diff -r 6641e941ef1e -r ff1a9aa48cfd app/django/contrib/sessions/backends/file.py
--- a/app/django/contrib/sessions/backends/file.py	Tue Oct 14 12:36:55 2008 +0000
+++ b/app/django/contrib/sessions/backends/file.py	Tue Oct 14 16:00:59 2008 +0000
@@ -1,9 +1,12 @@
+import errno
 import os
 import tempfile
+
 from django.conf import settings
-from django.contrib.sessions.backends.base import SessionBase
+from django.contrib.sessions.backends.base import SessionBase, CreateError
 from django.core.exceptions import SuspiciousOperation, ImproperlyConfigured
 
+
 class SessionStore(SessionBase):
     """
     Implements a file based session store.
@@ -15,10 +18,10 @@
 
         # Make sure the storage path is valid.
         if not os.path.isdir(self.storage_path):
-            raise ImproperlyConfigured("The session storage path %r doesn't exist. "\
-                                       "Please set your SESSION_FILE_PATH setting "\
-                                       "to an existing directory in which Django "\
-                                       "can store session data." % self.storage_path)
+            raise ImproperlyConfigured(
+                "The session storage path %r doesn't exist. Please set your"
+                " SESSION_FILE_PATH setting to an existing directory in which"
+                " Django can store session data." % self.storage_path)
 
         self.file_prefix = settings.SESSION_COOKIE_NAME
         super(SessionStore, self).__init__(session_key)
@@ -31,9 +34,11 @@
             session_key = self.session_key
 
         # Make sure we're not vulnerable to directory traversal. Session keys
-        # should always be md5s, so they should never contain directory components.
+        # should always be md5s, so they should never contain directory
+        # components.
         if os.path.sep in session_key:
-            raise SuspiciousOperation("Invalid characters (directory components) in session key")
+            raise SuspiciousOperation(
+                "Invalid characters (directory components) in session key")
 
         return os.path.join(self.storage_path, self.file_prefix + session_key)
 
@@ -42,28 +47,85 @@
         try:
             session_file = open(self._key_to_file(), "rb")
             try:
-                try:
-                    session_data = self.decode(session_file.read())
-                except(EOFError, SuspiciousOperation):
-                    self._session_key = self._get_new_session_key()
-                    self._session_cache = {}
-                    self.save()
-                    # Ensure the user is notified via a new cookie.
-                    self.modified = True
+                file_data = session_file.read()
+                # Don't fail if there is no data in the session file.
+                # We may have opened the empty placeholder file.
+                if file_data:
+                    try:
+                        session_data = self.decode(file_data)
+                    except (EOFError, SuspiciousOperation):
+                        self.create()
             finally:
                 session_file.close()
-        except(IOError):
+        except IOError:
             pass
         return session_data
 
-    def save(self):
-        try:
-            f = open(self._key_to_file(self.session_key), "wb")
+    def create(self):
+        while True:
+            self._session_key = self._get_new_session_key()
             try:
-                f.write(self.encode(self._session))
+                self.save(must_create=True)
+            except CreateError:
+                continue
+            self.modified = True
+            self._session_cache = {}
+            return
+
+    def save(self, must_create=False):
+        # Get the session data now, before we start messing
+        # with the file it is stored within.
+        session_data = self._get_session(no_load=must_create)
+
+        session_file_name = self._key_to_file()
+
+        try:
+            # Make sure the file exists.  If it does not already exist, an
+            # empty placeholder file is created.
+            flags = os.O_WRONLY | os.O_CREAT | getattr(os, 'O_BINARY', 0)
+            if must_create:
+                flags |= os.O_EXCL
+            fd = os.open(session_file_name, flags)
+            os.close(fd)
+
+        except OSError, e:
+            if must_create and e.errno == errno.EEXIST:
+                raise CreateError
+            raise
+
+        # Write the session file without interfering with other threads
+        # or processes.  By writing to an atomically generated temporary
+        # file and then using the atomic os.rename() to make the complete
+        # file visible, we avoid having to lock the session file, while
+        # still maintaining its integrity.
+        #
+        # Note: Locking the session file was explored, but rejected in part
+        # because in order to be atomic and cross-platform, it required a
+        # long-lived lock file for each session, doubling the number of
+        # files in the session storage directory at any given time.  This
+        # rename solution is cleaner and avoids any additional overhead
+        # when reading the session data, which is the more common case
+        # unless SESSION_SAVE_EVERY_REQUEST = True.
+        #
+        # See ticket #8616.
+        dir, prefix = os.path.split(session_file_name)
+
+        try:
+            output_file_fd, output_file_name = tempfile.mkstemp(dir=dir,
+                prefix=prefix + '_out_')
+            renamed = False
+            try:
+                try:
+                    os.write(output_file_fd, self.encode(session_data))
+                finally:
+                    os.close(output_file_fd)
+                os.rename(output_file_name, session_file_name)
+                renamed = True
             finally:
-                f.close()
-        except(IOError, EOFError):
+                if not renamed:
+                    os.unlink(output_file_name)
+
+        except (OSError, IOError, EOFError):
             pass
 
     def exists(self, session_key):
@@ -71,7 +133,11 @@
             return True
         return False
 
-    def delete(self, session_key):
+    def delete(self, session_key=None):
+        if session_key is None:
+            if self._session_key is None:
+                return
+            session_key = self._session_key
         try:
             os.unlink(self._key_to_file(session_key))
         except OSError: