Django

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Changeset 8750

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Timestamp:
08/30/08 15:50:41 (3 months ago)
Author:
jacob
Message:

Fixed #8616 (again): prevent a race condition in the session file backend. Many thanks to Warren Smith for help and the eventual fix.

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  • django/trunk/AUTHORS

    r8747 r8750  
    369369    sloonz <simon.lipp@insa-lyon.fr> 
    370370    SmileyChris <smileychris@gmail.com> 
     371    Warren Smith <warren@wandrsmith.net>  
    371372    smurf@smurf.noris.de 
    372373    Vsevolod Solovyov 

  • django/trunk/django/contrib/sessions/backends/file.py

    r8707 r8750  
    4848            session_file = open(self._key_to_file(), "rb") 
    4949            try: 
    50                 try: 
    51                     session_data = self.decode(session_file.read()) 
    52                 except (EOFError, SuspiciousOperation): 
    53                     self.create() 
     50                file_data = session_file.read() 
     51                # Don't fail if there is no data in the session file. 
     52                # We may have opened the empty placeholder file. 
     53                if file_data: 
     54                    try: 
     55                        session_data = self.decode(file_data) 
     56                    except (EOFError, SuspiciousOperation): 
     57                        self.create() 
    5458            finally: 
    5559                session_file.close() 
    …… 
    7074 
    7175    def save(self, must_create=False): 
    72         flags = os.O_WRONLY | os.O_CREAT | os.O_TRUNC | getattr(os, 'O_BINARY', 0) 
    73         if must_create: 
    74             flags |= os.O_EXCL 
    75         # Because this may trigger a load from storage, we must do it before 
    76         # truncating the file to save. 
     76        # Get the session data now, before we start messing 
     77        # with the file it is stored within. 
    7778        session_data = self._get_session(no_load=must_create) 
     79 
     80        session_file_name = self._key_to_file() 
     81 
    7882        try: 
    79             fd = os.open(self._key_to_file(self.session_key), flags) 
    80             try: 
    81                 os.write(fd, self.encode(session_data)) 
    82             finally: 
    83                 os.close(fd) 
     83            # Make sure the file exists.  If it does not already exist, an 
     84            # empty placeholder file is created. 
     85            flags = os.O_WRONLY | os.O_CREAT | getattr(os, 'O_BINARY', 0) 
     86            if must_create: 
     87                flags |= os.O_EXCL 
     88            fd = os.open(session_file_name, flags) 
     89            os.close(fd) 
     90 
    8491        except OSError, e: 
    8592            if must_create and e.errno == errno.EEXIST: 
    8693                raise CreateError 
    8794            raise 
    88         except (IOError, EOFError): 
     95 
     96        # Write the session file without interfering with other threads 
     97        # or processes.  By writing to an atomically generated temporary 
     98        # file and then using the atomic os.rename() to make the complete 
     99        # file visible, we avoid having to lock the session file, while 
     100        # still maintaining its integrity. 
     101        # 
     102        # Note: Locking the session file was explored, but rejected in part 
     103        # because in order to be atomic and cross-platform, it required a 
     104        # long-lived lock file for each session, doubling the number of 
     105        # files in the session storage directory at any given time.  This 
     106        # rename solution is cleaner and avoids any additional overhead 
     107        # when reading the session data, which is the more common case 
     108        # unless SESSION_SAVE_EVERY_REQUEST = True. 
     109        # 
     110        # See ticket #8616. 
     111        dir, prefix = os.path.split(session_file_name) 
     112 
     113        try: 
     114            output_file_fd, output_file_name = tempfile.mkstemp(dir=dir, 
     115                prefix=prefix + '_out_') 
     116            try: 
     117                try: 
     118                    os.write(output_file_fd, self.encode(session_data)) 
     119                finally: 
     120                    os.close(output_file_fd) 
     121                renamed = False 
     122                os.rename(output_file_name, session_file_name) 
     123                renamed = True 
     124            finally: 
     125                if not renamed: 
     126                    os.unlink(output_file_name) 
     127 
     128        except (OSError, IOError, EOFError): 
    89129            pass 
    90130