slapos.cookbook: Add _failsafe publish type recipes

In some contexts it's required to not stop buildout processing on publishing
errors, thus such recipe is required in those cases.

By providing -error-status-file it's possible to check status of the publish,
the file will be created on error and removed on successful call or during
uninstall. It's responsibility of the caller to handle such errors, for example
by using proper promises.

setup.py

@@ -131,6 +131,8 @@ setup(name=name,
           'publish-early = slapos.recipe.publish_early:Recipe',
           'publishsection = slapos.recipe.publish:PublishSection',
           'publishurl = slapos.recipe.publishurl:Recipe',
+          'publish_failsafe = slapos.recipe.publish:RecipeFailsafe',
+          'publish.serialised_failsafe = slapos.recipe.publish:SerialisedFailsafe',
           'random.time = slapos.recipe.random:Time',
           'random.integer = slapos.recipe.random:Integer',
           'readline = slapos.recipe.readline:Recipe',

slapos/recipe/publish.py

@@ -29,10 +29,12 @@ import zc.buildout
 from slapos.recipe.librecipe import wrap
 from slapos.recipe.librecipe import GenericSlapRecipe
 import six
+import os
 
 CONNECTION_PARAMETER_STRING = 'connection-'
 
 class Recipe(GenericSlapRecipe):
+  return_list = []
   def __init__(self, buildout, name, options):
     super(Recipe, self).__init__(buildout, name, options)
     # Tell buildout about the sections we will access during install.
@@ -55,7 +57,7 @@ class Recipe(GenericSlapRecipe):
       for k in publish:
         publish_dict[k] = section[k]
     self._setConnectionDict(publish_dict, self.options.get('-slave-reference'))
-    return []
+    return self.return_list
 
   def _setConnectionDict(self, publish_dict, slave_reference=None):
     return self.setConnectionDict(publish_dict, slave_reference)
@@ -65,6 +67,32 @@ class Serialised(Recipe):
     return super(Serialised, self)._setConnectionDict(wrap(publish_dict), slave_reference)
 
 
+class Failsafe(object):
+  def _setConnectionDict(self, publish_dict, slave_reference):
+    error_status_file = self.options.get('-error-status-file')
+    if error_status_file:
+      self.return_list = [error_status_file]
+    else:
+      self.return_list = []
+    try:
+      super(Failsafe, self)._setConnectionDict(publish_dict, slave_reference)
+    except Exception:
+      if error_status_file is not None:
+        with open(error_status_file, 'w') as fh:
+          fh.write('')
+    else:
+      if error_status_file is not None:
+        if os.path.exists(error_status_file):
+          os.unlink(error_status_file)
+
+
+class RecipeFailsafe(Failsafe, Recipe):
+  pass
+
+
+class SerialisedFailsafe(Failsafe, Serialised):
+  pass
+
 
 class PublishSection(GenericSlapRecipe):
   """