Added transaction support to write & delete files.

* if error occurs, disk image returned to state it was before the call

atrcopy/diskimages.py

@@ -377,6 +377,14 @@ class DiskImageBase(object):
 
     # file writing methods
 
+    def begin_transaction(self):
+        state = self.bytes[:], self.style[:]
+        return state
+
+    def rollback_transaction(self, state):
+        self.bytes[:], self.style[:] = state
+        return
+
     def write_file(self, filename, filetype, data):
         """Write data to a file on disk
 
@@ -384,6 +392,8 @@ class DiskImageBase(object):
         not enough space on disk or a free entry is not available in the
         catalog.
         """
+        state = self.begin_transaction()
+        try:
             directory = self.directory_class(self.bytes_per_sector)
             self.get_directory(directory)
             dirent = directory.add_dirent(filename, filetype)
@@ -396,6 +406,11 @@ class DiskImageBase(object):
             self.write_sector_list(vtoc)
             self.write_sector_list(directory)
             self.get_metadata()
+        except AtrError:
+            self.rollback_transaction(state)
+            raise
+        finally:
+            self.get_metadata()
 
     def write_sector_list(self, sector_list):
         for sector in sector_list:
@@ -404,6 +419,8 @@ class DiskImageBase(object):
             self.bytes[pos:pos + size] = sector.data
 
     def delete_file(self, filename):
+        state = self.begin_transaction()
+        try:
             directory = self.directory_class(self.bytes_per_sector)
             self.get_directory(directory)
             dirent = directory.find_dirent(filename)
@@ -415,6 +432,11 @@ class DiskImageBase(object):
             self.write_sector_list(vtoc)
             self.write_sector_list(directory)
             self.get_metadata()
+        except AtrError:
+            self.rollback_transaction(state)
+            raise
+        finally:
+            self.get_metadata()
 
 
 class WriteableSector(object):