Ticket #4048: before_activate_$plugin.action.diff

wp-admin/plugins.php

@@ -15 +15 @@
                         @include(ABSPATH . PLUGINDIR . '/' . $plugin);
                         $current[] = $plugin;
                         sort($current);
-                        update_option('active_plugins', $current);
+                        $can_activate = true; //flag that can be set to false by the plugin
+                        do_action('before_activate_' . $plugin);
+                        if($can_activate){ //if the plugin says we shouldn't activate, trust it
+                                update_option('active_plugins', $current);
+                        } else {
+                                exit; 
+                        }
                         do_action('activate_' . $plugin);
                 }
                 wp_redirect('plugins.php?activate=true'); // overrides the ?error=true one above