1"""Generic interface to all dbm clones.
2
3Use
4
5        import dbm
6        d = dbm.open(file, 'w', 0o666)
7
8The returned object is a dbm.gnu, dbm.ndbm or dbm.dumb object, dependent on the
9type of database being opened (determined by the whichdb function) in the case
10of an existing dbm. If the dbm does not exist and the create or new flag ('c'
11or 'n') was specified, the dbm type will be determined by the availability of
12the modules (tested in the above order).
13
14It has the following interface (key and data are strings):
15
16        d[key] = data   # store data at key (may override data at
17                        # existing key)
18        data = d[key]   # retrieve data at key (raise KeyError if no
19                        # such key)
20        del d[key]      # delete data stored at key (raises KeyError
21                        # if no such key)
22        flag = key in d # true if the key exists
23        list = d.keys() # return a list of all existing keys (slow!)
24
25Future versions may change the order in which implementations are
26tested for existence, and add interfaces to other dbm-like
27implementations.
28"""
29
30__all__ = ['open', 'whichdb', 'error']
31
32import io
33import os
34import struct
35import sys
36
37
38class error(Exception):
39    pass
40
41_names = ['dbm.gnu', 'dbm.ndbm', 'dbm.dumb']
42_defaultmod = None
43_modules = {}
44
45error = (error, OSError)
46
47try:
48    from dbm import ndbm
49except ImportError:
50    ndbm = None
51
52
53def open(file, flag='r', mode=0o666):
54    """Open or create database at path given by *file*.
55
56    Optional argument *flag* can be 'r' (default) for read-only access, 'w'
57    for read-write access of an existing database, 'c' for read-write access
58    to a new or existing database, and 'n' for read-write access to a new
59    database.
60
61    Note: 'r' and 'w' fail if the database doesn't exist; 'c' creates it
62    only if it doesn't exist; and 'n' always creates a new database.
63    """
64    global _defaultmod
65    if _defaultmod is None:
66        for name in _names:
67            try:
68                mod = __import__(name, fromlist=['open'])
69            except ImportError:
70                continue
71            if not _defaultmod:
72                _defaultmod = mod
73            _modules[name] = mod
74        if not _defaultmod:
75            raise ImportError("no dbm clone found; tried %s" % _names)
76
77    # guess the type of an existing database, if not creating a new one
78    result = whichdb(file) if 'n' not in flag else None
79    if result is None:
80        # db doesn't exist or 'n' flag was specified to create a new db
81        if 'c' in flag or 'n' in flag:
82            # file doesn't exist and the new flag was used so use default type
83            mod = _defaultmod
84        else:
85            raise error[0]("need 'c' or 'n' flag to open new db")
86    elif result == "":
87        # db type cannot be determined
88        raise error[0]("db type could not be determined")
89    elif result not in _modules:
90        raise error[0]("db type is {0}, but the module is not "
91                       "available".format(result))
92    else:
93        mod = _modules[result]
94    return mod.open(file, flag, mode)
95
96
97def whichdb(filename):
98    """Guess which db package to use to open a db file.
99
100    Return values:
101
102    - None if the database file can't be read;
103    - empty string if the file can be read but can't be recognized
104    - the name of the dbm submodule (e.g. "ndbm" or "gnu") if recognized.
105
106    Importing the given module may still fail, and opening the
107    database using that module may still fail.
108    """
109
110    # Check for ndbm first -- this has a .pag and a .dir file
111    try:
112        f = io.open(filename + ".pag", "rb")
113        f.close()
114        f = io.open(filename + ".dir", "rb")
115        f.close()
116        return "dbm.ndbm"
117    except OSError:
118        # some dbm emulations based on Berkeley DB generate a .db file
119        # some do not, but they should be caught by the bsd checks
120        try:
121            f = io.open(filename + ".db", "rb")
122            f.close()
123            # guarantee we can actually open the file using dbm
124            # kind of overkill, but since we are dealing with emulations
125            # it seems like a prudent step
126            if ndbm is not None:
127                d = ndbm.open(filename)
128                d.close()
129                return "dbm.ndbm"
130        except OSError:
131            pass
132
133    # Check for dumbdbm next -- this has a .dir and a .dat file
134    try:
135        # First check for presence of files
136        os.stat(filename + ".dat")
137        size = os.stat(filename + ".dir").st_size
138        # dumbdbm files with no keys are empty
139        if size == 0:
140            return "dbm.dumb"
141        f = io.open(filename + ".dir", "rb")
142        try:
143            if f.read(1) in (b"'", b'"'):
144                return "dbm.dumb"
145        finally:
146            f.close()
147    except OSError:
148        pass
149
150    # See if the file exists, return None if not
151    try:
152        f = io.open(filename, "rb")
153    except OSError:
154        return None
155
156    with f:
157        # Read the start of the file -- the magic number
158        s16 = f.read(16)
159    s = s16[0:4]
160
161    # Return "" if not at least 4 bytes
162    if len(s) != 4:
163        return ""
164
165    # Convert to 4-byte int in native byte order -- return "" if impossible
166    try:
167        (magic,) = struct.unpack("=l", s)
168    except struct.error:
169        return ""
170
171    # Check for GNU dbm
172    if magic in (0x13579ace, 0x13579acd, 0x13579acf):
173        return "dbm.gnu"
174
175    # Later versions of Berkeley db hash file have a 12-byte pad in
176    # front of the file type
177    try:
178        (magic,) = struct.unpack("=l", s16[-4:])
179    except struct.error:
180        return ""
181
182    # Unknown
183    return ""
184
185
186if __name__ == "__main__":
187    for filename in sys.argv[1:]:
188        print(whichdb(filename) or "UNKNOWN", filename)
189