? .cvsignore
? drbm.tmproj
? drupal_render.patch
? nbproject
? sessions.patch
? tmp.patch
? tmp.patch.patch
? modules/node/node-admin-display-overview-form.tpl.php
? modules/node/node.build_mode.inc
? sites/all/modules
? sites/default/files
Index: install.php
===================================================================
RCS file: /cvs/drupal/drupal/install.php,v
retrieving revision 1.148
diff -u -F^f -p -r1.148 install.php
--- install.php	6 Jan 2009 13:23:54 -0000	1.148
+++ install.php	18 Jan 2009 14:33:22 -0000
@@ -628,7 +628,7 @@ function install_tasks($profile, $task) 
   drupal_install_init_database();
 
   drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);
-  $_SESSION['messages'] = $messages;
+  drupal_set_session('messages', $messages);
 
   // URL used to direct page requests.
   $url = $base_url . '/install.php?locale=' . $install_locale . '&profile=' . $profile;
Index: update.php
===================================================================
RCS file: /cvs/drupal/drupal/update.php,v
retrieving revision 1.269
diff -u -F^f -p -r1.269 update.php
--- update.php	17 Jan 2009 18:55:29 -0000	1.269
+++ update.php	18 Jan 2009 14:33:23 -0000
@@ -293,7 +293,7 @@ function update_batch() {
 
   // During the update, toggle site maintenance so that schema changes do not
   // affect visiting users.
-  $_SESSION['site_offline'] = variable_get('site_offline', FALSE);
+  drupal_set_session('site_offline', variable_get('site_offline', FALSE));
   if ($_SESSION['site_offline'] == FALSE) {
     variable_set('site_offline', TRUE);
   }
@@ -327,9 +327,9 @@ function update_finished($success, $resu
   // clear the caches in case the data has been updated.
   drupal_flush_all_caches();
 
-  $_SESSION['update_results'] = $results;
-  $_SESSION['update_success'] = $success;
-  $_SESSION['updates_remaining'] = $operations;
+  drupal_set_session('update_results', $results);
+  drupal_set_session('update_success', $success);
+  drupal_set_session('updates_remaining', $operations);
 
   // Now that the update is done, we can disable site maintenance if it was
   // previously turned off.
Index: includes/batch.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/batch.inc,v
retrieving revision 1.30
diff -u -F^f -p -r1.30 batch.inc
--- includes/batch.inc	12 Jan 2009 06:23:57 -0000	1.30
+++ includes/batch.inc	18 Jan 2009 14:33:23 -0000
@@ -34,6 +34,11 @@ function _batch_page() {
     ':token' => drupal_get_token($_REQUEST['id']))
   )->fetchField();
 
+  if (!$batch) {
+    drupal_set_message(t('No active batch.'), 'error');
+    drupal_goto();
+  }
+
   $batch = unserialize($batch);
 
   // Register database update for the end of processing.
@@ -409,7 +414,7 @@ function _batch_finished() {
     // We get here if $form['#redirect'] was FALSE, or if the form is a
     // multi-step form. We save the final $form_state value to be retrieved
     // by drupal_get_form(), and redirect to the originating page.
-    $_SESSION['batch_form_state'] = $_batch['form_state'];
+    drupal_set_session('batch_form_state', $_batch['form_state']);
     drupal_goto($_batch['source_page']);
   }
 }
Index: includes/bootstrap.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/bootstrap.inc,v
retrieving revision 1.264
diff -u -F^f -p -r1.264 bootstrap.inc
--- includes/bootstrap.inc	14 Jan 2009 12:15:38 -0000	1.264
+++ includes/bootstrap.inc	18 Jan 2009 14:33:23 -0000
@@ -671,23 +671,41 @@ function variable_del($name) {
 /**
  * Retrieve the current page from the cache.
  *
- * Note: we do not serve cached pages when status messages are waiting (from
- * a redirected form submission which was completed).
+ * Note: we do not serve cached pages to authenticated users, or to anonymous
+ * users when $_SESSION is non-empty. $_SESSION may contain status messages
+ * from a form submission, the contents of a shopping cart, or other user-
+ * specific content that should not be cached and displayed to other users.
+ *
+ * @param $retrieve
+ *   If TRUE, look up and return the current page in the cache, or start output
+ *   buffering if the conditions for caching are satisfied. If FALSE, only
+ *   return a boolean value indicating whether the current request may be
+ *   cached.
+ * @return
+ *   The cache object, if the page was found in the cache; TRUE if the page was
+ *   not found, but output buffering was started in order to possibly cache the
+ *   current request; FALSE if the page was not found, and the current request
+ *   may not be cached (e.g. because it belongs to an authenticated user). If
+ *   $retrieve is TRUE, only return either TRUE or FALSE.
  */
-function page_get_cache() {
+function page_get_cache($retrieve) {
   global $user, $base_root;
+  static $ob_started = FALSE;
 
