Scenario:

• call margo_create(&h) to create a handle;
• call margo_ref_incr(h) to increment its internal refcount;
• call margo_destroy(h) to destroy it; The call to margo_destroy should decrease the reference count, NOT put the handle back in the cache. Otherwise when the handle is pulled back from the cache to be reused, HG_Reset will fail. This problem appears in Thallium, in particular. It is harmless, to some extents only: when resetting fails, margo falls back to creating a new handle from scratch, so we just get some Mercury-related errors on stderr and the cache quickly fills up with handles that cannot be reset.

The solution would be to have Jerome provide a HG_Get_refcount in Mercury so that margo can decide whether HG_Destroy or cache_put should be called within margo_destroy.