Add functionality to impersonate a user

Problem/Motivation

Currently Drupal has no unified API that allows safe user impersonation in code. This functionality is often needed by core and contrib and each module implements it in different ways some of them not secure. This may be considered a security improvement since:

1. It can help people avoid unsafe impersonation constructs as outlined in Safely Impersonating Another User.
2. It makes it possible to nest multiple impersonation levels without having the innermost impersonation logic incorrectly re-enable session saving when finished (see #21)

Proposed resolution

Add an AccountSwitcher class with the two methods switchTo($account) and switchBack(). Streamline the use of account "impersonations" throughout core (cron).

Remaining tasks

Commit.

User interface changes

None

API changes

API addition: a new service with two new methods for safely impersonating an account.

Original report by @drewish →

I'm trying to write tests for file.inc and several of the validation functions need to switch to a uid=1 and non-uid=1 users. I know about DrupalWebTestCase::drupalLogin() but it affects the web browser's user not the global $user;. chx pointed me toward [#218104] which provided the method for doing this in tests:

  /**
   * Test the file_validate_extensions() function for the root user.
   */
  function testFileValidateExtensionsUid1() {
    global $user;
    $original_user = $user;
    session_save_session(FALSE);
    $user = user_load(array('uid' => 1));

    // Run these test as user 1
    $file = new stdClass();
    $file->filename = 'asdf.txt';
    $errors = file_validate_extensions($file, 'asdf txt pork');
    $this->assertEqual(count($errors), 0, t("Valid extension accepted."));

    $file->filename = 'asdf.txt';
    $errors = file_validate_extensions($file, 'exe png');
    $this->assertEqual(count($errors), 0, t("Invalid extension also accepted -- they're uid 1."));

    $user = $original_user;
    session_save_session(TRUE);
  }

  /**
   * Test the file_validate_extensions() function for the root user.
   */
  function testFileValidateExtensionsUidNot1() {
    global $user;
    $original_user = $user;
    session_save_session(FALSE);
    $user = $this->drupalCreateUser();

    // Run these test as a regular user
    $file = new stdClass();
    $file->filename = 'asdf.txt';
    $errors = file_validate_extensions($file, 'asdf txt pork');
    $this->assertEqual(count($errors), 0, t("Valid extension accepted."));

    $file->filename = 'asdf.txt';
    $errors = file_validate_extensions($file, 'exe png');
    $this->assertEqual(count($errors), 1, t("Invalid extension blocked."));

    $user = $original_user;
    session_save_session(TRUE);
  }

It's just enough code to be a pain and clutter up the tests. It seems like it be better to have DrupalWebTestCase::drupalSetUser() and ::drupalRestoreUser() or something like that to make this a little saner.