Lib/dumbdbm.py

   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)