Improve performance when constructing custom fields for objects

Summary:
Ref T11404. This improves things by about 10%:

  - Use `PhutilClassMapQuery`, which has slightly better caching.
  - Do a little less work to generate pretty error messages.
  - Make the "disabled" code a little faster (and sort of clearer, too?) by doing less fancy stuff.

These are pretty minor adjustments and not the sort of optimizations I'd make normally, but this code gets called ~100x (once per revision) and generates ~10 fields normally, so even small savings can amount to something.

(I also want to try to make `arc` faster in the next update, and improving Conduit performance helps with that.)

Test Plan: Ran `differential.revision.search`, saw cost drop from ~195ms to ~170ms locally.

Reviewers: yelirekim, chad

Reviewed By: chad

Maniphest Tasks: T11404

Differential Revision: https://secure.phabricator.com/D16355

src/infrastructure/customfield/field/PhabricatorCustomField.php

@@ -112,14 +112,13 @@ abstract class PhabricatorCustomField extends Phobject {
     $object,
     array $options = array()) {
 
-    $field_objects = id(new PhutilSymbolLoader())
+    $field_objects = id(new PhutilClassMapQuery())
       ->setAncestorClass($base_class)
-      ->loadObjects();
+      ->execute();
 
     $fields = array();
-    $from_map = array();
     foreach ($field_objects as $field_object) {
-      $current_class = get_class($field_object);
+      $field_object = clone $field_object;
       foreach ($field_object->createFields($object) as $field) {
         $key = $field->getFieldKey();
         if (isset($fields[$key])) {
@@ -127,11 +126,10 @@ abstract class PhabricatorCustomField extends Phobject {
             pht(
               "Both '%s' and '%s' define a custom field with ".
               "field key '%s'. Field keys must be unique.",
-              $from_map[$key],
-              $current_class,
+              get_class($fields[$key]),
+              get_class($field),
               $key));
         }
-        $from_map[$key] = $current_class;
         $fields[$key] = $field;
       }
     }
@@ -146,11 +144,13 @@ abstract class PhabricatorCustomField extends Phobject {
 
     if (empty($options['withDisabled'])) {
       foreach ($fields as $key => $field) {
-        $config = idx($spec, $key, array()) + array(
-          'disabled' => $field->shouldDisableByDefault(),
-        );
+        if (isset($spec[$key]['disabled'])) {
+          $is_disabled = $spec[$key]['disabled'];
+        } else {
+          $is_disabled = $field->shouldDisableByDefault();
+        }
 
-        if (!empty($config['disabled'])) {
+        if ($is_disabled) {
           if ($field->canDisableField()) {
             unset($fields[$key]);
           }