Create chunks, strata in cache via temporary files

This avoids problems with files with the right names but partial
content, if morph is killed in the middle of writing the file.

1 files changed, 66 insertions, 0 deletions

diff --git a/morphlib/savefile.py b/morphlib/savefile.py
new file mode 100644
index 00000000..902a9923
--- /dev/null
+++ b/morphlib/savefile.py
@@ -0,0 +1,66 @@
+# Copyright (C) 2012  Codethink Limited
+# 
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; version 2 of the License.
+# 
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+# 
+# You should have received a copy of the GNU General Public License along
+# with this program; if not, write to the Free Software Foundation, Inc.,
+# 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+
+
+import os
+import tempfile
+
+class SaveFile(file):
+
+    '''Save files with a temporary name and rename when they're ready.
+    
+    This class acts exactly like the normal ``file`` class, except that
+    it is meant only for saving data to files. The data is written to
+    a temporary file, which gets renamed to the target name when the
+    open file is closed. This avoids readers of the file from getting
+    an incomplete file.
+    
+    Example:
+    
+        f = SaveFile('foo', 'w')
+        f.write(stuff)
+        f.close()
+        
+    The file will be called something like ``tmpCAFEBEEF`` until ``close``
+    is called, at which point it gets renamed to ``foo``.
+    
+    If the writer decides the file is not worth saving, they can call the
+    ``abort`` method, which deletes the temporary file.
+    
+    '''
+
+    def __init__(self, filename, *args, **kwargs):
+        self._savefile_filename = filename
+        dirname = os.path.dirname(filename)
+        fd, self._savefile_tempname = tempfile.mkstemp(dir=dirname)
+        os.close(fd)
+        file.__init__(self, self._savefile_tempname, *args, **kwargs)
+
+    def abort(self):
+        '''Abort file saving.
+        
+        The temporary file will be removed, and the universe is almost
+        exactly as if the file save had never started.
+        
+        '''
+
+        os.remove(self._savefile_tempname)
+        return file.close(self)
+
+    def close(self):
+        ret = file.close(self)
+        os.rename(self._savefile_tempname, self._savefile_filename)
+        return ret
+