[starvation] prefer shorter UI-thread traces

Summary: There may be several reasons why we think a method is on the UI thread.  Choose to keep the shortest via join.

Reviewed By: jeremydubreil

Differential Revision: D9806944

fbshipit-source-id: 89d27456d

infer/src/concurrency/starvationDomain.ml

@@ -224,7 +224,7 @@ module UIThreadExplanationDomain = struct
 
   type astate = t
 
-  let join lhs _ = lhs
+  let join lhs rhs = if List.length lhs.trace <= List.length rhs.trace then lhs else rhs
 
   let widen ~prev ~next ~num_iters:_ = join prev next