job pickling: plugins don't return their factories

Remove the need for plugins to find and return the factory they came
from. Also take the opportunity to combine source and element pickling
into a single 'plugin' pickling path.

This will make it easier for us to later support pickling plugins from
the 'first_pass_config' of projects.

1 files changed, 4 insertions, 5 deletions

diff --git a/src/buildstream/plugin.py b/src/buildstream/plugin.py
index e269a4b1a..c1ee333f7 100644
--- a/src/buildstream/plugin.py
+++ b/src/buildstream/plugin.py
@@ -712,11 +712,10 @@ class Plugin():
     # Return data necessary to reconstruct this object in a child job process.
     #
     # Returns:
-    #    (PluginContext, str, dict): A tuple of (factory, meta_kind, state),
-    #    where `factory` is an object that can use `meta_kind` to create an
-    #    instance of the same type as `self`. `state` is what we want
-    #    `self.__dict__` to be restored to after instantiation in the child
-    #    process.
+    #    (str, dict): A tuple of (meta_kind, state), where a factory can use
+    #    `meta_kind` to create an instance of the same type as `self`. `state`
+    #    is what we want `self.__dict__` to be restored to after instantiation
+    #    in the child process.
     #
     def _get_args_for_child_job_pickling(self):
         # Note that this is only to be implemented as a BuildStream internal,