The MONGO_INITIALIZER actions are stored in a hash map keyed by their string names. This is traversed to produce the inputs to the unstable topological sort that determines their execution order. They are inserted in some order determined by an executable's linker line. So, it's possible for a Graph that's missing a prerequisite constraint edge to slip through testing, since the input order will be kind of stable until an initializer changes its name or the link order changes.

Anyway, we should add randomness to the selections made by the topological sort so that if there's a missing dependency constraint we'll be likely to encounter it in testing.