With some clearer thinking, I believe this is the proper implementation of tmpDir. If there's something wrong with it, send me a note or write a test and it'll be fixed.

plugins/Infobot.py

@@ -104,7 +104,7 @@ class InfobotDB(object):
                           'I hear ya']
 
     def flush(self):
-        fd = utils.AtomicFile(filename)
+        fd = utils.transactionalFile(filename, 'wb')
         pickle.dump((self._is, self._are), fd)
         fd.close()
 

plugins/URL.py

@@ -149,7 +149,7 @@ class URLDB(object):
         return [url for (url, nick) in self.getUrlsAndNicks(p)]
 
     def vacuum(self):
-        out = utils.AtomicFile(self.filename)
+        out = utils.transactionalFile(self.filename)
         notAdded = 0
         urls = self.getUrlsAndNicks(lambda *args: True)
         seen = sets.Set()

src/cdb.py

@@ -136,7 +136,7 @@ def make(dbFilename, readFilename=None):
 class Maker(object):
     """Class for making CDB databases."""
     def __init__(self, filename):
-        self.fd = utils.AtomicFile(filename)
+        self.fd = utils.transactionalFile(filename)
         self.filename = filename
         self.fd.seek(2048)
         self.hashPointers = [(0, 0)] * 256

src/conf.py

@@ -526,11 +526,19 @@ registerGlobalValue(supybot.drivers, 'module',
 
 class Directory(registry.String):
     def __call__(self):
+        # ??? Should we perhaps always return an absolute path here?
         v = super(Directory, self).__call__()
         if not os.path.exists(v):
             os.mkdir(v)
         return v
 
+    def dirize(self, filename):
+        dir = self()
+        dirname = os.path.basename(filename)
+        if not dirname.endswith(dir): # ??? Should this be an "in" test instead?
+            return os.path.join(dir, os.path.basename(filename))
+        return filename
+
 class DataFilename(registry.String):
     def __call__(self):
         v = super(DataFilename, self).__call__()
@@ -557,12 +565,11 @@ registerGlobalValue(supybot.directories.data, 'tmp',
     DataFilenameDirectory('tmp', """Determines what directory temporary files
     are put into."""))
 
-# This is a hack, but it should work.
-utils._AtomicFile = utils.AtomicFile
-def AtomicFile(*args, **kwargs):
+# Remember, we're *meant* to replace this nice little wrapper.
+def transactionalFile(*args, **kwargs):
     kwargs['tmpDir'] = supybot.directories.data.tmp()
-    return utils._AtomicFile(*args, **kwargs)
-utils.AtomicFile = AtomicFile
+    return utils.AtomicFile(*args, **kwargs)
+utils.transactionalFile = transactionalFile
 
 class PluginDirectories(registry.CommaSeparatedListOfStrings):
     def __call__(self):

src/ircdb.py

@@ -571,7 +571,7 @@ class UsersDictionary(utils.IterableMap):
         if self.filename is not None:
             L = self.users.items()
             L.sort()
-            fd = utils.AtomicFile(self.filename)
+            fd = utils.transactionalFile(self.filename)
             for (id, u) in L:
                 fd.write('user %s' % id)
                 fd.write(os.linesep)
@@ -732,7 +732,7 @@ class ChannelsDictionary(utils.IterableMap):
     def flush(self):
         """Flushes the channel database to its file."""
         if self.filename is not None:
-            fd = utils.AtomicFile(self.filename)
+            fd = utils.transactionalFile(self.filename)
             for (channel, c) in self.channels.iteritems():
                 fd.write('channel %s' % channel)
                 fd.write(os.linesep)
@@ -792,7 +792,7 @@ class IgnoresDB(object):
 
     def flush(self):
         if self.filename is not None:
-            fd = utils.AtomicFile(self.filename)
+            fd = utils.transactionalFile(self.filename)
             for hostmask in self.hostmasks:
                 fd.write(hostmask)
                 fd.write(os.linesep)

src/registry.py

@@ -78,7 +78,7 @@ def open(filename, clear=False):
 def close(registry, filename, annotated=True, helpOnceOnly=False):
     first = True
     helpCache = sets.Set()
-    fd = utils.AtomicFile(filename)
+    fd = utils.transactionalFile(filename)
     for (name, value) in registry.getValues(getChildren=True):
         if annotated and hasattr(value,'help') and value.help:
             if not helpOnceOnly or value.help not in self.helpCache:

src/utils.py

@@ -673,34 +673,36 @@ def stackTrace():
 
 class AtomicFile(file):
     """Used for files that need to be atomically written -- i.e., if there's a
-    failure, the original file remains, unmodified.
-
-    Opens the file in 'w' mode."""
-    def __init__(self, filename, allowEmptyOverwrite=False, tmpDir=None):
-        self.filename = os.path.abspath(filename)
+    failure, the original file remains, unmodified.  mode must be 'w' or 'wb'"""
+    def __init__(self, filename, mode='w',
+                 allowEmptyOverwrite=False, tmpDir=None):
+        if mode not in ('w', 'wb'):
+            raise ValueError, 'Invalid mode: %r' % mode
         self.rolledback = False
         self.allowEmptyOverwrite = allowEmptyOverwrite
-        self.tempFilename = '%s.%s' % (self.filename, mktemp())
-        if tmpDir is not None:
-            tempFilename = os.path.dirname(self.tempFilename)
-            self.tempFilename = os.path.join(tmpDir, self.tempFilename)
-        super(self.__class__, self).__init__(self.tempFilename, 'w')
+        self.filename = filename
+        if tmpDir is None:
+            # If not given a tmpDir, we'll just put a random token on the end
+            # of our filename and put it in the same directory.
+            self.tempFilename = '%s.%s' % (self.filename, mktemp())
+        else:
+            # If given a tmpDir, we'll get the basename (just the filename, no
+            # directory), put our random token on the end, and put it in tmpDir
+            tempFilename = '%s.%s' % (os.path.basename(self.filename), mktemp())
+            self.tempFilename = os.path.join(tmpDir, tempFilename)
+        super(AtomicFile, self).__init__(self.tempFilename, mode)
 
     def rollback(self):
-        #print 'AtomicFile.rollback'
         if not self.closed:
-            super(self.__class__, self).close()
+            super(AtomicFile, self).close()
             if os.path.exists(self.tempFilename):
-                #print 'AtomicFile: Removing %s.' % self.tempFilename
                 os.remove(self.tempFilename)
             self.rolledback = True
 
     def close(self):
-        #print 'AtomicFile.close'
         if not self.rolledback:
-            #print 'AtomicFile.close: actually closing.'
-            super(self.__class__, self).close()
-            size = os.stat(self.tempFilename).st_size
+            super(AtomicFile, self).close()
+            size = os.path.getsize(self.tempFilename)
             if size or self.allowEmptyOverwrite:
                 if os.path.exists(self.tempFilename):
                     # We use shutil.move here instead of os.rename because
@@ -710,9 +712,16 @@ class AtomicFile(file):
                     shutil.move(self.tempFilename, self.filename)
 
     def __del__(self):
-        #print 'AtomicFile.__del__'
+        # We rollback because if we're deleted without being explicitly closed,
+        # that's bad.  We really should log this here, but as of yet we've got
+        # no logging facility in utils.  I've got some ideas for this, though.
         self.rollback()
 
+def transactionalFile(*args, **kwargs):
+    # This exists so it can be replaced by a function that provides the tmpDir.
+    # We do that replacement in conf.py.
+    return AtomicFile(*args, **kwargs)
+
 if __name__ == '__main__':
     import sys, doctest
     doctest.testmod(sys.modules['__main__'])