[thread-safety] fix bug in expanding callee access paths in caller

Summary:
It looks like the old code for expanding access paths assumed that `FormalMap.get_formals_indexes` assumed the returned list tuples would be sorted by index, but it's actually sorted by var name.
As a consequence, formals might be expanded into the wrong actuals.
This diff fixes the problem by not relying on `get_formals_indexes`.

Reviewed By: jberdine

Differential Revision: D6056365

fbshipit-source-id: 09f3208

infer/src/checkers/ThreadSafety.ml

@@ -569,6 +569,9 @@ module TransferFunctions (CFG : ProcCfg.S) = struct
       exps ~init:accesses
 
   let expand_actuals actuals accesses pdesc =
+    let open Domain in
+    if AccessDomain.is_empty accesses then accesses
+    else
       let rec get_access_path = function
         | HilExp.AccessPath ap
          -> Some ap
@@ -577,23 +580,21 @@ module TransferFunctions (CFG : ProcCfg.S) = struct
         | _
          -> None
       in
-    let open Domain in
+      let formal_map = FormalMap.make pdesc in
-    let formals, _ = FormalMap.make pdesc |> FormalMap.get_formals_indexes |> List.unzip in
-    let n = min (List.length formals) (List.length actuals) in
-    let fmls_actls =
-      List.zip_exn (List.take formals n) (List.take actuals n)
-      |> List.filter_map ~f:(fun (fml, act) ->
-             match get_access_path act with Some path -> Some (fml, path) | _ -> None )
-    in
-    let fmap =
-      List.fold fmls_actls ~init:AccessPath.BaseMap.empty ~f:(fun acc (fml, act) ->
-          AccessPath.BaseMap.add fml act acc )
-    in
       let expand_path (base, accesses as path) =
-      try
+        match FormalMap.get_formal_index base formal_map with
-        let actual = AccessPath.BaseMap.find base fmap in
+        | Some formal_index -> (
-        AccessPath.append actual accesses
+          match List.nth actuals formal_index with
-      with Not_found -> path
+          | Some actual_exp -> (
+            match get_access_path actual_exp with
+            | Some actual
+             -> AccessPath.append actual accesses
+            | None
+             -> path )
+          | None
+           -> path )
+        | None
+         -> path
       in
       let expand_pre accesses =
         let sinks =