Ticket #4048: 4048.r6603.diff

plugin.php

@@ -99 +99 @@
                                 wp_redirect(add_query_arg('_error_nonce', wp_create_nonce('plugin-activation-error_' . $plugin), $redirect)); // we'll override this later if the plugin can be included without fatal error
                         ob_start();
                         @include(ABSPATH . PLUGINDIR . '/' . $plugin);
-                        $current[] = $plugin;
-                        sort($current);
-                        update_option('active_plugins', $current);
-                        do_action('activate_' . $plugin);
+                        $can_activate = apply_filters( 'before_activate_' . $plugin, true );
+                        if( $can_activate ) {
+                                $current[] = $plugin;
+                                sort($current);
+                                update_option('active_plugins', $current);
+                                do_action('activate_' . $plugin);
+                        }
                         ob_end_clean();
                 }