dune clean fail with DUNE_BUILD_DIR set to an absolute path

[Et7f3]

Expected Behavior

dune clean succeed when DUNE_BUILD_DIR is set to an absolute path.

Actual Behavior

it doesn't

Reproduction

• PR with a reproducing test: not yet

1. open a folder with dune-project
2. set DUNE_BUILD_DIR to $PWD/some/folders you should get a absolute path
3. dune clean

% ../dune/./_esy/default/store/b/dune-f684ad2b/install/default/bin/dune.exe clean
�[1;31mInternal error, please report upstream including the contents of _build/log.�[0m
Description:
  ("Path.rm_rf called on external dir",
  { fn =
      "C:/Users/Et7f3/ppx_import/_esy/default/store/b/ppx__import-e1393642"
  })
Raised at Stdune__Code_error.raise in file "otherlibs/stdune/code_error.ml",
  line 11, characters 30-62
Called from Stdune__Fpath.rm_rf in file "otherlibs/stdune/fpath.ml", line
  131, characters 4-78
Called from Cmdliner_term.app.(fun) in file
  "vendor/cmdliner/src/cmdliner_term.ml", line 25, characters 19-24
Called from Cmdliner.Term.run in file "vendor/cmdliner/src/cmdliner.ml", line
  117, characters 32-39
Called from Cmdliner.Term.term_eval in file
  "vendor/cmdliner/src/cmdliner.ml", line 147, characters 18-36
Called from Cmdliner.Term.Group.eval in file
  "vendor/cmdliner/src/cmdliner.ml", line 367, characters 22-48
Called from Dune__exe__Main in file "bin/main.ml", line 90, characters 10-50

I must not crash.  Uncertainty is the mind-killer. Exceptions are the
little-death that brings total obliteration.  I will fully express my cases.
Execution will pass over me and through me.  And when it has gone past, I
will unwind the stack along its path.  Where the cases are handled there will
be nothing.  Only I will remain.
[build ppx_import] % echo $DUNE_BUILD_DIR/
C:/Users/Et7f3/ppx_import/_esy/default/store/b/ppx__import-e1393642

% echo $DUNE_BUILD_DIR/
C:/Users/Et7f3/ppx_import/_esy/default/store/b/ppx__import-e1393642/
% echo $PWD
/cygdrive/c/Users/Et7f3/ppx_import

Specifications

• Version of dune (output of dune --version): ef1ee9a
• Version of ocaml (output of ocamlc --version) 4.13.1
• Operating system (distribution and version): windows

Additional information

• Link to gist with verbose output (run dune with the --verbose flag):

[bobot]

Is it because the BUILD_DIR chosen is absolute or absolute and outside the source directory (i.e. outside the dune-project scope)?

[Et7f3]

$PWD/dune-project exists so I don't understand the difference

[bobot]

So the culprit is really the fact that an absolute path is used, please could you update the description and title to make that obvious?

[rgrinberg]

Fixed by #5507