Fix problem where :0>3s produced a string of all zeros for null fields.

Add a confirmation dialog to the backup pushbutton

src/calibre/gui2/preferences/misc.py

@@ -97,6 +97,9 @@ def mark_dirty(self):
         db = self.gui.library_view.model().db
         ids = [id for id in db.data.iterallids()]
         db.dirtied(ids)
+        info_dialog(self, _('Backup metadata'),
+            _('Metadata will be backed up while calibre is running, at the '
+              'rate of 30 books per minute.'), show=True)
 
     def debug_device_detection(self, *args):
         from calibre.gui2.preferences.device_debug import DebugDevice

src/calibre/gui2/preferences/misc.ui

@@ -127,7 +127,7 @@
    <item row="10" column="0" colspan="2">
     <widget class="QPushButton" name="button_all_books_dirty">
      <property name="text">
-      <string>Back up metadata of all books (while you are working)</string>
+      <string>Back up metadata of all books</string>
      </property>
     </widget>
    </item>

src/calibre/library/caches.py

@@ -75,6 +75,7 @@ def run(self):
             # Give the GUI thread a chance to do something. Python threads don't
             # have priorities, so this thread would naturally keep the processor
             # until some scheduling event happens. The sleep makes such an event
+            print 'do one'
             time.sleep(0.1)
             try:
                 raw = metadata_to_opf(mi)

src/calibre/utils/formatter.py

@@ -100,7 +100,7 @@ def format_field(self, val, fmt):
                 val = func[1](self, val)
             else:
                 val = func[1](self, val, *args)
-        else:
+        elif val:
             val = string.Formatter.format_field(self, val, fmt)
         if not val:
             return ''