Provide an Editor extension point for transaction validation

Summary:
Depends on D20040. Ref T13242. See PHI1039. See PHI873. Two reasonable cases have arisen recently where extending validation rules would help solve problems.

We can do this in a pretty straightforward way with a standard extension pattern.

Test Plan:
Used this extension to keep ducks away from projects:

```lang=php
<?php

final class NoDucksEditorExtension
  extends PhabricatorEditorExtension {

  const EXTENSIONKEY = 'no.ducks';

  public function getExtensionName() {
    return pht('No Ducks!');
  }

  public function supportsObject(
    PhabricatorApplicationTransactionEditor $editor,
    PhabricatorApplicationTransactionInterface $object) {
    return ($object instanceof PhabricatorProject);
  }

  public function validateTransactions($object, array $xactions) {
    $errors = array();

    $name_type = PhabricatorProjectNameTransaction::TRANSACTIONTYPE;

    $old_value = $object->getName();
    foreach ($xactions as $xaction) {
      if ($xaction->getTransactionType() !== $name_type) {
        continue;
      }

      $new_value = $xaction->getNewValue();
      if ($old_value === $new_value) {
        continue;
      }

      if (preg_match('/duck/i', $new_value)) {
        $errors[] = $this->newInvalidTransactionError(
          $xaction,
          pht('Project names must not contain the substring "duck".'));
      }
    }

    return $errors;
  }

}
```

{F6162585}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13242

Differential Revision: https://secure.phabricator.com/D20041

src/__phutil_library_map__.php

@@ -3061,6 +3061,8 @@ phutil_register_library_map(array(
     'PhabricatorEditPage' => 'applications/transactions/editengine/PhabricatorEditPage.php',
     'PhabricatorEditType' => 'applications/transactions/edittype/PhabricatorEditType.php',
     'PhabricatorEditor' => 'infrastructure/PhabricatorEditor.php',
+    'PhabricatorEditorExtension' => 'applications/transactions/engineextension/PhabricatorEditorExtension.php',
+    'PhabricatorEditorExtensionModule' => 'applications/transactions/engineextension/PhabricatorEditorExtensionModule.php',
     'PhabricatorEditorMailEngineExtension' => 'applications/transactions/engineextension/PhabricatorEditorMailEngineExtension.php',
     'PhabricatorEditorMultipleSetting' => 'applications/settings/setting/PhabricatorEditorMultipleSetting.php',
     'PhabricatorEditorSetting' => 'applications/settings/setting/PhabricatorEditorSetting.php',
@@ -8927,6 +8929,8 @@ phutil_register_library_map(array(
     'PhabricatorEditPage' => 'Phobject',
     'PhabricatorEditType' => 'Phobject',
     'PhabricatorEditor' => 'Phobject',
+    'PhabricatorEditorExtension' => 'Phobject',
+    'PhabricatorEditorExtensionModule' => 'PhabricatorConfigModule',
     'PhabricatorEditorMailEngineExtension' => 'PhabricatorMailEngineExtension',
     'PhabricatorEditorMultipleSetting' => 'PhabricatorSelectSetting',
     'PhabricatorEditorSetting' => 'PhabricatorStringSetting',

src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php

@@ -88,6 +88,7 @@ abstract class PhabricatorApplicationTransactionEditor
   private $hasRequiredMFA = false;
   private $request;
   private $cancelURI;
+  private $extensions;
 
   const STORAGE_ENCODING_BINARY = 'binary';
 
@@ -1013,6 +1014,7 @@ abstract class PhabricatorApplicationTransactionEditor
       }
 
       $errors[] = $this->validateAllTransactions($object, $xactions);
+      $errors[] = $this->validateTransactionsWithExtensions($object, $xactions);
       $errors = array_mergev($errors);
 
       $continue_on_missing = $this->getContinueOnMissingFields();
@@ -4975,4 +4977,60 @@ abstract class PhabricatorApplicationTransactionEditor
     return $xactions;
   }
 
