137 lines
5.4 KiB
Racket
137 lines
5.4 KiB
Racket
#lang racket/base
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;; Timer drivers for os2.rkt.
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;; Uses mutable state internally, but because the scope of the
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;; mutation is limited to each timer process alone, it's easy to show
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;; correct linear use of the various pointers.
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(require data/heap)
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(require racket/set)
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(require racket/match)
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(require "os2.rkt")
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(provide (struct-out set-timer)
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(struct-out timer-expired)
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timer-driver
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timer-relay)
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;; (set-timer Any Number (or 'relative 'absolute))
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;; The timer driver and timer relays listen for messages of this type,
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;; and when they hear one, they set an alarm that will later send a
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;; corresponding timer-expired message.
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(struct set-timer (label msecs kind) #:prefab)
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;; (timer-expired Any Number)
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;; Message sent by the timer driver or a timer relay upon expiry of a
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;; timer. Contains the label specified in the corresponding set-timer
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;; message, and also the current absolute time from the outside world.
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(struct timer-expired (label msecs) #:prefab)
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;; (pending-timer AbsoluteSeconds Any Boolean)
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;; An outstanding timer being managed by the timer-driver.
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(struct pending-timer (deadline label) #:transparent)
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;; (driver-state Symbol Maybe<EID> Heap<PendingTimer>)
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;; State of a timer-driver, including the identifier of the driver,
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;; the currently-active subscription to ground time events (if any),
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;; and the heap of all remaining timers.
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(struct driver-state (self-id heap) #:transparent)
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;; (relay-state ExactPositiveInteger Hash<ExactPositiveInteger,Any>)
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;; State of a timer-relay, including the next timer number and a
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;; mapping from timer number to timer label.
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(struct relay-state (next-counter active-timers) #:transparent)
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;; Note that (set-timer 'current-time 0 #f) causes an immediate reply
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;; of (timer-expired 'current-time (current-inexact-milliseconds)),
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;; which can be used for an event-oriented interface to reading the
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;; system clock.
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;; Racket's alarm-evt is almost the right design for timeouts: its
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;; synchronisation value should be the (or some) value of the clock
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;; after the asked-for time. That way it serves as timeout and
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;; clock-reader in one.
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(define (timer-evt msecs)
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(wrap-evt (alarm-evt msecs)
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(lambda (_) (current-inexact-milliseconds))))
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;; -> Heap<PendingTimer>
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(define (make-timer-heap)
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(make-heap (lambda (t1 t2) (<= (pending-timer-deadline t1) (pending-timer-deadline t2)))))
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;; Heap<PendingTimer> -> Maybe<PendingTimer>
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;; Retrieves the earliest-deadline timer from the heap, if there is
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;; one.
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(define (next-timer! heap)
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(if (zero? (heap-count heap))
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#f
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(heap-min heap)))
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;; Heap<PendingTimer> AbsoluteSeconds -> ListOf<TimerExpired>
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;; Retrieves (and removes) all timers from the heap that have deadline
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;; earlier or equal to the time passed in.
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(define (fire-timers! heap now)
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(if (zero? (heap-count heap))
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'()
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(let ((m (heap-min heap)))
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(if (<= (pending-timer-deadline m) now)
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(begin (heap-remove-min! heap)
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(cons (send-message (timer-expired (pending-timer-label m) now))
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(fire-timers! heap now)))
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'()))))
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;; [Symbol] -> BootK
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;; Process for mapping this-level timer requests to ground-level timer
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;; events and back.
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(define (timer-driver self-id)
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(transition (driver-state self-id (make-timer-heap))
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(role 'relay-down (set (topic-subscriber (set-timer (wild) (wild) (wild)))
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(topic-publisher (timer-expired (wild) (wild))))
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#:state state
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[(set-timer label msecs 'relative)
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(install-timer! state label (+ (current-inexact-milliseconds) msecs))]
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[(set-timer label msecs 'absolute)
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(install-timer! state label msecs)])))
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;; DriverState Any AbsoluteSeconds -> Transition
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(define (install-timer! state label deadline)
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(heap-add! (driver-state-heap state) (pending-timer deadline label))
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(update-time-listener! state))
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;; DriverState -> Transition
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(define (update-time-listener! state)
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(define next (next-timer! (driver-state-heap state)))
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(transition state
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(delete-role 'time-listener)
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(and next
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(role 'time-listener (topic-subscriber (cons (timer-evt (pending-timer-deadline next))
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(wild)))
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#:state state
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[(cons (? evt?) now)
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(define to-send (fire-timers! (driver-state-heap state) now))
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;; Note: compute to-send before recursing, because of side-effects on heap
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(sequence-actions state
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update-time-listener!
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to-send)]))))
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;; Symbol -> BootK
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;; Process for mapping this-level timer requests to meta-level timer
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;; requests. Useful when running nested VMs: essentially extends timer
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;; support up the branches of the VM tree toward the leaves.
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(define (timer-relay self-id)
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(transition (relay-state 0 (hash))
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(at-meta-level
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(role 'relay-up (topic-subscriber (timer-expired (wild) (wild)))
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#:state (relay-state next-counter active-timers)
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[(timer-expired (list (== self-id) counter) now)
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(transition (relay-state next-counter (hash-remove active-timers counter))
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(and (hash-has-key? active-timers counter)
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(send-message (timer-expired (hash-ref active-timers counter) now))))]))
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(role 'relay-down (set (topic-subscriber (set-timer (wild) (wild) (wild)))
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(topic-publisher (timer-expired (wild) (wild))))
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#:state (relay-state next-counter active-timers)
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[(set-timer label msecs kind)
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(transition (relay-state (+ next-counter 1) (hash-set active-timers next-counter label))
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(at-meta-level
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(send-message (set-timer (list self-id next-counter) msecs kind))))])))
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