OCaml-LSP hangs for some on-hover queries in Irmin's codebase

[voodoos]

In Irmin, hovering at position 43:40 in file irmin_pack_mem.ml causes ocaml-lsp to hang at 100% cpu.
Cancellation is not working and the process must be manually killed.

Merlin does answer fast (with index 0, see #1191):

❯ ocamlmerlin single type-enclosing -position 43:40 -index 0 \
      -filename src/irmin-pack/mem/irmin_pack_mem.ml <src/irmin-pack/mem/irmin_pack_mem.ml | jq '.timing'
{
  "clock": 263,
  "cpu": 207,
  "query": 40,
  "pp": 0,
  "reader": 1,
  "ppx": 23,
  "typer": 144,
  "error": 0
}

[voodoos]

It looks like it is actually the document query (that ocaml-lsp runs along type-enclosing with each type-on-hover request) that hangs.

Running merlin directly also hangs:

❯ ocamlmerlin single document -position 43:40 -filename src/irmin-pack/mem/irmin_pack_mem.ml <src/irmin-pack/mem/irmin_pack_mem.ml | jq '.timing'