+
+/* -(  Extensions  )--------------------------------------------------------- */
+
+
+  private function validateTransactionsWithExtensions(
+    PhabricatorLiskDAO $object,
+    array $xactions) {
+    $errors = array();
+
+    $extensions = $this->getEditorExtensions();
+    foreach ($extensions as $extension) {
+      $extension_errors = $extension
+        ->setObject($object)
+        ->validateTransactions($object, $xactions);
+
+      assert_instances_of(
+        $extension_errors,
+        'PhabricatorApplicationTransactionValidationError');
+
+      $errors[] = $extension_errors;
+    }
+
+    return array_mergev($errors);
+  }
+
+  private function getEditorExtensions() {
+    if ($this->extensions === null) {
+      $this->extensions = $this->newEditorExtensions();
+    }
+    return $this->extensions;
+  }
+
+  private function newEditorExtensions() {
+    $extensions = PhabricatorEditorExtension::getAllExtensions();
+
+    $actor = $this->getActor();
+    $object = $this->object;
+    foreach ($extensions as $key => $extension) {
+
+      $extension = id(clone $extension)
+        ->setViewer($actor)
+        ->setEditor($this)
+        ->setObject($object);
+
+      if (!$extension->supportsObject($this, $object)) {
+        unset($extensions[$key]);
+        continue;
+      }
+
+      $extensions[$key] = $extension;
+    }
+
+    return $extensions;
+  }
+
+
 }

src/applications/transactions/engineextension/PhabricatorEditorExtension.php

@@ -0,0 +1,83 @@
+<?php
+
+abstract class PhabricatorEditorExtension
+  extends Phobject {
+
+  private $viewer;
+  private $editor;
+  private $object;
+
+  final public function getExtensionKey() {
+    return $this->getPhobjectClassConstant('EXTENSIONKEY');
+  }
+
+  final public function setEditor(
+    PhabricatorApplicationTransactionEditor $editor) {
+    $this->editor = $editor;
+    return $this;
+  }
+
+  final public function getEditor() {
+    return $this->editor;
+  }
+
+  final public function setViewer(PhabricatorUser $viewer) {
+    $this->viewer = $viewer;
+    return $this;
+  }
+
+  final public function getViewer() {
+    return $this->viewer;
+  }
+
+  final public function setObject(
+    PhabricatorApplicationTransactionInterface $object) {
+    $this->object = $object;
+    return $this;
+  }
+
+  final public static function getAllExtensions() {
+    return id(new PhutilClassMapQuery())
+      ->setAncestorClass(__CLASS__)
+      ->setUniqueMethod('getExtensionKey')
+      ->execute();
+  }
+
+  abstract public function getExtensionName();
+
+  public function supportsObject(
+    PhabricatorApplicationTransactionEditor $editor,
+    PhabricatorApplicationTransactionInterface $object) {
+    return true;
+  }
+
+  public function validateTransactions($object, array $xactions) {
+    return array();
+  }
+
+  final protected function newTransactionError(
+    PhabricatorApplicationTransaction $xaction,
+    $title,
+    $message) {
+    return new PhabricatorApplicationTransactionValidationError(
+      $xaction->getTransactionType(),
+      $title,
+      $message,
+      $xaction);
+  }
+
+  final protected function newRequiredTransasctionError(
+    PhabricatorApplicationTransaction $xaction,
+    $message) {
+    return $this->newError($xaction, pht('Required'), $message)
+      ->setIsMissingFieldError(true);
+  }
+
+  final protected function newInvalidTransactionError(
+    PhabricatorApplicationTransaction $xaction,
+    $message) {
+    return $this->newTransactionError($xaction, pht('Invalid'), $message);
+  }
+
+
+}

src/applications/transactions/engineextension/PhabricatorEditorExtensionModule.php

@@ -0,0 +1,40 @@
+<?php
+
+final class PhabricatorEditorExtensionModule
+  extends PhabricatorConfigModule {
+
+  public function getModuleKey() {
+    return 'editor';
+  }
+
+  public function getModuleName() {
+    return pht('Engine: Editor');
+  }
+
+  public function renderModuleStatus(AphrontRequest $request) {
+    $viewer = $request->getViewer();
+
+    $extensions = PhabricatorEditorExtension::getAllExtensions();
+
+    $rows = array();
+    foreach ($extensions as $extension) {
+      $rows[] = array(
+        get_class($extension),
+        $extension->getExtensionName(),
+      );
+    }
+
+    return id(new AphrontTableView($rows))
+      ->setHeaders(
+        array(
+          pht('Class'),
+          pht('Name'),
+        ))
+      ->setColumnClasses(
+        array(
+          null,
+          'wide pri',
+        ));
+  }
+
+}