1 """A dumb and slow but simple dbm clone.

     2 

     3 For database spam, spam.dir contains the index (a text file),

     4 spam.bak *may* contain a backup of the index (also a text file),

     5 while spam.dat contains the data (a binary file).

     6 

     7 XXX TO DO:

     8 

     9 - seems to contain a bug when updating...

    10 

    11 - reclaim free space (currently, space once occupied by deleted or expanded

    12 items is never reused)

    13 

    14 - support concurrent access (currently, if two processes take turns making

    15 updates, they can mess up the index)

    16 

    17 - support efficient access to large databases (currently, the whole index

    18 is read when the database is opened, and some updates rewrite the whole index)

    19 

    20 - support opening for read-only (flag = 'm')

    21 

    22 """

    23 

    24 import os as _os

    25 import __builtin__

    26 import UserDict

    27 

    28 _open = __builtin__.open

    29 

    30 _BLOCKSIZE = 512

    31 

    32 error = IOError                         # For anydbm

    33 

    34 class _Database(UserDict.DictMixin):

    35 

    36     # The on-disk directory and data files can remain in mutually

    37     # inconsistent states for an arbitrarily long time (see comments

    38     # at the end of __setitem__).  This is only repaired when _commit()

    39     # gets called.  One place _commit() gets called is from __del__(),

    40     # and if that occurs at program shutdown time, module globals may

    41     # already have gotten rebound to None.  Since it's crucial that

    42     # _commit() finish successfully, we can't ignore shutdown races

    43     # here, and _commit() must not reference any globals.

    44     _os = _os       # for _commit()

    45     _open = _open   # for _commit()

    46 

    47     def __init__(self, filebasename, mode):

    48         self._mode = mode

    49 

    50         # The directory file is a text file.  Each line looks like

    51         #    "%r, (%d, %d)\n" % (key, pos, siz)

    52         # where key is the string key, pos is the offset into the dat

    53         # file of the associated value's first byte, and siz is the number

    54         # of bytes in the associated value.

    55         self._dirfile = filebasename + _os.extsep + 'dir'

    56 

    57         # The data file is a binary file pointed into by the directory

    58         # file, and holds the values associated with keys.  Each value

    59         # begins at a _BLOCKSIZE-aligned byte offset, and is a raw

    60         # binary 8-bit string value.

    61         self._datfile = filebasename + _os.extsep + 'dat'

    62         self._bakfile = filebasename + _os.extsep + 'bak'

    63 

    64         # The index is an in-memory dict, mirroring the directory file.

    65         self._index = None  # maps keys to (pos, siz) pairs

    66 

    67         # Mod by Jack: create data file if needed

    68         try:

    69             f = _open(self._datfile, 'r')

    70         except IOError:

    71             f = _open(self._datfile, 'w')

    72             self._chmod(self._datfile)

    73         f.close()

    74         self._update()

    75 

    76     # Read directory file into the in-memory index dict.

    77     def _update(self):

    78         self._index = {}

    79         try:

    80             f = _open(self._dirfile)

    81         except IOError:

    82             pass

    83         else:

    84             for line in f:

    85                 line = line.rstrip()

    86                 key, pos_and_siz_pair = eval(line)

    87                 self._index[key] = pos_and_siz_pair

    88             f.close()

    89 

    90     # Write the index dict to the directory file.  The original directory

    91     # file (if any) is renamed with a .bak extension first.  If a .bak

    92     # file currently exists, it's deleted.

    93     def _commit(self):

    94         # CAUTION:  It's vital that _commit() succeed, and _commit() can

    95         # be called from __del__().  Therefore we must never reference a

    96         # global in this routine.

    97         if self._index is None:

    98             return  # nothing to do

    99 

   100         try:

   101             self._os.unlink(self._bakfile)

   102         except self._os.error:

   103             pass

   104 

   105         try:

   106             self._os.rename(self._dirfile, self._bakfile)

   107         except self._os.error:

   108             pass

   109 

   110         f = self._open(self._dirfile, 'w')

   111         self._chmod(self._dirfile)

   112         for key, pos_and_siz_pair in self._index.iteritems():

   113             f.write("%r, %r\n" % (key, pos_and_siz_pair))

   114         f.close()

   115 

   116     sync = _commit

   117 

   118     def __getitem__(self, key):

   119         pos, siz = self._index[key]     # may raise KeyError

   120         f = _open(self._datfile, 'rb')

   121         f.seek(pos)

   122         dat = f.read(siz)

   123         f.close()

   124         return dat

   125 

   126     # Append val to the data file, starting at a _BLOCKSIZE-aligned

   127     # offset.  The data file is first padded with NUL bytes (if needed)

