c++ compiler detection fails under --sandbox symlink

[emillon]

Hi,

@MisterDA and I found the following sandboxing issue: when --sandbox symlink is passed, the piece of code that tries to detect the type of C++ compiler fails.
One way to test this is to add --sandbox symlink to that line:

dune/test/blackbox-tests/test-cases/cxx-flags.t/run.t

Line 111 in 3869922

$ dune exec ./main.exe

It seems that the .dune/ccomp rules are missing some dependency information or something like that.

[rgrinberg]

Which dependencies are absent?

[emillon]

It's the .h file. The error message is:

  File ".dune/ccomp/_unknown_", line 1, characters 0-0:
  cc1: fatal error: $TESTCASE_ROOT/_build/default/.dune/ccomp/header_check.h: No such file or directory
  compilation terminated.
  [1]

[rgrinberg]

Sorry for asking you to spell it out, but then why don't we add a dependency on the header?

diff --git a/src/dune_rules/cxx_rules.ml b/src/dune_rules/cxx_rules.ml
index 456b8aa14..f5812f170 100644
--- a/src/dune_rules/cxx_rules.ml
+++ b/src/dune_rules/cxx_rules.ml
@@ -31,7 +31,7 @@ let rules ~sctx ~dir =
     [ (match Ocaml_config.ccomp_type ocfg with
       | Msvc -> A "/EP"
       | Other _ -> As [ "-E"; "-P" ])
-    ; A Path.(to_absolute_filename (build header_file))
+    ; Dep (Path.build header_file)
     ]
   in
   let action =

[emillon]

Yes I just saw that as I was replying. This works. I'll update the PR.