Allow other modules to return search results on solr failure

Here's a first cut of a patch to do this.

Wow, added some stuff I didn't mean to there. Here's a cleaner patch, sorry about that...

I have re-rolled the patch for D7 in regards to the updated roadmap.

I've rolled another patch that will account for search modules using alternate paths (I wasn't aware of this change in the API, learn something new every day :)).

I don't think arguments are included in the $querystring variable. If you look where apachesolr_failure() is called you can see only the keys variable is being passed, not the full query array:

<?php

/**
 * Implementation of hook_search_execute().
 */
function apachesolr_search_search_execute($keys = NULL, $conditions = NULL) {
  // ...
  catch (Exception $e) {
    watchdog('Apache Solr', nl2br(check_plain($e->getMessage())), NULL, WATCHDOG_ERROR);
    apachesolr_failure(t('Solr search'), $keys);
  }
}
?>

Rerolled with changes as requested with the exception of drupal_goto(). I'm not actually sure what the problem you're referring to is. The code is essentially the same as what was already in the module:

- drupal_goto('search/node/' . drupal_encode_path($querystring));
+ drupal_goto('search/' . $search_info['path'] . '/' . drupal_encode_path($querystring));

Should we create a new issue for the drupal_goto() change? I'm happy to implement it here but it's kind of out of scope for what this issue was originally trying to provide.

For cross reference, I've created a new issue for the drupal_goto() array here: http://drupal.org/node/1105584

Here's a re-rolled patch with the above changes.

Good point! Here's another patch with namespaced variables for show_error and show_no_results.

Patch with update hook to reset variables based on new naming conventions.

boom

Originally I needed this functionality for a client that's on D6, so marking as patch to be ported, will upload a port shortly.

I'm not sure the way I'm showing the module name is optimal but it seems to be the closest thing to system_get_info() in D6 that we can get without having to parse all the info files from all the modules (when really we're only interested in the search modules).

Actually, that case is caught above:

 function apachesolr_failure($search_name, $querystring) {
-  $fail_rule = variable_get('apachesolr_failure', 'show_error');
+  $fail_rule = variable_get('apachesolr_failure', 'apachesolr:show_error');
 
   switch ($fail_rule) {
-    case 'show_error':
+    case 'apachesolr:show_error':
       drupal_set_message(t('The Apache Solr search engine is not available. Please contact your site administrator.'), 'error');
       break;
-    case 'show_drupal_results':
-      drupal_set_message(t("%search_name is not available. Your search is being redirected.", array('%search_name' => $search_name)));
-      drupal_goto('search/node/' . drupal_urlencode($querystring));
-      break;
-    case 'show_no_results':
+    case 'apachesolr:show_no_results':
+      // Do nothing.
       return;
+    default:
+      // If we're failing over to another module make sure the search is available.
+      if (module_exists($fail_rule)) {
+        drupal_set_message(t("%search_name is not available. Your search is being redirected.", array('%search_name' => $search_name)));
+        drupal_goto('search/' . $fail_rule . '/' . drupal_urlencode($querystring));
+      }
+      break;
   }

The only time 'default' is reached is if a search module is selected as the failover, not one of the two predefined apachesolr cases. Additionally, the default case checks to make sure that the fail rule is a valid module before firing the drupal_goto().