add reproduction for regression with exec + watch

test/blackbox-tests/test-cases/exec-watch/dune

@@ -7,9 +7,9 @@
   ;; sometimes go undetected. Adding a delay (e.g. `sleep 1`) before modifying the
   ;; program seems to guarantee a rebuild will be triggered, but that is too
   ;; unreliable to depend on in a test so these tests are disabled on macos.
-  ;; (<> "macosx" %{ocaml-config:system})
+  (<> "macosx" %{ocaml-config:system})
   ;; disabled until it works in CI
-  false))
+  ))
 
 (cram
  (deps wait-for-file.sh))

test/blackbox-tests/test-cases/exec-watch/exec-watch-basic.t/run.t

@@ -18,7 +18,7 @@ between each change to its code.
   1 | let () = print_endline "baz
                              ^
   Error: String literal not terminated
-  Had errors, waiting for filesystem changes...
+  Had 1 error, waiting for filesystem changes...
   Success, waiting for filesystem changes...
   baz
   $ PID=$!

test/blackbox-tests/test-cases/exec-watch/exec-watch-multi-levels.t/bin/dune

@@ -0,0 +1,3 @@
+(executable
+ (name main)
+ (libraries unix))

test/blackbox-tests/test-cases/exec-watch/exec-watch-multi-levels.t/bin/main.ml

@@ -0,0 +1,9 @@
+let () =
+  let touch path =
+    let fd = Unix.openfile path [ Unix.O_CREAT ] 777 in
+    Unix.close fd
+  in
+  print_endline "foo"; 
+  match Sys.argv.(1)  with
+  | exception _ -> ()
+  | path -> touch path

test/blackbox-tests/test-cases/exec-watch/exec-watch-multi-levels.t/dune-project

@@ -0,0 +1 @@
+(lang dune 3.6)

test/blackbox-tests/test-cases/exec-watch/exec-watch-multi-levels.t/run.t

@@ -0,0 +1,27 @@
+
+exec + watch works fine when invoked at the root level
+  $ DONE_FLAG=_build/done_flag
+  $ dune exec --watch ./bin/main.exe $DONE_FLAG &
+  Success, waiting for filesystem changes...
+  foo
+  $ PID=$!
+
+Wait for the $DONE_FLAG file to exist, then delete the file. This file is
+created by the program being exec'd, so when it exists we know that it's safe to
+change the code and proceed with the test.
+  $ ../wait-for-file.sh $DONE_FLAG
+  $ kill $PID
+
+
+It's broken when invoked in a child folder
+  $ cd bin && dune exec --root .. --watch ./bin/main.exe &
+  Entering directory '..'
+  Error: posix_spawn(): No such file or directory
+  Leaving directory '..'
+  $ wait
+
+But it works fine without watch mode
+  $ cd bin && dune exec --root .. ./bin/main.exe
+  Entering directory '..'
+  Leaving directory '..'
+  foo