-  $cache = NULL;
-
-  if (!$user->uid && ($_SERVER['REQUEST_METHOD'] == 'GET' || $_SERVER['REQUEST_METHOD'] == 'HEAD') && count(drupal_set_message()) == 0) {
+  if ($user->uid || ($_SERVER['REQUEST_METHOD'] != 'GET' && $_SERVER['REQUEST_METHOD'] != 'HEAD') || count(drupal_get_messages(NULL, FALSE))) {
+    return FALSE;
+  }
+  if ($retrieve) {
     $cache = cache_get($base_root . request_uri(), 'cache_page');
-
-    if (empty($cache)) {
+    if ($cache) {
+      return $cache;
+    }
+    else {
       ob_start();
+      $ob_started = TRUE;
     }
   }
-
-  return $cache;
+  return $ob_started;
 }
 
 /**
@@ -956,7 +974,7 @@ function watchdog($type, $message, $vari
 function drupal_set_message($message = NULL, $type = 'status', $repeat = TRUE) {
   if ($message) {
     if (!isset($_SESSION['messages'])) {
-      $_SESSION['messages'] = array();
+      drupal_set_session('messages', array());
     }
 
     if (!isset($_SESSION['messages'][$type])) {
@@ -1093,7 +1111,7 @@ function drupal_get_bootstrap_phase() {
 }
 
 function _drupal_bootstrap($phase) {
-  global $conf;
+  global $conf, $user;
 
   switch ($phase) {
 
@@ -1139,7 +1157,16 @@ function _drupal_bootstrap($phase) {
     case DRUPAL_BOOTSTRAP_SESSION:
       require_once DRUPAL_ROOT . '/' . variable_get('session_inc', 'includes/session.inc');
       session_set_save_handler('_sess_open', '_sess_close', '_sess_read', '_sess_write', '_sess_destroy_sid', '_sess_gc');
-      session_start();
+      // If a session cookie exists, initialize the session. Otherwise the
+      // session is only started on demand in drupal_session_start(), making
+      // anonymous users not use a session cookie unless something is stored in
+      // $_SESSION. This allows HTTP proxies to cache anonymous pageviews.
+      if (isset($_COOKIE[session_name()])) {
+        drupal_session_start();
+      }
+      else {
+        $user = drupal_anonymous_user();
+      }
       break;
 
     case DRUPAL_BOOTSTRAP_VARIABLES:
@@ -1150,15 +1177,19 @@ function _drupal_bootstrap($phase) {
     case DRUPAL_BOOTSTRAP_LATE_PAGE_CACHE:
       $cache_mode = variable_get('cache', CACHE_DISABLED);
       // Get the page from the cache.
-      $cache = $cache_mode == CACHE_DISABLED ? '' : page_get_cache();
+      $cache = $cache_mode == CACHE_DISABLED ? FALSE : page_get_cache(TRUE);
       // If the skipping of the bootstrap hooks is not enforced, call hook_boot.
-      if (!$cache || $cache_mode != CACHE_AGGRESSIVE) {
+      if (!is_object($cache) || $cache_mode != CACHE_AGGRESSIVE) {
         // Load module handling.
         require_once DRUPAL_ROOT . '/includes/module.inc';
         module_invoke_all('boot');
       }
       // If there is a cached page, display it.
-      if ($cache) {
+      if (is_object($cache)) {
+        // Destroy empty anonymous sessions.
+        if (drupal_session_is_started() && empty($_SESSION)) {
+          session_destroy();
+        }
         drupal_page_cache_header($cache);
         // If the skipping of the bootstrap hooks is not enforced, call hook_exit.
         if ($cache_mode != CACHE_AGGRESSIVE) {
@@ -1169,6 +1200,13 @@ function _drupal_bootstrap($phase) {
       }
       // Prepare for non-cached page workflow.
       drupal_page_header();
+      // If the session has not already been started and output buffering is
+      // not enabled, the session must be started now before the HTTP headers
+      // are sent. If output buffering is enabled, the session may be started
+      // at any time using drupal_session_start().
+      if ($cache === FALSE) {
+        drupal_session_start();
+      }
       break;
 
     case DRUPAL_BOOTSTRAP_LANGUAGE:
Index: includes/common.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/common.inc,v
retrieving revision 1.848
diff -u -F^f -p -r1.848 common.inc
--- includes/common.inc	14 Jan 2009 21:13:41 -0000	1.848
+++ includes/common.inc	18 Jan 2009 14:33:23 -0000
@@ -339,9 +339,11 @@ function drupal_goto($path = '', $query 
     module_invoke_all('exit', $url);
   }
 
-  // Even though session_write_close() is registered as a shutdown function, we
-  // need all session data written to the database before redirecting.
-  session_write_close();
+  if (drupal_session_is_started()) {
+    // Even though session_write_close() is registered as a shutdown function,
+    // we need all session data written to the database before redirecting.
+    session_write_close();
+  }
 
   header('Location: ' . $url, TRUE, $http_response_code);
 
