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         f = open(filename + os.extsep + "dat", "rb")
  54         f.close()
  55         f = open(filename + os.extsep + "dir", "rb")
  56         try:
  57             if f.read(1) in ["'", '"']:
  58                 return "dumbdbm"
  59         finally:
  60             f.close()
  61     except IOError:
  62         pass
  63
  64     # See if the file exists, return None if not
  65     try:
  66         f = open(filename, "rb")
  67     except IOError:
  68         return None
  69
  70     # Read the start of the file -- the magic number
  71     s16 = f.read(16)
  72     f.close()
  73     s = s16[0:4]
  74
  75     # Return "" if not at least 4 bytes
  76     if len(s) != 4:
  77         return ""
  78
  79     # Convert to 4-byte int in native byte order -- return "" if impossible
  80     try:
  81         (magic,) = struct.unpack("=l", s)
  82     except struct.error:
  83         return ""
  84
  85     # Check for GNU dbm
  86     if magic == 0x13579ace:
  87         return "gdbm"
  88
  89     # Check for BSD hash
  90     if magic in (0x00061561, 0x61150600):
  91         return "dbhash"
  92
  93     # BSD hash v2 has a 12-byte NULL pad in front of the file type
  94     try:
  95         (magic,) = struct.unpack("=l", s16[-4:])
  96     except struct.error:
  97         return ""
  98
  99     # Check for BSD hash
 100     if magic in (0x00061561, 0x61150600):
 101         return "dbhash"
 102
 103     # Unknown
 104     return ""