Lib/whichdb.py

   1 """Guess which db package to use to open a db file."""
   2
   3 import os
   4 import struct
   5
   6 try:
   7     import dbm
   8     _dbmerror = dbm.error
   9 except ImportError:
  10     dbm = None
  11     # just some sort of valid exception which might be raised in the
  12     # dbm test
  13     _dbmerror = IOError
  14
  15 def whichdb(filename):
  16     """Guess which db package to use to open a db file.
  17
  18     Return values:
  19
  20     - None if the database file can't be read;
  21     - empty string if the file can be read but can't be recognized
  22     - the module name (e.g. "dbm" or "gdbm") if recognized.
  23
  24     Importing the given module may still fail, and opening the
  25     database using that module may still fail.
  26     """
  27
  28     # Check for dbm first -- this has a .pag and a .dir file
  29     try:
  30         f = open(filename + os.extsep + "pag", "rb")
  31         f.close()
  32         f = open(filename + os.extsep + "dir", "rb")
  33         f.close()
  34         return "dbm"
  35     except IOError:
  36         # some dbm emulations based on Berkeley DB generate a .db file
  37         # some do not, but they should be caught by the dbhash checks
  38         try:
  39             f = open(filename + os.extsep + "db", "rb")
  40             f.close()
  41             # guarantee we can actually open the file using dbm
  42             # kind of overkill, but since we are dealing with emulations
  43             # it seems like a prudent step
  44             if dbm is not None:
  45                 d = dbm.open(filename)
  46                 d.close()
  47                 return "dbm"
  48         except (IOError, _dbmerror):
  49             pass
  50
  51     # Check for dumbdbm next -- this has a .dir and and a .dat file
  52     try:
  53         # First check for presence of files
  54         os.stat(filename + os.extsep + "dat")
  55         size = os.stat(filename + os.extsep + "dir").st_size
  56         # dumbdbm files with no keys are empty
  57         if size == 0:
  58             return "dumbdbm"
  59         f = open(filename + os.extsep + "dir", "rb")
  60         try:
  61             if f.read(1) in ["'", '"']:
  62                 return "dumbdbm"
  63         finally:
  64             f.close()
  65     except (OSError, IOError):
  66         pass
  67
  68     # See if the file exists, return None if not
  69     try:
  70         f = open(filename, "rb")
  71     except IOError:
  72         return None
  73
  74     # Read the start of the file -- the magic number
  75     s16 = f.read(16)
  76     f.close()
  77     s = s16[0:4]
  78
  79     # Return "" if not at least 4 bytes
  80     if len(s) != 4:
  81         return ""
  82
  83     # Convert to 4-byte int in native byte order -- return "" if impossible
  84     try:
  85         (magic,) = struct.unpack("=l", s)
  86     except struct.error:
  87         return ""
  88
  89     # Check for GNU dbm
  90     if magic == 0x13579ace:
  91         return "gdbm"
  92
  93     # Check for old Berkeley db hash file format v2
  94     if magic in (0x00061561, 0x61150600):
  95         return "bsddb185"
  96
  97     # Later versions of Berkeley db hash file have a 12-byte pad in
  98     # front of the file type
  99     try:
 100         (magic,) = struct.unpack("=l", s16[-4:])
 101     except struct.error:
 102         return ""
 103
 104     # Check for BSD hash
 105     if magic in (0x00061561, 0x61150600):
 106         return "dbhash"
 107
 108     # Unknown
 109     return ""