Index: includes/bootstrap.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/bootstrap.inc,v
retrieving revision 1.453
diff -u -p -r1.453 bootstrap.inc
--- includes/bootstrap.inc	11 Dec 2010 21:03:47 -0000	1.453
+++ includes/bootstrap.inc	14 Dec 2010 08:02:50 -0000
@@ -546,7 +546,7 @@ function drupal_valid_http_host($host) {
  * session name correctly.
  */
 function drupal_settings_initialize() {
-  global $base_url, $base_path, $base_root;
+  global $base_url, $base_path, $base_root, $cookie_domain_is_custom;
 
   // Export the following settings.php variables to the global namespace
   global $databases, $cookie_domain, $conf, $installed_profile, $update_free_access, $db_url, $db_prefix, $drupal_hash_salt, $is_https, $base_secure_url, $base_insecure_url;
@@ -592,6 +592,8 @@ function drupal_settings_initialize() {
   if ($cookie_domain) {
     // If the user specifies the cookie domain, also use it for session name.
     $session_name = $cookie_domain;
+    // Also record that $cookie_domain is custom and not auto-generated.
+    $cookie_domain_is_custom = TRUE;
   }
   else {
     // Otherwise use $base_url as session name, without the protocol
Index: includes/update.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/update.inc,v
retrieving revision 1.83
diff -u -p -r1.83 update.inc
--- includes/update.inc	6 Dec 2010 06:57:40 -0000	1.83
+++ includes/update.inc	14 Dec 2010 08:02:50 -0000
@@ -156,6 +156,18 @@ function update_prepare_d7_bootstrap() {
       ),
     );
     update_extra_requirements($requirements);
+
+    // Allow a D6 session to work, since the upgrade has not been performed yet.
+    $d6_session_name = update_get_d6_session_name();
+    if (!empty($_COOKIE[$d6_session_name])) {
+      require_once DRUPAL_ROOT . '/includes/session.inc';
+      $sid = $_COOKIE[$d6_session_name];
+
+      // Respect the D6 sid and read an existing session with it.
+      $_COOKIE[session_name()] = $sid;
+      session_id($sid);
+      _drupal_session_read($sid);
+    }
   }
 
   // Create the registry tables.
@@ -829,6 +841,35 @@ function update_parse_db_url($db_url, $d
 }
 
 /**
+ * Constructs a session name compatible with a D6 environment.
+ *
+ * @return
+ *   D6-compatible session name string.
+ *
+ * @see drupal_settings_initialize()
+ */
+function update_get_d6_session_name() {
+  global $base_url, $cookie_domain, $cookie_domain_is_custom;
+
+  // If the cookie domain was specified in settings.php, also use it for
+  // session name.
+  if ($cookie_domain_is_custom) {
+    $session_name = $cookie_domain;
+  }
+  else {
+    // Otherwise use $base_url as session name, without the protocol
+    // to use the same session identifiers across http and https.
+    list( , $session_name) = explode('://', $base_url, 2);
+  }
+
+  if (ini_get('session.cookie_secure')) {
+    $session_name .= 'SSL';
+  }
+
+  return 'SESS' . md5($session_name);
+}
+
+/**
  * Perform one update and store the results for display on finished page.
  *
  * If an update function completes successfully, it should return a message
Index: modules/simpletest/tests/upgrade/upgrade.test
===================================================================
RCS file: /cvs/drupal/drupal/modules/simpletest/tests/upgrade/upgrade.test,v
retrieving revision 1.10
diff -u -p -r1.10 upgrade.test
--- modules/simpletest/tests/upgrade/upgrade.test	5 Nov 2010 19:05:02 -0000	1.10
+++ modules/simpletest/tests/upgrade/upgrade.test	14 Dec 2010 08:02:50 -0000
@@ -34,6 +34,9 @@ abstract class UpgradePathTestCase exten
   protected function setUp() {
     global $user, $language, $conf;
 
+    // Load the Update API.
+    require_once DRUPAL_ROOT . '/includes/update.inc';
+
     // Reset flags.
     $this->upgradedSite = FALSE;
     $this->upgradeErrors = array();
@@ -106,10 +109,11 @@ abstract class UpgradePathTestCase exten
     drupal_save_session(FALSE);
     $user = db_query('SELECT * FROM {users} WHERE uid = :uid', array(':uid' => 1))->fetchObject();
 
-    // Generate and set a session cookie.
+    // Generate and set a D6-compatible session cookie.
     $this->curlInitialize();
     $sid = drupal_hash_base64(uniqid(mt_rand(), TRUE) . drupal_random_bytes(55));
-    curl_setopt($this->curlHandle, CURLOPT_COOKIE, rawurlencode($this->session_name) . '=' . rawurlencode($sid));
+    $session_name = update_get_d6_session_name();
+    curl_setopt($this->curlHandle, CURLOPT_COOKIE, rawurlencode($session_name) . '=' . rawurlencode($sid));
 
     // Force our way into the session of the child site.
     drupal_save_session(TRUE);