@@ -1831,6 +1833,15 @@ function l($text, $path, array $options 
  * react to the closing of the page by calling hook_exit().
  */
 function drupal_page_footer() {
+  global $user;
+
+  // Destroy empty anonymous sessions if possible.
+  if (!headers_sent() && drupal_session_is_started() && empty($_SESSION) && !$user->uid) {
+    session_destroy();
+  }
+  elseif (!empty($_SESSION) && !drupal_session_is_started()) {
+    watchdog('session', '$_SESSION is non-empty yet no code has called drupal_session_start().', array(), WATCHDOG_NOTICE);
+  }
 
   if (variable_get('cache', CACHE_DISABLED) != CACHE_DISABLED) {
     page_set_cache();
@@ -2971,26 +2982,24 @@ function _drupal_bootstrap_full() {
 function page_set_cache() {
   global $user, $base_root;
 
-  if (!$user->uid && ($_SERVER['REQUEST_METHOD'] == 'GET' || $_SERVER['REQUEST_METHOD'] == 'HEAD') && count(drupal_get_messages(NULL, FALSE)) == 0) {
-    // This will fail in some cases, see page_get_cache() for the explanation.
-    if ($data = ob_get_contents()) {
-      $cache = TRUE;
-      if (variable_get('page_compression', TRUE) && function_exists('gzencode')) {
-        // We do not store the data in case the zlib mode is deflate.
-        // This should be rarely happening.
-        if (zlib_get_coding_type() == 'deflate') {
-          $cache = FALSE;
-        }
-        elseif (zlib_get_coding_type() == FALSE) {
-          $data = gzencode($data, 9, FORCE_GZIP);
-        }
-        // The remaining case is 'gzip' which means the data is
-        // already compressed and nothing left to do but to store it.
-      }
-      ob_end_flush();
-      if ($cache && $data) {
-        cache_set($base_root . request_uri(), $data, 'cache_page', CACHE_TEMPORARY, drupal_get_headers());
-      }
+  if (page_get_cache(FALSE)) {
+    $cache = TRUE;
+    $data = ob_get_contents();
+    if (variable_get('page_compression', TRUE) && function_exists('gzencode')) {
+      // We do not store the data in case the zlib mode is deflate. This should
+      // be rarely happening.
+      if (zlib_get_coding_type() == 'deflate') {
+        $cache = FALSE;
+      }
+      elseif (zlib_get_coding_type() == FALSE) {
+        $data = gzencode($data, 9, FORCE_GZIP);
+      }
+      // The remaining case is 'gzip' which means the data is already
+      // compressed and nothing left to do but to store it.
+    }
+    ob_end_flush();
+    if ($cache && $data) {
+      cache_set($base_root . request_uri(), $data, 'cache_page', CACHE_TEMPORARY, drupal_get_headers());
     }
   }
 }
Index: includes/form.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/form.inc,v
retrieving revision 1.313
diff -u -F^f -p -r1.313 form.inc
--- includes/form.inc	18 Jan 2009 06:51:41 -0000	1.313
+++ includes/form.inc	18 Jan 2009 14:33:24 -0000
@@ -2476,7 +2476,7 @@ function form_clean_id($id = NULL, $flus
  *   foreach ($results as $result) {
  *     $items[] = t('Loaded node %title.', array('%title' => $result));
  *   }
- *   $_SESSION['my_batch_results'] = $items;
+ *   drupal_set_session('my_batch_results', $items);
  * }
  * @endcode
  */
Index: includes/session.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/session.inc,v
retrieving revision 1.64
diff -u -F^f -p -r1.64 session.inc
--- includes/session.inc	24 Nov 2008 06:12:45 -0000	1.64
+++ includes/session.inc	18 Jan 2009 14:33:24 -0000
@@ -124,11 +124,10 @@ function _sess_read($key) {
 function _sess_write($key, $value) {
   global $user;
 
-  // If saving of session data is disabled or if the client doesn't have a session,
-  // and one isn't being created ($value), do nothing. This keeps crawlers out of
-  // the session table. This reduces memory and server load, and gives more useful
-  // statistics. We can't eliminate anonymous session table rows without breaking
-  // the "Who's Online" block.
+  // If saving of session data is disabled, or if a new empty anonymous session
+  // has been started, do nothing. This keeps anonymous users, including
+  // crawlers, out of the session table, unless they actually have something
+  // stored in $_SESSION.
   if (!drupal_save_session() || ($user->uid == 0 && empty($_COOKIE[session_name()]) && empty($value))) {
     return TRUE;
   }
@@ -159,6 +158,72 @@ function _sess_write($key, $value) {
 }
 
