Don't let command run by Dune observe INSIDE_EMACS (#4680)

To improve reproducibility

Author: jeremiedimino

CHANGES.md

@@ -147,6 +147,10 @@ Unreleased
 - Do not log `live_words` and `free_words` in trace file. This allows using
   `Gc.quick_stat` which does not scan the heap. (#4643, @emillon)
 
+- Don't let command run by Dune observe the environment variable
+  `INSIDE_EMACS` in order to improve reproducibility (#4680,
+  @jeremiedimino)
+
 2.9.0 (unreleased)
 ------------------
 

src/dune_rules/global.ml

@@ -14,6 +14,12 @@ let init ~capture_outputs =
        else
          Colors.setup_env_for_colors Env.initial
      in
-     Env.add env ~var:"INSIDE_DUNE" ~value:"1")
+     let env = Env.add env ~var:"INSIDE_DUNE" ~value:"1" in
+     (* To improve reproducibility, we don't let command executed by Dune
+        observe whether Dune is run inside emacs or not. One such program that
+        behave differently when run inside emacs is Dune itself and we sometimes
+        run Dune from inside Dune, for instnace in cram tests, so it is
+        important to do this. *)
+     Env.remove env ~var:"INSIDE_EMACS")
 
 let env () = Fdecl.get env