jinja2: Prevent 'once' from overwriting 'rendered'

See merge request nexedi/slapos.recipe.template!5

slapos/recipe/template/README.jinja2.txt

@@ -555,3 +555,40 @@ Removing the canary allows template to be re-rendered::
     >>> cat('foo_once')
     dummy
 
+It's also possible to use the same file for ``rendered`` and ``once``::
+
+    >>> write('buildout.cfg',
+    ... '''
+    ... [buildout]
+    ... parts = template
+    ...
+    ... [template]
+    ... recipe = slapos.recipe.template:jinja2
+    ... template = inline:initial content
+    ... rendered = rendered
+    ... once = ${:rendered}
+    ... ''')
+    >>> run_buildout() # doctest: +ELLIPSIS
+    Uninstalling template.
+    Installing template.
+    The template install returned None.  A path or iterable os paths should be returned.
+
+Template was rendered::
+
+    >>> cat('rendered')
+    initial content
+
+When buildout options are modified, the template will not be rendered again::
+
+    >>> with open('buildout.cfg', 'a') as f:
+    ...     f.writelines(['template = inline:something different'])
+
+    >>> run_buildout()
+    Uninstalling template.
+    Installing template.
+    The template install returned None.  A path or iterable os paths should be returned.
+
+Even though we used a different template, the file still contain the first template::
+
+    >>> cat('rendered')
+    initial content

slapos/recipe/template/jinja2_template.py

@@ -266,7 +266,7 @@ class Recipe(object):
         finally:
             os.close(fd)
         if self.once:
-            open(self.once, 'wb').close()
+            open(self.once, 'ab').close()
             return
         return self.rendered