 /**
+ * Propagate $_SESSION and set session cookie if not already set. This function
+ * should be called before writing to $_SESSION, usually via
+ * drupal_set_session().
+ *
+ * @param $start
+ *   If FALSE, the session is not actually started. This is only used by
+ *   drupal_session_is_started().
+ * @return
+ *   TRUE if session has already been started, or FALSE if it has not.
+ */
+function drupal_session_start($start = TRUE) {
+  static $started = FALSE;
+  if ($start && !$started) {
+    $started = TRUE;
+    session_start();
+  }
+  return $started;
+}
+
+/**
+ * Return whether a session has been started and the $_SESSION variable is
+ * available.
+ */
+function drupal_session_is_started() {
+  return drupal_session_start(FALSE);
+}
+
+/**
+ * Get a session variable.
+ *
+ * @param $name
+ *   The name of the variable to get. If not supplied, all variables are returned.
+ * @return
+ *   The value of the variable, or FALSE if the variable is not set.
+ */
+function drupal_get_session($name = NULL) {
+  if (is_null($name)) {
+    return $_SESSION;
+  }
+  elseif (isset($_SESSION[$name])) {
+    return $_SESSION[$name];
+  }
+  else {
+    return FALSE;
+  }
+}
+
+/**
+ * Set a session variable. The variable becomes accessible via $_SESSION[$name]
+ * in the current and later requests. If there is no active PHP session prior
+ * to the call, one is started automatically.
+ *
+ * Anonymous users generate less server load if their $_SESSION variable is
+ * empty, so unused entries should be unset using unset($_SESSION['foo']).
+ *
+ * @param $name
+ *   The name of the variable to set.
+ * @param $value
+ *   The value to set.
+ */
+function drupal_set_session($name, $value) {
+  drupal_session_start();
+  $_SESSION[$name] = $value;
+}
+
+/**
  * Called when an anonymous user becomes authenticated or vice-versa.
  */
 function drupal_session_regenerate() {
@@ -211,6 +276,9 @@ function _sess_destroy_sid($sid) {
   db_delete('sessions')
     ->condition('sid', $sid)
     ->execute();
+  // Unset cookie.
+  extract(session_get_cookie_params());
+  setcookie(session_name(), '', time() - 3600, $path, $domain, $secure, $httponly);
 }
 
 /**
Index: modules/book/book.install
===================================================================
RCS file: /cvs/drupal/drupal/modules/book/book.install,v
retrieving revision 1.26
diff -u -F^f -p -r1.26 book.install
--- modules/book/book.install	30 Dec 2008 16:43:16 -0000	1.26
+++ modules/book/book.install	18 Jan 2009 14:33:24 -0000
@@ -129,8 +129,8 @@ function book_update_6000() {
 
       db_create_table($ret, 'book', $schema['book']);
 
-      $_SESSION['book_update_6000_orphans']['from'] = 0;
-      $_SESSION['book_update_6000'] = array();
+      drupal_set_session('book_update_6000_orphans', array('from' => 0));
+      drupal_set_session('book_update_6000', array());
       $result = db_query("SELECT * from {book_temp} WHERE parent = 0");
 
       // Collect all books - top-level nodes.
Index: modules/dblog/dblog.admin.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/dblog/dblog.admin.inc,v
retrieving revision 1.10
diff -u -F^f -p -r1.10 dblog.admin.inc
--- modules/dblog/dblog.admin.inc	11 Jan 2009 21:19:17 -0000	1.10
+++ modules/dblog/dblog.admin.inc	18 Jan 2009 14:33:24 -0000
@@ -262,8 +262,10 @@ function _dblog_format_message($dblog) {
  * @see dblog_filter_form_validate()
  */
 function dblog_filter_form() {
+  if (!isset($_SESSION['dblog_overview_filter'])) {
+    drupal_set_session('dblog_overview_filter', array());
+  }
   $session = &$_SESSION['dblog_overview_filter'];
-  $session = is_array($session) ? $session : array();
   $filters = dblog_filters();
 
   $form['filters'] = array(
@@ -319,12 +321,15 @@ function dblog_filter_form_submit($form,
     case t('Filter'):
       foreach ($filters as $name => $filter) {
         if (isset($form_state['values'][$name])) {
+          if (!isset($_SESSION['dblog_overview_filter'])) {
+            drupal_set_session('dblog_overview_filter', array());
+          }
           $_SESSION['dblog_overview_filter'][$name] = $form_state['values'][$name];
         }
       }
       break;
     case t('Reset'):
-      $_SESSION['dblog_overview_filter'] = array();
+      drupal_set_session('dblog_overview_filter', array());
       break;
   }
   return 'admin/reports/dblog';
Index: modules/node/node.admin.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/node/node.admin.inc,v
retrieving revision 1.36
diff -u -F^f -p -r1.36 node.admin.inc
--- modules/node/node.admin.inc	14 Jan 2009 21:16:20 -0000	1.36
+++ modules/node/node.admin.inc	18 Jan 2009 14:33:25 -0000
@@ -204,8 +204,10 @@ function node_build_filter_query() {
  * Return form for node administration filters.
  */
 function node_filter_form() {
+  if (!isset($_SESSION['node_overview_filter'])) {
+    drupal_set_session('node_overview_filter', array());
+  }
   $session = &$_SESSION['node_overview_filter'];
-  $session = is_array($session) ? $session : array();
   $filters = node_filters();
 
   $i = 0;
@@ -320,6 +322,9 @@ function node_filter_form_submit($form, 
         $flat_options = form_options_flatten($filters[$filter]['options']);
 
         if (isset($flat_options[$form_state['values'][$filter]])) {
+          if (!isset($_SESSION['node_overview_filter'])) {
+            drupal_set_session('node_overview_filter', array());
+          }
           $_SESSION['node_overview_filter'][] = array($filter, $form_state['values'][$filter]);
         }
       }
@@ -328,7 +333,7 @@ function node_filter_form_submit($form, 
       array_pop($_SESSION['node_overview_filter']);
       break;
     case t('Reset'):
-      $_SESSION['node_overview_filter'] = array();
+      drupal_set_session('node_overview_filter', array());
       break;
   }
 }
Index: modules/openid/openid.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/openid/openid.module,v
retrieving revision 1.36
diff -u -F^f -p -r1.36 openid.module
--- modules/openid/openid.module	14 Jan 2009 21:13:41 -0000	1.36
+++ modules/openid/openid.module	18 Jan 2009 14:33:25 -0000
@@ -161,6 +161,9 @@ function openid_begin($claimed_id, $retu
   }
 
