Nested dune call fix (executable resolution part)

src/dune_rules/pkg_rules.ml

@@ -920,27 +920,32 @@ module Action_expander = struct
              let dir = t.paths.source_dir in
              Memo.return @@ Ok (Path.relative dir program)
            | In_path ->
-             let* artifacts = t.artifacts in
-             (match Filename.Map.find artifacts program with
-              | Some s -> Memo.return @@ Ok s
-              | None ->
-                (let path = Global.env () |> Env_path.path in
-                 Which.which ~path program)
-                >>= (function
-                 | Some p -> Memo.return (Ok p)
+             (match program with
+              | "dune" ->
+                let dune = Path.of_string Sys.executable_name in
+                Memo.return @@ Ok dune
+              | program ->
+                let* artifacts = t.artifacts in
+                (match Filename.Map.find artifacts program with
+                 | Some s -> Memo.return @@ Ok s
                  | None ->
-                   let+ depexts = filtered_depexts t in
-                   let hint =
-                     Run_with_path.depexts_hint depexts
-                     |> Option.map ~f:(fun pp -> Format.asprintf "%a" Pp.to_fmt pp)
-                   in
-                   Error
-                     (Action.Prog.Not_found.create
-                        ?hint
-                        ~program
-                        ~context:t.context
-                        ~loc:(Some loc)
-                        ()))))
+                   (let path = Global.env () |> Env_path.path in
+                    Which.which ~path program)
+                   >>= (function
+                    | Some p -> Memo.return (Ok p)
+                    | None ->
+                      let+ depexts = filtered_depexts t in
+                      let hint =
+                        Run_with_path.depexts_hint depexts
+                        |> Option.map ~f:(fun pp -> Format.asprintf "%a" Pp.to_fmt pp)
+                      in
+                      Error
+                        (Action.Prog.Not_found.create
+                           ?hint
+                           ~program
+                           ~context:t.context
+                           ~loc:(Some loc)
+                           ())))))
       in
       Result.map prog ~f:(map_exe t)
     ;;

test/blackbox-tests/test-cases/pkg/different-dune-in-path.t

@@ -89,12 +89,18 @@ Remember the digests, to not to have to call nested Dunes:
 Call Dune with an absolute PATH as argv[0]:
 
   $ PATH=$fakepath $DUNE build "$pkg_root/$foo_digest/target/"
-  Fake dune! (args: build -p foo @install)
   $ PATH=$fakepath $DUNE build "$pkg_root/$bar_digest/target/"
   Fake dune! (args: build -p bar @install)
 
-Make sure that fake dune is not picked up when dune is called with argv[0] = "dune":
+argv[0] is set by the calling program (like a shell or cram test runner) and
+could be wrong, hence it cannot always be trusted. In the examples above we
+launch dune with an absolute path, thus one could just use argv[0] to get the
+exact path to the `dune` binary.
+
+To make sure that we pick up the right dune even when argv[0] is being set to
+unhelpful values we launch the binary but set the value to a relative value,
+namely argv[0] = "dune". This is exactly what happens if `dune` is in the PATH
+and the user launches `dune` in a shell.
 
   $ dune clean
   $ PATH=$fakepath dune_cmd exec-a "dune" $DUNE build "$pkg_root/$foo_digest/target/"
-  Fake dune! (args: build -p foo @install)