Fix sandboxing when the build directory is absolute

Fix #979

Signed-off-by: Jeremie Dimino <jeremie@dimino.org>

src/build_system.ml

@@ -790,12 +790,7 @@ let rec compile_rule t ?(copy_source=false) pre_rule =
           match sandbox_dir with
           | Some sandbox_dir ->
             Path.rm_rf sandbox_dir;
-            let sandboxed path =
-              if Path.is_managed path then
-                Path.append sandbox_dir path
-              else
-                path
-            in
+            let sandboxed path = Path.sandbox_managed_paths ~sandbox_dir path in
             make_local_parent_dirs t all_deps ~map_path:sandboxed;
             make_local_parent_dirs t targets  ~map_path:sandboxed;
             Action.sandbox action

src/stdune/path.ml

@@ -764,6 +764,15 @@ let drop_optional_build_context t =
   | None -> t
   | Some (_, t) -> t
 
+let local_src   = Local.of_string "src"
+let local_build = Local.of_string "build"
+
+let sandbox_managed_paths ~sandbox_dir t =
+  match t with
+  | External _ -> t
+  | In_source_tree p -> append_local sandbox_dir (Local.append local_src   p)
+  | In_build_dir   p -> append_local sandbox_dir (Local.append local_build p)
+
 let split_first_component t =
   match kind t, is_root t with
   | Local t, false ->

src/stdune/path.mli

@@ -103,6 +103,10 @@ val drop_build_context_exn : t -> t
 (** Drop the "_build/blah" prefix if present, return [t] otherwise *)
 val drop_optional_build_context : t -> t
 
+(** Transform managed paths so that they are descedant of
+    [sandbox_dir]. *)
+val sandbox_managed_paths : sandbox_dir:t -> t -> t
+
 val explode : t -> string list option
 val explode_exn : t -> string list