mkdir the initial working directory in sandbox

CHANGES.md

@@ -113,6 +113,9 @@ Unreleased
   to be produced by the action. Old message is still produced on ENOENT, but other
   errors deserve a more detailed report. (#4501, @aalekseyev)
 
+- Fixed a bug where a sandboxed action would fail if it declares no dependencies in
+  its initial working directory or any directory it `chdir`s into. (#4509, @aalekseyev)
+
 2.9.0 (unreleased)
 ------------------
 

src/dune_engine/build_system.ml

@@ -1439,6 +1439,7 @@ end = struct
           let sandbox_suffix = rule_digest |> Digest.to_string in
           (Path.Build.relative sandbox_dir sandbox_suffix, mode))
     in
+    let chdirs = Action.chdirs action in
     let* sandboxed, action =
       match sandbox with
       | None -> Fiber.return (None, action)
@@ -1450,7 +1451,7 @@ end = struct
         let* () =
           Fiber.parallel_iter_set
             (module Path.Set)
-            (Dep.Facts.dirs deps)
+            (Path.Set.union (Dep.Facts.dirs deps) chdirs)
             ~f:(fun path ->
               Memo.Build.run
                 (match Path.as_in_build_dir path with
@@ -1470,7 +1471,6 @@ end = struct
         ( Some sandboxed
         , Action.sandbox action ~sandboxed ~mode:sandbox_mode ~deps )
     and* () =
-      let chdirs = Action.chdirs action in
       Fiber.parallel_iter_set
         (module Path.Set)
         chdirs

test/blackbox-tests/test-cases/depend-on/no-deps-in-cwd.t/run.t

@@ -0,0 +1,31 @@
+
+  $ cat >dune-project <<EOF
+  > (lang dune 3.0)
+  > EOF
+
+You can `chdir` into a directory without declaring any dependencies
+and dune makes sure that the directory exists inside the sandbox.
+
+  $ mkdir a
+  $ echo contents > x
+  $ cat >a/dune <<EOF
+  > (rule
+  >   (alias a)
+  >   (deps ../x)
+  >   (action (bash "cat ../x"))
+  > )
+  > EOF
+  $ dune build @a --sandbox=copy
+          bash alias a/a
+  contents
+
+  $ cat >dune <<EOF
+  > (rule
+  >   (alias root)
+  >   (deps x)
+  >   (action (chdir a (bash "cat ../x")))
+  > )
+  > EOF
+  $ dune build @root --sandbox=copy
+          bash alias root
+  contents