Add Lwt_condition.broadcast_exn

[nojb]

No description provided.

[aantron]

Thanks! Looks reasonable, but, being new to maintaining Lwt, I will ponder this for a little while.

[aantron]

By the way, can you sketch out, at your leisure, your use case for this function?

[nojb]

When you have a set of listeners waiting on a condition the only way there is to signal failure to all of them is to introduce an auxiliary thread stopper and instead of using Lwt_condition.wait use something like:

let stopper, stop = Lwt.wait ()
let wait_with_cancellation cond =
  Lwt.pick [Lwt_condition.wait cond; stopper]

Then you can fail all waiting threads with exception exn by doing Lwt.wakeup_exn stop exn. If then you want to do this again, you need to introduce a new stopper, etc. It just seemed cleaner to offer this idiom from inside Lwt_condition.

[aantron]

Committed as a44d7a0. Thanks!