Implement lazy file opening for diff generation

rdiff_backup/opener.py

+class LazyFile:
+	def __init__(self, path, *args, **kwargs):
+		self.path = path
+		self.args = args
+		self.kwargs = kwargs
+		self.file = None
+
+	def read(self, length=-1):
+		self.open()
+		return self.file.read(length)
+
+	def open(self):
+		if self.file is None:
+			self.file = open(self.path, *self.args, **self.kwargs)
+
+	def close(self):
+		if self.file is None:
+			return None
+		value = self.file.close()
+		self.file = None
+		return value
+
+
+def lazy_open(path, *args, **kwargs):
+	return LazyFile(path, *args, **kwargs)

rdiff_backup/restore.py

@@ -24,6 +24,7 @@ from __future__ import generators
 from pool import Pool as ThreadPool
 import tempfile, os, cStringIO, itertools
 import static, rorpiter, FilenameMapping, connection
+import opener
 
 class RestoreError(Exception): pass
 
@@ -294,7 +295,8 @@ class MirrorStruct:
 				# a file is already there, we can create a diff
 				signature_vf = target_rorp.conn.Rdiff.get_signature_vf(target_rorp)
 				target_signature = connection.VirtualFile(target_rorp.conn, signature_vf)
-				delta_fp = mir_rorp.get_delta(target_signature, open(file_fp.name, 'rb'))
+				file_fobj = opener.lazy_open(file_fp.name, 'rb')
+				delta_fp = mir_rorp.get_delta(target_signature, file_fobj)
 				mir_rorp.setfile(delta_fp)
 			else:
 				# standard rdiff-backup behavior, transfer everything