A field in a message that refers to a capability.

ref_leak_detected thread_id fn is called when a promise or capability is GC'd while its ref-count is non-zero, indicating that resources may have been leaked. fn () will log a warning about this and free the resources itself. The reason for going via ref_leak_detected rather than calling fn directly is because the OCaml GC may detect the problem at any point (e.g. while we're sending another message). The implementation should arrange for fn to be called at a safe point in thread thread_id (e.g. when returning to the thread's main loop). Unit-tests may wish to call fn immediately to show the error and then fail the test.