Commits on Jan 21, 2022

1. selectors: fix for edge case around recursion clauses with an immedia… …

   …te edge.
   
   See also the diff in the ipld/ipld repo for the new fixture,
   which includes some explanation.
   
   This would be a somewhat silly selector to write, but,
   it doesn't seem to be something we should reject at compile time, either;
   so, we must handle it gracefully.
   
   Along the way, I documented some code that was sparse in comments.
   
   I also added a shortcut to traversals so that a selector that
   explicitly states it's not interested in anything (by returning a
   non-nil but empty slice for its interests) is treated differently
   than a nil return for interests (which means "I don't know; hit me
   with everything you've got and lemme see").  This means that our
   edge case here with edges (...heh) doesn't cause the traversal
   to create an iterator that it doesn't really need, etc.
   
   We turn out to need both that shortcut, *and* less panicky methods on
   ExploreRecursiveEdge, *and* the edge case branch in ExploreRecursive...
   because traversals are pre-order.  Decide is called first, then
   Interests, and then Explore.  Therefore informing the Explore method
   alone about this situation is not sufficient.
   
   Previously, a recursion clause with an edge as its immediate and only
   child would get a passing grade by the compiler (same as now),
   but would panic when actually used (oh dear).

   

   warpfork committed Jan 21, 2022
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