Add a test reproducing issue ocaml#1647

tests/test-dirs/with-ppx/issue1647-cache-invalidation.t

@@ -0,0 +1,89 @@
+Make sure that this test doesn't depend on previous state
+  $ $MERLIN server stop-server
+
+
+  $ cat >dune-project <<EOF
+  > (lang dune 2.0)
+  > EOF
+
+  $ cat > my_ppx.ml <<EOF
+  > open Ppxlib
+  > let rule =
+  >    let ppx =
+  >      let ast_context = Extension.Context.expression in
+  >      let pl = Ast_pattern.(pstr nil) in
+  >      let expand_fn ~ctxt =
+  >        let loc = Expansion_context.Extension.extension_point_loc ctxt in
+  >        Ast_builder.Default.eint ~loc 0 in
+  >      Extension.V3.declare "just_zero" ast_context pl expand_fn
+  > in
+  > Ppxlib.Context_free.Rule.extension ppx
+  > let () = Driver.register_transformation ~rules:[ rule ] "just_zero"
+  > EOF
+
+  $ cat > main.ml <<EOF
+  > let () = print_int [%just_zero]
+  > EOF
+
+  $ cat >dune <<EOF
+  > (library
+  >  (name my_ppx)
+  >  (kind ppx_rewriter)
+  >  (modules my_ppx)
+  >  (libraries ppxlib))
+  > 
+  > (executable
+  >  (name main)
+  >  (modules main)
+  >  (preprocess
+  >   (pps my_ppx)))
+  > EOF
+
+  $ cat > .merlin <<EOF
+  > FLG -ppx '_build/default/.ppx/a56cb746ce56d7c281c7d9796f4166ed/ppx.exe -as-ppx
+  > USE_PPX_CACHE
+  > EOF
+
+  $ dune exec ./main.exe
+  0
+
+`errors` requires the typedtree and thus types the buffer
+  $ $MERLIN server errors -log-file merlin_logs \
+  > -filename main.ml 1> /dev/null <main.ml
+  $ cat merlin_logs | grep 'Phase cache' -A 1 | sed "s/[0-9]*//g"
+  # . Phase cache - Reader phase
+  Cache wasn't populated
+  --
+  # . Phase cache - PPX phase
+  Cache wasn't populated
+
+Now we edit the file. This should invalidate the caches
+  $ cat > main.ml <<EOF
+  > let () = print_string [%just_zero]
+  > EOF
+
+
+The `dump source` command only requires the parsetree and only invalidates the
+reader cache:
+  $ $MERLIN server dump -what source  -log-file merlin_logs \
+  > -filename main.ml <main.ml | tr -d '\n' | jq '.value'
+  "let () = print_string ([%just_zero ])"
+
+  $ cat merlin_logs | grep 'Phase cache' -A 1 | sed "s/[0-9]*//g"
+  # . Phase cache - Reader phase
+  Cache invalidation
+
+FIXME: The `dump ppxed-source` wrongly reuse the cached version since this
+behavior is based ont he reader cahce being hit or not
+  $ $MERLIN server dump -what ppxed-source  -log-file merlin_logs \
+  > -filename main.ml <main.ml | tr -d '\n' | jq '.value'
+  "let () = print_int 0"
+
+  $ cat merlin_logs | grep 'Phase cache' -A 1 | sed "s/[0-9]*//g"
+  # . Phase cache - Reader phase
+  Cache hit
+  # . Phase cache - PPX phase
+  Cache hit
+
+Stop server
+  $ $MERLIN server stop-server