default: allow 'install' to install no file/dir

README.rst

@@ -222,6 +222,49 @@ Setting `location` option is incompatible::
     Initializing section script.
   Error: When shared=true, option 'location' can't be set
 
+option: location
+----------------
+
+If empty or unset, the value is initialized automatically according to rules
+defined above (`shared` option), and before the `init` code is executed.
+This way, it's possible to initialize other values of the section depending
+on the actual value of location.
+
+If not shared, the value can be customized in `init`. This is actually the
+only way to empty location, which is useful if there's nothing file/directory
+to track but you need to distinguish install from update::
+
+  >>> write(sample_buildout, 'buildout.cfg', """
+  ... [buildout]
+  ... parts = script
+  ...
+  ... [script]
+  ... recipe = slapos.recipe.build
+  ... init =
+  ...   options['location'] = ''
+  ... install =
+  ...   print("install")
+  ... update =
+  ...   print("update")
+  ... """)
+
+  >>> print(system(buildout))
+  Uninstalling script.
+  Installing script.
+  install
+  >>> print(system(buildout))
+  Updating script.
+  update
+  >>> cat('.installed.cfg')
+  [buildout]
+  ...
+  [script]
+  __buildout_installed__ =
+  __buildout_signature__ = ...
+
+If install & update run the same code, `install` can be unset (or empty)
+and you can ignore `location`.
+
 
 =============================
  slapos.recipe.build:download

slapos/recipe/build/__init__.py

@@ -263,7 +263,11 @@ class Script(EnvironMixin):
 
   def install(self):
     if self._install:
+      shared = self._shared
+      if shared.location:
         return self._shared.install(self.__install)
+      self._exec(self._install)
+    else:
       self.update()
     return ()