A handle to a pending request that can be used to cancel it.

create () is a fresh broadcast queue.

suspend t fn arranges for fn () to be called on resume_all.

fn () may be called from the caller's context, or by resume_all, so it needs to be able to cope with running in any context where that can run. For example, fn must be safe to call from a signal handler if resume_all can be called from one. fn must not raise.

The returned request can be used to cancel. It can be None in the (unlikely) event that t got resumed before the function returned.

resume_all t calls all non-cancelled callbacks attached to t, in the order in which they were suspended.

This function is lock-free and can be used safely even from a signal handler or GC finalizer.

cancel request attempts to remove a pending request.

It returns true if the request was cancelled, or false if it got resumed before that could happen.

Display the internal state of a queue, for debugging.