   // Store discovered information in the users' session so we don't have to rediscover.
+  if (!isset($_SESSION['openid'])) {
+    drupal_set_session('openid', array());
+  }
   $_SESSION['openid']['service'] = $services[0];
   // Store the claimed id
   $_SESSION['openid']['claimed_id'] = $claimed_id;
@@ -408,6 +411,9 @@ function openid_authentication($response
       // We were unable to register a valid new user, redirect to standard
       // user/register and prefill with the values we received.
       drupal_set_message(t('OpenID registration failed for the reasons listed. You may register now, or if you already have an account you can <a href="@login">log in</a> now and add your OpenID under "My Account"', array('@login' => url('user/login'))), 'error');
+      if (!isset($_SESSION['openid'])) {
+        drupal_set_session('openid', array());
+      }
       $_SESSION['openid']['values'] = $form_state['values'];
       // We'll want to redirect back to the same place.
       $destination = drupal_get_destination();
Index: modules/simpletest/drupal_web_test_case.php
===================================================================
RCS file: /cvs/drupal/drupal/modules/simpletest/drupal_web_test_case.php,v
retrieving revision 1.79
diff -u -F^f -p -r1.79 drupal_web_test_case.php
--- modules/simpletest/drupal_web_test_case.php	6 Jan 2009 12:44:20 -0000	1.79
+++ modules/simpletest/drupal_web_test_case.php	18 Jan 2009 14:33:25 -0000
@@ -919,8 +919,9 @@ class DrupalWebTestCase {
       $user = $this->originalUser;
       drupal_save_session(TRUE);
 
-      // Ensure that the internal logged in variable is reset.
+      // Ensure that internal logged in variable and cURL options are reset.
       $this->isLoggedIn = FALSE;
+      $this->additionalCurlOptions = array();
 
       // Reload module list and implementations to ensure that test module hooks
       // aren't called after tests.
Index: modules/simpletest/simpletest.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/simpletest/simpletest.module,v
retrieving revision 1.33
diff -u -F^f -p -r1.33 simpletest.module
--- modules/simpletest/simpletest.module	12 Jan 2009 06:23:57 -0000	1.33
+++ modules/simpletest/simpletest.module	18 Jan 2009 14:33:25 -0000
@@ -436,7 +436,7 @@ function _simpletest_batch_operation($te
 
 function _simpletest_batch_finished($success, $results, $operations, $elapsed) {
   if (isset($results['test_id'])) {
-    $_SESSION['test_id'] = $results['test_id'];
+    drupal_set_session('test_id', $results['test_id']);
   }
   if ($success) {
     drupal_set_message(t('The tests finished in @elapsed.', array('@elapsed' => $elapsed)));
Index: modules/simpletest/tests/session.test
===================================================================
RCS file: /cvs/drupal/drupal/modules/simpletest/tests/session.test,v
retrieving revision 1.9
diff -u -F^f -p -r1.9 session.test
--- modules/simpletest/tests/session.test	18 Dec 2008 00:42:55 -0000	1.9
+++ modules/simpletest/tests/session.test	18 Jan 2009 14:33:25 -0000
@@ -146,6 +146,89 @@ class SessionTestCase extends DrupalWebT
   }
 
