download*: new alternate-url option

README.rst

@@ -339,6 +339,17 @@ In case of checksum mismatch::
   Error: MD5 checksum mismatch downloading '...'
   >>> ls('parts')
 
+option: alternate-url
+---------------------
+
+Alternate URL. If supported by Buildout, it is used as fallback if the main
+URL (`url` option) fails at HTTP level.
+
+Useful when a version of a resource can only be downloaded with a temporary
+URL as long as it's the last version, and this version is then moved to a
+permanent place when a newer version is released: `url` shall be the final URL
+and `alternate-url` the temporary one.
+
 option: mode
 ------------
 
@@ -462,6 +473,11 @@ option: md5sum
 An MD5 checksum can be specified to check the downloaded file, like for the
 ``download`` recipe. However, if unset, updating the part does nothing.
 
+option: alternate-url
+---------------------
+
+See the ``download`` recipe.
+
 option: environment
 -------------------
 

slapos/recipe/download.py

@@ -36,6 +36,7 @@ class Recipe(object):
   def __init__(self, buildout, name, options):
     self._buildout = buildout['buildout']
     self._url = options['url']
+    self._alternate = options.get('alternate-url')
     self._md5sum = options.get('md5sum') or None
     self._name = name
     mode = options.get('mode')
@@ -72,8 +73,10 @@ class Recipe(object):
     return [destination]
 
   def _download(self):
+    alternate = self._alternate
     download.Download(self._buildout, hash_name=True)(
-      self._url, self._md5sum, self._destination)
+      self._url, self._md5sum, self._destination,
+      **({'alternate_url': alternate} if alternate else {}))
     if self._mode is not None:
       os.chmod(self._destination, self._mode)
 

slapos/recipe/downloadunpacked.py

@@ -61,8 +61,10 @@ class Recipe(EnvironMixin):
 
   def _install(self):
     location = self._shared.location
+    alternate = self.options.get('alternate-url')
     path, is_temp = download.Download(self.buildout['buildout'],
-      hash_name=True)(self._url, self.options.get('md5sum') or None)
+      hash_name=True)(self._url, self.options.get('md5sum') or None,
+                      **({'alternate_url': alternate} if alternate else {}))
     try:
       archive_util.extraction_drivers = patched_extraction_drivers
       # ad-hoc support for .xz and .lz archive