Ticket #8616: cross_platform_session_file_read_write_locking.diff

django/contrib/sessions/backends/file.py

@@ -5 +5 @@
 from django.conf import settings
 from django.contrib.sessions.backends.base import SessionBase, CreateError
 from django.core.exceptions import SuspiciousOperation, ImproperlyConfigured
+from django.core.files import locks
 
+IO_LOCK_SUFFIX = "_iolock"
 
 class SessionStore(SessionBase):
     """
@@ -42 +44 @@
 
         return os.path.join(self.storage_path, self.file_prefix + session_key)
 
+    def _key_to_io_lock_file(self, session_key=None):
+        """
+        Get the I/O lock file associated with this session key.
+        """
+        return self._key_to_file(session_key) + IO_LOCK_SUFFIX
+
     def load(self):
         session_data = {}
         try:
+            # Open and acquire a shared lock on the I/O lock file before
+            # attempting to read the session file.  This makes us wait to read
+            # the session file until another thread or process is finished
+            # writing it.
+            io_lock_file = open(self._key_to_io_lock_file(), "rb")
+            locks.lock(io_lock_file, locks.LOCK_SH)
+            
             session_file = open(self._key_to_file(), "rb")
             try:
                 try:
@@ -53 +68 @@
                     self.create()
             finally:
                 session_file.close()
+                io_lock_file.close()
         except IOError:
             pass
         return session_data
@@ -76 +92 @@
         # truncating the file to save.
         session_data = self._get_session(no_load=must_create)
         try:
+            # Open and acquire an exclusive lock on the I/O lock file before
+            # attempting to write the session file.  This makes other threads
+            # or processes wait to read or write the session file until we are
+            # finished writing it.
+            io_lock_file = open(self._key_to_io_lock_file(), "wb")
+            locks.lock(io_lock_file, locks.LOCK_EX)
+            
             fd = os.open(self._key_to_file(self.session_key), flags)
             try:
                 os.write(fd, self.encode(session_data))
             finally:
                 os.close(fd)
+                io_lock_file.close()
         except OSError, e:
             if must_create and e.errno == errno.EEXIST:
                 raise CreateError
@@ -100 +124 @@
             session_key = self._session_key
         try:
             os.unlink(self._key_to_file(session_key))
+            os.unlink(self._key_to_io_lock_file(session_key))
         except OSError:
             pass