   /**
+   * Test that empty anonymous sessions are destroyed.
+   */
+  function testEmptyAnonymousSession() {
+    // With caching disabled, a session is always started.
+    $this->drupalGet('');
+    $this->assertSessionCookie(FALSE);
+    $this->assertSessionStarted(TRUE);
+    $this->assertSessionEmpty(TRUE);
+
+    variable_set('cache', CACHE_NORMAL);
+
+    // During this request the session is destroyed in drupal_page_footer(),
+    // and the session cookie is unset.
+    $this->drupalGet('');
+    $this->assertSessionCookie(TRUE);
+    $this->assertSessionStarted(TRUE);
+    $this->assertSessionEmpty(TRUE);
+    $this->assertFalse($this->drupalGetHeader('ETag'), t('Page was not cached.'));
+    // When PHP deletes a cookie, it sends "Set-Cookie: cookiename=deleted;
+    // expires=..."
+    $this->assertTrue(preg_match('/SESS\w+=deleted/', $this->drupalGetHeader('Set-Cookie')), t('Session cookie was deleted.'));
+
+    // Verify that the session cookie was actually deleted.
+    $this->drupalGet('');
+    $this->assertSessionCookie(FALSE);
+    $this->assertSessionStarted(FALSE);
+    $this->assertFalse($this->drupalGetHeader('Set-Cookie'), t('New session was not started.'));
+
+    // Start a new session by setting a message.
+    $this->drupalGet('session-test/set-message');
+    $this->assertSessionCookie(FALSE);
+    $this->assertSessionStarted(FALSE);
+    $this->assertTrue($this->drupalGetHeader('Set-Cookie'), t('New session was started.'));
+
+    // Display the message.
+    $this->drupalGet('');
+    $this->assertSessionCookie(TRUE);
+    $this->assertSessionStarted(TRUE);
+    $this->assertSessionEmpty(FALSE);
+    $this->assertFalse($this->drupalGetHeader('ETag'), t('Page was not cached.'));
+    $this->assertText(t('This is a dummy message.'), t('Message was displayed.'));
+
+    // During this request the session is destroyed in _drupal_bootstrap(),
+    // and the session cookie is unset.
+    $this->drupalGet('');
+    $this->assertSessionCookie(TRUE);
+    $this->assertSessionStarted(TRUE);
+    $this->assertSessionEmpty(TRUE);
+    $this->assertTrue($this->drupalGetHeader('ETag'), t('Page was cached.'));
+    $this->assertNoText(t('This is a dummy message.'), t('Message was not cached.'));
+    $this->assertTrue(preg_match('/SESS\w+=deleted/', $this->drupalGetHeader('Set-Cookie')), t('Session cookie was deleted.'));
+
+    // Verify that session was destroyed.
+    $this->drupalGet('');
+    $this->assertSessionCookie(FALSE);
+    $this->assertSessionStarted(FALSE);
+    $this->assertTrue($this->drupalGetHeader('ETag'), t('Page was cached.'));
+    $this->assertFalse($this->drupalGetHeader('Set-Cookie'), t('New session was not started.'));
+
+    // Verify that modifying $_SESSION without having started a session
+    // generates a watchdog message, and that no messages have been generated
+    // so far.
+    $this->assertEqual($this->getWarningCount(), 0, t('No watchdog messages have been generated'));
+    $this->drupalGet('/session-test/set-not-started');
+    $this->assertSessionCookie(FALSE);
+    $this->assertSessionStarted(FALSE);
+    $this->assertEqual($this->getWarningCount(), 1, t('1 watchdog messages has been generated'));
+  }
+
+  /**
+   * Count watchdog messages about modifying $_SESSION without having started a
+   * session.
+   */
+  function getWarningCount() {
+    return db_select('watchdog')
+      ->condition('type', 'session')
+      ->condition('message', '$_SESSION is non-empty yet no code has called drupal_session_start().')
+      ->countQuery()
+      ->execute()
+      ->fetchField();
+  }
+
+  /**
    * Reset the cookie file so that it refers to the specified user.
    *
    * @param $uid User id to set as the active session.
@@ -161,4 +244,40 @@ class SessionTestCase extends DrupalWebT
     $this->drupalGet('session-test/get');
     $this->assertResponse(200, t('Session test module is correctly enabled.'), t('Session'));
   }
+
+  /**
+   * Assert whether the SimpleTest browser sent a session cookie.
+   */
+  function assertSessionCookie($sent) {
+    if ($sent) {
+      $this->assertIdentical($this->drupalGetHeader('X-Session-Cookie'), '1', t('Session cookie was sent.'));
+    }
+    else {
+      $this->assertIdentical($this->drupalGetHeader('X-Session-Cookie'), '0', t('Session cookie was not sent.'));
+    }
+  }
+
+  /**
+   * Assert whether session was started during the bootstrap process.
+   */
+  function assertSessionStarted($started) {
+    if ($started) {
+      $this->assertIdentical($this->drupalGetHeader('X-Session-Started'), '1', t('Session was started.'));
+    }
+    else {
+      $this->assertIdentical($this->drupalGetHeader('X-Session-Started'), '0', t('Session was not started.'));
+    }
+  }
+
+  /**
+   * Assert whether $_SESSION is empty at the beginning of the request.
+   */
+  function assertSessionEmpty($empty) {
+    if ($empty) {
+      $this->assertIdentical($this->drupalGetHeader('X-Session-Empty'), '1', t('Session was empty.'));
+    }
+    else {
+      $this->assertIdentical($this->drupalGetHeader('X-Session-Empty'), '0', t('Session was not empty.'));
+    }
+  }
 }
Index: modules/simpletest/tests/session_test.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/simpletest/tests/session_test.module,v
retrieving revision 1.4
diff -u -F^f -p -r1.4 session_test.module
--- modules/simpletest/tests/session_test.module	24 Nov 2008 06:12:45 -0000	1.4
+++ modules/simpletest/tests/session_test.module	18 Jan 2009 14:33:25 -0000
@@ -31,11 +31,42 @@ function session_test_menu() {
     'access arguments' => array('access content'),
     'type' => MENU_CALLBACK,
   );
+  $items['session-test/set-message'] = array(
+    'title' => t('Session value'),
+    'page callback' => '_session_test_set_message',
+    'access arguments' => array('access content'),
+    'type' => MENU_CALLBACK,
+  );
+  $items['session-test/set-not-started'] = array(
+    'title' => t('Session value'),
+    'page callback' => '_session_test_set_not_started',
+    'access arguments' => array('access content'),
+    'type' => MENU_CALLBACK,
+  );
 
   return $items;
 }
 
 /**
+ * Implement hook_boot().
+ */
+function session_test_boot() {
+  header('X-Session-Cookie: ' . intval(isset($_COOKIE[session_name()])));
+  header('X-Session-Started: ' . intval(drupal_session_is_started()));
+  header('X-Session-Empty: ' . intval(empty($_SESSION)));
+}
+
+/**
+ * Implement hook_init().
+ */
+function session_test_init() {
+  // hook_init() is called later in the bootstrap process, but not in cached
+  // requests. Here the header set in hook_boot() is overwritten, so the
+  // session state is reported as late in the bootstrap process as possible.
+  header('X-Session-Started: ' . intval(drupal_session_is_started()));
+}
+
+/**
  * Page callback, prints the stored session value to the screen.
  */
 function _session_test_get() {
@@ -51,7 +82,7 @@ function _session_test_get() {
  * Page callback, stores a value in $_SESSION['session_test_value'].
  */
 function _session_test_set($value) {
-  $_SESSION['session_test_value'] = $value;
+  drupal_set_session('session_test_value', $value);
   return t('The current value of the stored session variable has been set to %val', array('%val' => $value));
 }
 
@@ -73,6 +104,27 @@ function _session_test_id() {
 }
 
 /**
+ * Menu callback, sets a message to me displayed on the following page.
+ */
+function _session_test_set_message() {
+  drupal_set_message(t('This is a dummy message.'));
+  print t('A message was set.');
+  // Do not return anything, so the current request does not result in a themed
+  // page with messages. The message will be displayed in the following request
+  // instead.
+}
+
+/**
+ * Menu callback, stores a value in $_SESSION['session_test_value'] without
+ * having started the session in advance.
+ */
+function _session_test_set_not_started() {
+  if (!drupal_session_is_started()) {
+    $_SESSION['session_test_value'] = t('Session was not started'); 
+  }
+}
+
+/**
  * Implementation of hook_user().
  */
 function session_test_user_login($edit = array(), $user = NULL) {
Index: modules/system/system.admin.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.admin.inc,v
retrieving revision 1.117
diff -u -F^f -p -r1.117 system.admin.inc
--- modules/system/system.admin.inc	15 Jan 2009 16:07:51 -0000	1.117
+++ modules/system/system.admin.inc	18 Jan 2009 14:33:26 -0000
@@ -1308,6 +1308,18 @@ function system_performance_settings() {
 
   $description = '<p>' . t("The normal cache mode is suitable for most sites and does not cause any side effects. The aggressive cache mode causes Drupal to skip the loading (boot) and unloading (exit) of enabled modules when serving a cached page. This results in an additional performance boost but can cause unwanted side effects.") . '</p>';
 
+  // Check if the "Who's online" block is enabled.
+  $online_block_enabled = db_select('block')
+    ->condition('module', 'user')
+    ->condition('delta', 'online')
+    ->condition('status', 1)
+    ->countQuery()
+    ->execute()
+    ->fetchField();
+  if ($online_block_enabled) {
+    $description .= '<p>' . t("When caching is enabled, anonymous user sessions are only saved to the database when needed, so the \"Who's online\" block does not display the number of anonymous users.") . '</p>';
+  }
+
   $problem_modules = array_unique(array_merge(module_implements('boot'), module_implements('exit')));
   sort($problem_modules);
 
Index: modules/system/system.install
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.install,v
retrieving revision 1.297
diff -u -F^f -p -r1.297 system.install
--- modules/system/system.install	14 Jan 2009 21:13:41 -0000	1.297
+++ modules/system/system.install	18 Jan 2009 14:33:26 -0000
@@ -1899,10 +1899,10 @@ function system_update_6021() {
   // Multi-part update
   if (!isset($_SESSION['system_update_6021'])) {
     db_add_field($ret, 'menu', 'converted', array('type' => 'int', 'unsigned' => TRUE, 'not null' => TRUE, 'default' => 0, 'size' => 'tiny'));
-    $_SESSION['system_update_6021_max'] = db_result(db_query('SELECT COUNT(*) FROM {menu}'));
-    $_SESSION['menu_menu_map'] = array(1 => 'navigation');
+    drupal_set_session('system_update_6021_max', db_result(db_query('SELECT COUNT(*) FROM {menu}')));
+    drupal_set_session('menu_menu_map', array(1 => 'navigation'));
     // 0 => FALSE is for new menus, 1 => FALSE is for the navigation.
-    $_SESSION['menu_item_map'] = array(0 => FALSE, 1 => FALSE);
+    drupal_set_session('menu_item_map', array(0 => FALSE, 1 => FALSE));
     $table = array(
       'fields' => array(
         'menu_name'   => array('type' => 'varchar', 'length' => 32, 'not null' => TRUE, 'default' => ''),
@@ -1913,7 +1913,7 @@ function system_update_6021() {
     );
     db_create_table($ret, 'menu_custom', $table);
     db_query("INSERT INTO {menu_custom} (menu_name, title, description) VALUES ('%s', '%s', '%s')", $menus['navigation']);
-    $_SESSION['system_update_6021'] = 0;
+    drupal_set_session('system_update_6021', 0);
   }
 
