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"""Guess which db package to use to open a db file."""
import os
import struct
import sys
try:
import dbm
_dbmerror = dbm.error
except ImportError:
dbm = None
# just some sort of valid exception which might be raised in the
# dbm test
_dbmerror = IOError
def whichdb(filename):
"""Guess which db package to use to open a db file.
Return values:
- None if the database file can't be read;
- empty string if the file can be read but can't be recognized
- the module name (e.g. "dbm" or "gdbm") if recognized.
Importing the given module may still fail, and opening the
database using that module may still fail.
"""
# Check for dbm first -- this has a .pag and a .dir file
try:
f = open(filename + os.extsep + "pag", "rb")
f.close()
# dbm linked with gdbm on OS/2 doesn't have .dir file
if not (dbm.library == "GNU gdbm" and sys.platform == "os2emx"):
f = open(filename + os.extsep + "dir", "rb")
f.close()
return "dbm"
except IOError:
# some dbm emulations based on Berkeley DB generate a .db file
# some do not, but they should be caught by the dbhash checks
try:
f = open(filename + os.extsep + "db", "rb")
f.close()
# guarantee we can actually open the file using dbm
# kind of overkill, but since we are dealing with emulations
# it seems like a prudent step
if dbm is not None:
d = dbm.open(filename)
d.close()
return "dbm"
except (IOError, _dbmerror):
pass
# Check for dumbdbm next -- this has a .dir and and a .dat file
try:
# First check for presence of files
os.stat(filename + os.extsep + "dat")
size = os.stat(filename + os.extsep + "dir").st_size
# dumbdbm files with no keys are empty
if size == 0:
return "dumbdbm"
f = open(filename + os.extsep + "dir", "rb")
try:
if f.read(1) in ["'", '"']:
return "dumbdbm"
finally:
f.close()
except (OSError, IOError):
pass
# See if the file exists, return None if not
try:
f = open(filename, "rb")
except IOError:
return None
# Read the start of the file -- the magic number
s16 = f.read(16)
f.close()
s = s16[0:4]
# Return "" if not at least 4 bytes
if len(s) != 4:
return ""
# Convert to 4-byte int in native byte order -- return "" if impossible
try:
(magic,) = struct.unpack("=l", s)
except struct.error:
return ""
# Check for GNU dbm
if magic == 0x13579ace:
return "gdbm"
# Check for old Berkeley db hash file format v2
if magic in (0x00061561, 0x61150600):
return "bsddb185"
# Later versions of Berkeley db hash file have a 12-byte pad in
# front of the file type
try:
(magic,) = struct.unpack("=l", s16[-4:])
except struct.error:
return ""
# Check for BSD hash
if magic in (0x00061561, 0x61150600):
return "dbhash"
# Unknown
return ""
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