Fix the use of Path.reach for program names

src/action.ml

@@ -160,7 +160,7 @@ module Mini_shexp = struct
     in
     let env = Context.extend_env ~vars:env_extra ~env in
     Future.run Strict ~dir:(Path.to_string dir) ~env ~stdout_to
-      (Path.reach ~from:dir prog) args
+      (Path.reach_for_running ~from:dir prog) args
 
   let rec exec t ~dir ~env ~env_extra ~stdout_to ~tail =
     match t with

src/path.ml

@@ -261,6 +261,21 @@ let reach t ~from =
       ]
   | true, true -> Local.reach t ~from
 
+let reach_for_running t ~from =
+  match is_local t, is_local from with
+  | false, _ -> t
+  | true, false ->
+    Sexp.code_error "Path.reach_for_running called with invalid combination"
+      [ "t"   , sexp_of_t t
+      ; "from", sexp_of_t from
+      ]
+  | true, true ->
+    let s = Local.reach t ~from in
+    if String.is_prefix s ~prefix:"../" then
+      s
+    else
+      "./" ^ s
+
 let descendant t ~of_ =
   if is_local t && is_local of_ then
     Local.descendant t ~of_

src/path.mli

@@ -49,6 +49,7 @@ val relative : t -> string -> t
 val absolute : string -> t
 
 val reach : t -> from:t -> string
+val reach_for_running : t -> from:t -> string
 
 val descendant : t -> of_:t -> t option