Cancelling +an asynchronous request

Most asynchronous service providers support a cancel function. This allows +an outstanding request to be cancelled before it is complete.

A single thread that is waiting for a single asynchronous request to complete +cannot cancel that request because the thread is asleep until it completes. +However, when multiple requests are outstanding, the handling of a request +completion may involve cancelling some or all of the other outstanding requests.

Notes

A cancel should only +be issued from the thread that issued the request.
It is convention that +cancel functions provided by asynchronous service providers have Cancel somewhere +in the name, but need not necessarily be called Cancel().
An asynchronous service +provider must make certain guarantees about cancellation:
- it must complete quickly — +otherwise, the User::WaitForRequest() above would take a +long time to complete, and cause the program to become unresponsive
- it must not violate +the guarantee that each request produces precisely one signal
The service provider +does not have to guarantee to cancel the actual request: it may already have +completed — asynchronously, by definition — by the time the client +thread issues the cancel.
Although the cancel +must return quickly, the service initiated by the request may not have completed. +For instance, if data were requested from a network drive, it may not be returned +until after the cancel. Because of the cancel, the service provider must discard +such data.