Original bug ID: 7810
Reporter: @stedolan
Status: new
Resolution: open
Priority: normal
Severity: minor
Category: runtime system and C interface

Bug description

This function creates an ephemeron, fills it in, causes a GC and finally inspects the ephemeron. The ephemeron should not be cleared because the key is still live, since it is part of the return value.

let ephe x =
  let open Ephemeron.K1 in
  let e = create () in
  set_key e x;
  set_data e 42;
  Gc.full_major ();
  (x, get_data e)

The second component of this pair should never be None, e.g.:

# ephe (ref 1000);;
- : int ref * int option = ({contents = 1000}, Some 42)

However, it is None if the argument has Infix_tag (i.e. is a member of a set of mutually recursive functions other than the first):

# ephe (let rec f x = () and g x = () in g);;
Warning 26: unused variable f.
- : ('_a -> unit) * int option = (<fun>, None)

The bug is this check in minor_gc.c (which appears a couple of times):

if (Hd_val (*key) == 0){ /* Value copied to major heap */

If *key has Infix_tag, Hd_val (*key) can be nonzero even though it is copied to the major heap.