jinja2: by default, rerender on update only if the template may have changed

slapos/recipe/template/README.jinja2.txt

@@ -485,6 +485,53 @@ path)::
     b1foo
     bc1foo
 
+``update`` - force rerendering on update
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+By default, and like the default recipe, nothing is done on update if the
+template is known in advance to be the same, either because it's inline or
+a md5sum is given::
+
+    >>> write('buildout.cfg',
+    ... '''
+    ... [buildout]
+    ... parts = template
+    ...
+    ... [template]
+    ... recipe = slapos.recipe.template:jinja2
+    ... inline = {{ os.environ['FOO'] }}
+    ... output = foo
+    ... context = import os os
+    ... ''')
+    >>> os.environ['FOO'] = '1'
+    >>> run_buildout()
+    Uninstalling template.
+    Installing template.
+    >>> cat('foo')
+    1
+    >>> os.environ['FOO'] = '2'
+    >>> run_buildout()
+    Updating template.
+    >>> cat('foo')
+    1
+
+But Jinja2 is such that the output may depend on other things than buildout
+data and it may be wanted to force update in such case::
+
+    >>> with open('buildout.cfg', 'a') as f:
+    ...     f.writelines(['update = true'])
+    >>> run_buildout()
+    Uninstalling template.
+    Installing template.
+    >>> cat('foo')
+    2
+    >>> os.environ['FOO'] = '1'
+    >>> run_buildout()
+    Updating template.
+    >>> cat('foo')
+    1
+    >>> del os.environ['FOO']
+
 ``encoding``
 ~~~~~~~~~~~~
 

slapos/recipe/template/__init__.py

@@ -61,6 +61,9 @@ def get_umask():
     get_umask = lambda: umask
     return umask
 
+def is_true(value, default=False):
+    return default if value is None else ('false', 'true').index(value)
+
 
 class Recipe(object):
 

slapos/recipe/template/jinja2_template.py

@@ -30,7 +30,7 @@ import zc.buildout
 from jinja2 import Environment, StrictUndefined, \
     BaseLoader, TemplateNotFound, PrefixLoader
 import six
-from . import Recipe
+from . import is_true, Recipe
 
 DEFAULT_CONTEXT = {x.__name__: x for x in (
     abs, all, any, bin, bool, bytes, callable, chr, complex, dict, divmod,
@@ -145,6 +145,7 @@ class Recipe(Recipe):
         args = self.buildout, name, options
         self.once = options.get('once')
         self.encoding = options.get('encoding', 'utf-8')
+        self._update = is_true(options.get('update'))
         import_delimiter = options.get('import-delimiter',
             DEFAULT_IMPORT_DELIMITER)
         import_dict = {}
@@ -223,4 +224,8 @@ class Recipe(Recipe):
             return
         return installed
 
-    update = install
+    def update(self):
+        if self._update:
+            self.install()
+        else:
+            super(Recipe, self).update()