symlinks in directory targets don't go in shared cache

[emillon]

When a directory target contains a symlink, it prevents it from going to the shared cache:

  $ export DUNE_CACHE_ROOT=$PWD/.cache
  $ export DUNE_CACHE=enabled

  $ cat > dune-project << EOF
  > (lang dune 3.0)
  > (using directory-targets 0.1)
  > EOF

  $ cat > dune << EOF
  > (rule
  >  (target (dir d))
  >  (action
  >   (progn
  >    (run mkdir -p d)
  >    (chdir d
  >     (progn
  >      (echo building)
  >      (run touch target)
  >      (run ln -s target symlink))))))
  > EOF

  $ dune build d
  building

  $ dune clean

  $ dune build d
  building

In this example, the rule is executed the second time (we can see it with the
echo). If we remove the run ln ... action, it is correctly cached (second
echo is not visible).

I would expect this rule to be able to go to the shared cache; maybe with some
restrictions about the symlink targets.

This seems to prevent some pkg rules to cache correctly, for example
ocp-indent.1.8.1 has some symlinks and it gets redownloaded after dune clean.

[ejgallego]

@emillon do you know if this is this a recent bug or one that has been present since longer?

[emillon]

@ejgallego the current behavior is present in dune 3.13.0 which introduced caching for directory targets.