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Add support for a generalized interruptibility facility in the server.
This offers a generalized interruptibility facility, trialed in
Future<T> and ProducerConsumerQueue<T>.
It offers 3 major concepts:
Notifyable: A type which can notified off-thread, causing a wake up
from some kind of blocking wait
Waitable: A type which is Notifyable, and also can perform work while in
a ready-to-receive notification state. static methods offer support for
running underneath condition_variable::wait's. The chief implementer is
the transport layer baton type
Interruptible: A type which can wait on condition variables, and offers:
- deadlines. This means the type integrates some sort of clock source
- interruptibility. This means the type offers a way of noticing
that it should no longer run via status or exception
Additionally, Interruptible's offer special scoped guards which
offer
- Exemption from interruption in a region defined by the lifetime
of a guard object
- Subsidiary deadlines which can trigger recursively, offering
specialized timeout and status return support.
The series of virtual types allows us to slice the interface between
opCtx and future such that opctx can use future and future can use
opctx. Additionally, cutting out more functionality allows us to flow a
noop interruptibility type which unifies our waiting behind a common
api.
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