   $limit = 50;
Index: modules/user/user.admin.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/user/user.admin.inc,v
retrieving revision 1.35
diff -u -F^f -p -r1.35 user.admin.inc
--- modules/user/user.admin.inc	11 Jan 2009 21:19:19 -0000	1.35
+++ modules/user/user.admin.inc	18 Jan 2009 14:33:26 -0000
@@ -33,8 +33,10 @@ function user_admin($callback_arg = '') 
  * @see user_filter_form_submit()
  */
 function user_filter_form() {
+  if (!isset($_SESSION['user_overview_filter'])) {
+    drupal_set_session('user_overview_filter', array());
+  }
   $session = &$_SESSION['user_overview_filter'];
-  $session = is_array($session) ? $session : array();
   $filters = user_filters();
 
   $i = 0;
@@ -101,6 +103,9 @@ function user_filter_form_submit($form, 
         // Merge an array of arrays into one if necessary.
         $options = $filter == 'permission' ? call_user_func_array('array_merge', $filters[$filter]['options']) : $filters[$filter]['options'];
         if (isset($options[$form_state['values'][$filter]])) {
+          if (!isset($_SESSION['user_overview_filter'])) {
+            drupal_set_session('user_overview_filter', array());
+          }
           $_SESSION['user_overview_filter'][] = array($filter, $form_state['values'][$filter]);
         }
       }
@@ -109,7 +114,7 @@ function user_filter_form_submit($form, 
       array_pop($_SESSION['user_overview_filter']);
       break;
     case t('Reset'):
-      $_SESSION['user_overview_filter'] = array();
+      drupal_set_session('user_overview_filter', array());
       break;
     case t('Update'):
       return;
Index: modules/user/user.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/user/user.module,v
retrieving revision 1.954
diff -u -F^f -p -r1.954 user.module
--- modules/user/user.module	13 Jan 2009 06:27:01 -0000	1.954
+++ modules/user/user.module	18 Jan 2009 14:33:27 -0000
@@ -812,6 +812,7 @@ function user_block_configure($delta = '
       $period = drupal_map_assoc(array(30, 60, 120, 180, 300, 600, 900, 1800, 2700, 3600, 5400, 7200, 10800, 21600, 43200, 86400), 'format_interval');
       $form['user_block_seconds_online'] = array('#type' => 'select', '#title' => t('User activity'), '#default_value' => variable_get('user_block_seconds_online', 900), '#options' => $period, '#description' => t('A user is considered online for this long after they have last viewed a page.'));
       $form['user_block_max_list_count'] = array('#type' => 'select', '#title' => t('User list length'), '#default_value' => variable_get('user_block_max_list_count', 10), '#options' => drupal_map_assoc(array(0, 5, 10, 15, 20, 25, 30, 40, 50, 75, 100)), '#description' => t('Maximum number of currently online users to display.'));
+      $form['user_block_cache'] = array('#markup' => '<p>If page caching is disabled, the block shows the number of anonymous and authenticated users, respectively. If page caching is enabled, only the number of authenticated users is displayed.</p>');
       return $form;
   }
 }
@@ -877,15 +878,22 @@ function user_block_view($delta = '') {
 
         // Perform database queries to gather online user lists. We use s.timestamp
         // rather than u.access because it is much faster.
-        $anonymous_count = drupal_session_count($interval);
         $authenticated_count = db_query("SELECT COUNT(DISTINCT s.uid) FROM {sessions} s WHERE s.timestamp >= :timestamp AND s.uid > 0", array(':timestamp' => $interval))->fetchField();
 
-        // Format the output with proper grammar.
-        if ($anonymous_count == 1 && $authenticated_count == 1) {
-          $output = t('There is currently %members and %visitors online.', array('%members' => format_plural($authenticated_count, '1 user', '@count users'), '%visitors' => format_plural($anonymous_count, '1 guest', '@count guests')));
+        // When page caching is enabled, sessions are only created for
+        // anonymous users when needed.
+        if (variable_get('cache', CACHE_DISABLED) == CACHE_DISABLED) {
+          $anonymous_count = drupal_session_count($interval);
+          // Format the output with proper grammar.
+          if ($anonymous_count == 1 && $authenticated_count == 1) {
+            $output = t('There is currently %members and %visitors online.', array('%members' => format_plural($authenticated_count, '1 user', '@count users'), '%visitors' => format_plural($anonymous_count, '1 guest', '@count guests')));
+          }
+          else {
+            $output = t('There are currently %members and %visitors online.', array('%members' => format_plural($authenticated_count, '1 user', '@count users'), '%visitors' => format_plural($anonymous_count, '1 guest', '@count guests')));
+          }
         }
         else {
-          $output = t('There are currently %members and %visitors online.', array('%members' => format_plural($authenticated_count, '1 user', '@count users'), '%visitors' => format_plural($anonymous_count, '1 guest', '@count guests')));
+          $output = format_plural($authenticated_count, 'There is currently 1 user online.', 'There are currently @count users online.');
         }
 
         // Display a list of currently online users.