   128     # to get to an aligned offset.  Return pair

   129     #     (starting offset of val, len(val))

   130     def _addval(self, val):

   131         f = _open(self._datfile, 'rb+')

   132         f.seek(0, 2)

   133         pos = int(f.tell())

   134         npos = ((pos + _BLOCKSIZE - 1) // _BLOCKSIZE) * _BLOCKSIZE

   135         f.write('\0'*(npos-pos))

   136         pos = npos

   137         f.write(val)

   138         f.close()

   139         return (pos, len(val))

   140 

   141     # Write val to the data file, starting at offset pos.  The caller

   142     # is responsible for ensuring that there's enough room starting at

   143     # pos to hold val, without overwriting some other value.  Return

   144     # pair (pos, len(val)).

   145     def _setval(self, pos, val):

   146         f = _open(self._datfile, 'rb+')

   147         f.seek(pos)

   148         f.write(val)

   149         f.close()

   150         return (pos, len(val))

   151 

   152     # key is a new key whose associated value starts in the data file

   153     # at offset pos and with length siz.  Add an index record to

   154     # the in-memory index dict, and append one to the directory file.

   155     def _addkey(self, key, pos_and_siz_pair):

   156         self._index[key] = pos_and_siz_pair

   157         f = _open(self._dirfile, 'a')

   158         self._chmod(self._dirfile)

   159         f.write("%r, %r\n" % (key, pos_and_siz_pair))

   160         f.close()

   161 

   162     def __setitem__(self, key, val):

   163         if not type(key) == type('') == type(val):

   164             raise TypeError, "keys and values must be strings"

   165         if key not in self._index:

   166             self._addkey(key, self._addval(val))

   167         else:

   168             # See whether the new value is small enough to fit in the

   169             # (padded) space currently occupied by the old value.

   170             pos, siz = self._index[key]

   171             oldblocks = (siz + _BLOCKSIZE - 1) // _BLOCKSIZE

   172             newblocks = (len(val) + _BLOCKSIZE - 1) // _BLOCKSIZE

   173             if newblocks <= oldblocks:

   174                 self._index[key] = self._setval(pos, val)

   175             else:

   176                 # The new value doesn't fit in the (padded) space used

   177                 # by the old value.  The blocks used by the old value are

   178                 # forever lost.

   179                 self._index[key] = self._addval(val)

   180 

   181             # Note that _index may be out of synch with the directory

   182             # file now:  _setval() and _addval() don't update the directory

   183             # file.  This also means that the on-disk directory and data

   184             # files are in a mutually inconsistent state, and they'll

   185             # remain that way until _commit() is called.  Note that this

   186             # is a disaster (for the database) if the program crashes

   187             # (so that _commit() never gets called).

   188 

   189     def __delitem__(self, key):

   190         # The blocks used by the associated value are lost.

   191         del self._index[key]

   192         # XXX It's unclear why we do a _commit() here (the code always

   193         # XXX has, so I'm not changing it).  _setitem__ doesn't try to

   194         # XXX keep the directory file in synch.  Why should we?  Or

   195         # XXX why shouldn't __setitem__?

   196         self._commit()

   197 

   198     def keys(self):

   199         return self._index.keys()

   200 

   201     def has_key(self, key):

   202         return key in self._index

   203 

   204     def __contains__(self, key):

   205         return key in self._index

   206 

   207     def iterkeys(self):

   208         return self._index.iterkeys()

   209     __iter__ = iterkeys

   210 

   211     def __len__(self):

   212         return len(self._index)

   213 

   214     def close(self):

   215         self._commit()

   216         self._index = self._datfile = self._dirfile = self._bakfile = None

   217 

   218     __del__ = close

   219 

   220     def _chmod (self, file):

   221         if hasattr(self._os, 'chmod'):

   222             self._os.chmod(file, self._mode)

   223 

   224 

   225 def open(file, flag=None, mode=0666):

   226     """Open the database file, filename, and return corresponding object.

   227 

   228     The flag argument, used to control how the database is opened in the

   229     other DBM implementations, is ignored in the dumbdbm module; the

   230     database is always opened for update, and will be created if it does

   231     not exist.

   232 

   233     The optional mode argument is the UNIX mode of the file, used only when

   234     the database has to be created.  It defaults to octal code 0666 (and

   235     will be modified by the prevailing umask).

   236 

   237     """

   238     # flag argument is currently ignored

   239 

   240     # Modify mode depending on the umask

   241     try:

   242         um = _os.umask(0)

   243         _os.umask(um)

   244     except AttributeError:

   245         pass

   246     else:

   247         # Turn off any bits that are set in the umask

   248         mode = mode & (~um)

   249 

   250     return _Database(file, mode)