Comment author: @stedolan

I had a look into this, and it seems to be an issue with the implementation of Lwt_react.with_finaliser rather than with the OCaml GC. That function is implemented as follows:

let with_finaliser f event =
let r = ref () in
Gc.finalise (finalise f) r;
map (fun x -> ignore r; x) event

The idea is to add a finaliser to the reference 'r', and ensure that the reference 'r' is reachable from the returned event. However, flambda (correctly!) notices that 'r' is not used in the returned event (since it is ignored), and optimises the use of 'r' away. This means that the GC is free to collect 'r' and run the finaliser immediately.

The simplest fix is to ensure that the reference to 'r' is not optimised away, by hiding the fact that it is unused from the optimiser using Sys.opaque_identity:

map (fun x -> ignore (Sys.opaque_identity r); x) event

I've made an issue on the Lwt bugtracker about this at ocsigen/lwt#626. I'm afraid I won't have time to follow up on this immediately (I'm just about to go on vacation for a couple of weeks), but I'll have a look later on.

Comment author: antron

This should be fixed in Lwt master in ocsigen/lwt#629.