Fix a dune crash when subdir is an absolute path

CHANGES.md

@@ -94,6 +94,8 @@ Unreleased
 
 - Add support for sandboxing using hard links (#4360, @snowleopard)
 
+- Fix dune crash when `subdir` is an absolute path (#4366, @anmonteiro)
+
 2.8.2 (21/01/2021)
 ------------------
 

src/dune_engine/sub_dirs.ml

@@ -188,9 +188,14 @@ module Dir_map = struct
   let merge_all = List.fold_left ~f:merge ~init:empty
 end
 
-let descedant_path =
+let descendant_path =
   Dune_lang.Decoder.plain_string (fun ~loc fn ->
-      Path.Local.parse_string_exn ~loc fn |> Path.Local.explode)
+      if Filename.is_relative fn then
+        Path.Local.parse_string_exn ~loc fn |> Path.Local.explode
+      else
+        let msg = [ Pp.textf "invalid sub-directory path %S" fn ] in
+        let hints = [ Pp.textf "sub-directory path must be relative" ] in
+        User_error.raise ~loc ~hints msg)
 
 let strict_subdir field_name =
   let open Dune_lang.Decoder in
@@ -263,7 +268,7 @@ let decode =
   in
   let rec subdir () =
     let* () = Dune_lang.Syntax.since Stanza.syntax (2, 5) in
-    let* subdir = descedant_path in
+    let* subdir = descendant_path in
     let+ node = fields (decode ~allow_ignored_subdirs:false) in
     Dir_map.make_at_path subdir node
   and decode ~allow_ignored_subdirs =

test/blackbox-tests/test-cases/subdir-stanza.t/run.t

@@ -118,3 +118,19 @@ Include stanzas within subdir stanzas
   $ dune build --root . a/hello.txt
   $ cat _build/default/a/hello.txt
   Hello!
+
+
+  $ echo "(lang dune 2.5)" > dune-project
+  $ cat >dune <<EOF
+  > (rule (with-stdout-to foo.txt (echo "bar")))
+  > (subdir /absolute/path/to/bar
+  >  (rule (with-stdout-to foo.txt (echo "bar"))))
+  > EOF
+  $ dune build ./foo.txt ./bar/foo.txt
+  File "dune", line 2, characters 8-29:
+  2 | (subdir /absolute/path/to/bar
+              ^^^^^^^^^^^^^^^^^^^^^
+  Error: invalid sub-directory path "/absolute/path/to/bar"
+  Hint: sub-directory path must be relative
+  [1]
+