: add option --db-connection to "node check" (Ian)
         repmgr: report database connection error if the --optformat option was

doc/appendix-release-notes.xml

@@ -60,6 +60,15 @@
               displayed correctly.
             </para>
           </listitem>
+
+          <listitem>
+            <para>
+              <link linkend="repmgr-primary-unregister">repmgr primary unregister</link>:
+              the <option>--force</option> can be provided to unregister an active primary node, provided
+              it has no registered standby nodes.
+            </para>
+          </listitem>
+
           <listitem>
             <para>
               <link linkend="repmgr-standby-clone">repmgr standby clone</link>: new option

doc/repmgr-primary-unregister.xml

@@ -60,6 +60,17 @@
         </listitem>
       </varlistentry>
 
+     <varlistentry>
+        <term><option>--force</option></term>
+        <listitem>
+          <para>
+            Forcibly unregister the node if it is registered as an active primary,
+            as long as it has no registered standbys; or if it is registered as
+            a primary but running as a standby.
+          </para>
+        </listitem>
+      </varlistentry>
+
     </variablelist>
 
   </refsect1>

repmgr-action-primary.c

@@ -454,9 +454,9 @@ do_primary_unregister(void)
 
 			/*
 			 * This appears to be the cluster primary - cowardly refuse to
-			 * delete the record
+			 * delete the record, unless --force is supplied.
 			 */
-			if (primary_node_info.node_id == target_node_info_ptr->node_id)
+			if (primary_node_info.node_id == target_node_info_ptr->node_id && !runtime_options.force)
 			{
 				log_error(_("node \"%s\" (ID: %i) is the current primary node, unable to unregister"),
 						  target_node_info_ptr->node_name,