fix: copy sandboxes should copy symlinks as well

doc/changes/9282.md

@@ -0,0 +1 @@
+- Copying mode for sandboxes will now follow symbolic links (#9282, @rgrinberg)

src/dune_engine/sandbox.ml

@@ -45,6 +45,7 @@ module Item = struct
   type t =
     | File
     | Directory of { perms : int }
+    | Link
     | Other of Unix.file_kind
 
   let of_path path =
@@ -59,6 +60,7 @@ module Item = struct
     match kind with
     | S_DIR -> Directory { perms = (Path.Untracked.stat_exn path).st_perm }
     | S_REG -> File
+    | S_LNK -> Link
     | _ -> Other kind
   ;;
 end
@@ -71,6 +73,12 @@ let copy_recursively =
   in
   let rec loop item ~src ~dst =
     match (item : Item.t) with
+    | Link ->
+      (match Path.Untracked.stat_exn src with
+       | { Unix.st_kind = S_REG; _ } -> Io.copy_file ~chmod:chmod_file ~src ~dst ()
+       | { Unix.st_kind = S_DIR; st_perm = perms; _ } ->
+         loop (Directory { perms }) ~src ~dst
+       | { Unix.st_kind; _ } -> loop (Other st_kind) ~src ~dst)
     | File -> Io.copy_file ~chmod:chmod_file ~src ~dst ()
     | Directory { perms } ->
       (match Path.Untracked.readdir_unsorted_with_kinds src with

test/blackbox-tests/test-cases/directory-targets/copy-sandboxing.t

@@ -103,15 +103,7 @@ Now we demonstrate that symlinks aren't supported:
   > }
 
   $ runtest "touch foo && ln -s foo bar"
-  File "dune", line 1, characters 0-63:
-  1 | (rule
-  2 |  (alias foo)
-  3 |  (action (echo test))
-  4 |  (deps sub/targetdir))
-  Error: Failed to copy file _build/default/sub/targetdir/bar of kind "symbolic
-  link" while creating a copy sandbox
-  Hint: Re-run Dune to delete the stale artifact, or manually delete this file
-  [1]
+  test
 
   $ runtest "mkdir bar && touch bar/somefileinbar && ln -s bar symlinktobar"
   File "sub/dune", line 1, characters 0-151: