refactor: Replace use of CoroutineContext by Dispatcher

This change replaces the use of `CoroutineContext` for passing the
`SimulationDispatcher` across the different modules of OpenDC by the
lightweight `Dispatcher` interface of the OpenDC common module.

2 files changed, 350 insertions, 0 deletions

diff --git a/opendc-common/src/main/java/org/opendc/common/util/Pacer.java b/opendc-common/src/main/java/org/opendc/common/util/Pacer.java
new file mode 100644
index 00000000..5b8d8cb0
--- /dev/null
+++ b/opendc-common/src/main/java/org/opendc/common/util/Pacer.java
@@ -0,0 +1,94 @@
+/*
+ * Copyright (c) 2022 AtLarge Research
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package org.opendc.common.util;
+
+import java.util.function.LongConsumer;
+import org.opendc.common.Dispatcher;
+import org.opendc.common.DispatcherHandle;
+
+/**
+ * Helper class to pace the incoming scheduling requests.
+ */
+public final class Pacer {
+    private final Dispatcher dispatcher;
+    private final long quantumMs;
+    private final LongConsumer process;
+
+    /**
+     * The current {@link DispatcherHandle} representing the pending scheduling cycle.
+     */
+    private DispatcherHandle handle;
+
+    /**
+     * Construct a {@link Pacer} instance.
+     *
+     * @param dispatcher The {@link Dispatcher} to schedule future invocations.
+     * @param quantumMs The scheduling quantum in milliseconds.
+     * @param process The process to invoke for the incoming requests.
+     */
+    public Pacer(Dispatcher dispatcher, long quantumMs, LongConsumer process) {
+        this.dispatcher = dispatcher;
+        this.quantumMs = quantumMs;
+        this.process = process;
+    }
+
+    /**
+     * Determine whether a scheduling cycle is pending.
+     */
+    public boolean isPending() {
+        return handle != null;
+    }
+
+    /**
+     * Enqueue a new scheduling cycle.
+     */
+    public void enqueue() {
+        if (handle != null) {
+            return;
+        }
+
+        final Dispatcher dispatcher = this.dispatcher;
+        long quantumMs = this.quantumMs;
+        long now = dispatcher.getTimeSource().millis();
+
+        // We assume that the scheduler runs at a fixed slot every time quantum (e.g t=0, t=60, t=120).
+        // We calculate here the delay until the next scheduling slot.
+        long timeUntilNextSlot = quantumMs - (now % quantumMs);
+
+        handle = dispatcher.scheduleCancellable(timeUntilNextSlot, () -> {
+            process.accept(now + timeUntilNextSlot);
+            handle = null;
+        });
+    }
+
+    /**
+     * Cancel the currently pending scheduling cycle.
+     */
+    public void cancel() {
+        final DispatcherHandle handle = this.handle;
+        if (handle != null) {
+            this.handle = null;
+            handle.cancel();
+        }
+    }
+}
diff --git a/opendc-common/src/main/java/org/opendc/common/util/TimerScheduler.java b/opendc-common/src/main/java/org/opendc/common/util/TimerScheduler.java
new file mode 100644
index 00000000..a85605e9
--- /dev/null
+++ b/opendc-common/src/main/java/org/opendc/common/util/TimerScheduler.java
@@ -0,0 +1,256 @@
+/*
+ * Copyright (c) 2022 AtLarge Research
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package org.opendc.common.util;
+
+import java.util.ArrayDeque;
+import java.util.HashMap;
+import java.util.PriorityQueue;
+import org.jetbrains.annotations.NotNull;
+import org.opendc.common.Dispatcher;
+import org.opendc.common.DispatcherHandle;
+
+/**
+ * A {@link TimerScheduler} facilitates scheduled execution of future tasks.
+ */
+public final class TimerScheduler<T> {
+    private final Dispatcher dispatcher;
+
+    /**
+     * The stack of the invocations to occur in the future.
+     */
+    private final ArrayDeque<Invocation> invocations = new ArrayDeque<>();
+
+    /**
+     * A priority queue containing the tasks to be scheduled in the future.
+     */
+    private final PriorityQueue<Timer<T>> queue = new PriorityQueue<Timer<T>>();
+
+    /**
+     * A map that keeps track of the timers.
+     */
+    private final HashMap<T, Timer<T>> timers = new HashMap<>();
+
+    /**
+     * Construct a {@link TimerScheduler} instance.
+     *
+     * @param dispatcher The {@link Dispatcher} to schedule future invocations.
+     */
+    public TimerScheduler(Dispatcher dispatcher) {
+        this.dispatcher = dispatcher;
+    }
+
+    /**
+     * Start a timer that will invoke the specified [block] after [delay].
+     * <p>
+     * Each timer has a key and if a new timer with same key is started the previous is cancelled.
+     *
+     * @param key The key of the timer to start.
+     * @param delay The delay before invoking the block.
+     * @param block The block to invoke.
+     */
+    public void startSingleTimer(T key, long delay, Runnable block) {
+        startSingleTimerTo(key, dispatcher.getTimeSource().millis() + delay, block);
+    }
+
+    /**
+     * Start a timer that will invoke the specified [block] at [timestamp].
+     * <p>
+     * Each timer has a key and if a new timer with same key is started the previous is cancelled.
+     *
+     * @param key The key of the timer to start.
+     * @param timestamp The timestamp at which to invoke the block.
+     * @param block The block to invoke.
+     */
+    public void startSingleTimerTo(T key, long timestamp, Runnable block) {
+        long now = dispatcher.getTimeSource().millis();
+        final PriorityQueue<Timer<T>> queue = this.queue;
+        final ArrayDeque<Invocation> invocations = this.invocations;
+
+        if (timestamp < now) {
+            throw new IllegalArgumentException("Timestamp must be in the future");
+        }
+
+        timers.compute(key, (k, old) -> {
+            if (old != null && old.timestamp == timestamp) {
+                // Fast-path: timer for the same timestamp already exists
+                old.block = block;
+                return old;
+            } else {
+                // Slow-path: cancel old timer and replace it with new timer
+                Timer<T> timer = new Timer<T>(key, timestamp, block);
+
+                if (old != null) {
+                    old.isCancelled = true;
+                }
+                queue.add(timer);
+                trySchedule(now, invocations, timestamp);
+
+                return timer;
+            }
+        });
+    }
+
+    /**
+     * Check if a timer with a given key is active.
+     *
+     * @param key The key to check if active.
+     * @return `true` if the timer with the specified [key] is active, `false` otherwise.
+     */
+    public boolean isTimerActive(T key) {
+        return timers.containsKey(key);
+    }
+
+    /**
+     * Cancel a timer with a given key.
+     * <p>
+     * If canceling a timer that was already canceled, or key never was used to start
+     * a timer this operation will do nothing.
+     *
+     * @param key The key of the timer to cancel.
+     */
+    public void cancel(T key) {
+        final Timer<T> timer = timers.remove(key);
+
+        // Mark the timer as cancelled
+        if (timer != null) {
+            timer.isCancelled = true;
+        }
+    }
+
+    /**
+     * Cancel all timers.
+     */
+    public void cancelAll() {
+        queue.clear();
+        timers.clear();
+
+        // Cancel all pending invocations
+        for (final Invocation invocation : invocations) {
+            invocation.cancel();
+        }
+
+        invocations.clear();
+    }
+
+    /**
+     * Try to schedule an engine invocation at the specified [target].
+     *
+     * @param now The current virtual timestamp.
+     * @param target The virtual timestamp at which the engine invocation should happen.
+     * @param scheduled The queue of scheduled invocations.
+     */
+    private void trySchedule(long now, ArrayDeque<Invocation> scheduled, long target) {
+        final Invocation head = scheduled.peek();
+
+        // Only schedule a new scheduler invocation in case the target is earlier than all other pending
+        // scheduler invocations
+        if (head == null || target < head.timestamp) {
+            final DispatcherHandle handle = dispatcher.scheduleCancellable(target - now, this::doRunTimers);
+            scheduled.addFirst(new Invocation(target, handle));
+        }
+    }
+
+    /**
+     * This method is invoked when the earliest timer expires.
+     */
+    private void doRunTimers() {
+        final ArrayDeque<Invocation> invocations = this.invocations;
+        final Invocation invocation = invocations.remove();
+
+        final PriorityQueue<Timer<T>> queue = this.queue;
+        final HashMap<T, Timer<T>> timers = this.timers;
+        long now = invocation.timestamp;
+
+        while (!queue.isEmpty()) {
+            final Timer<T> timer = queue.peek();
+
+            long timestamp = timer.timestamp;
+            boolean isCancelled = timer.isCancelled;
+
+            assert timestamp >= now : "Found task in the past";
+
+            if (timestamp > now && !isCancelled) {
+                // Schedule a task for the next event to occur.
+                trySchedule(now, invocations, timestamp);
+                break;
+            }
+
+            queue.poll();
+
+            if (!isCancelled) {
+                timers.remove(timer.key);
+                timer.run();
+            }
+        }
+    }
+
+    /**
+     * A task that is scheduled to run in the future.
+     */
+    private static class Timer<T> implements Comparable<Timer<T>> {
+        final T key;
+        final long timestamp;
+        Runnable block;
+
+        /**
+         * A flag to indicate that the task has been cancelled.
+         */
+        boolean isCancelled;
+
+        /**
+         * Construct a {@link Timer} instance.
+         */
+        public Timer(T key, long timestamp, Runnable block) {
+            this.key = key;
+            this.timestamp = timestamp;
+            this.block = block;
+        }
+
+        /**
+         * Run the task.
+         */
+        void run() {
+            block.run();
+        }
+
+        @Override
+        public int compareTo(@NotNull Timer<T> other) {
+            return Long.compare(timestamp, other.timestamp);
+        }
+    }
+
+    /**
+     * A future engine invocation.
+     * <p>
+     * This class is used to keep track of the future engine invocations created using the {@link Dispatcher} instance.
+     * In case the invocation is not needed anymore, it can be cancelled via [cancel].
+     */
+    private record Invocation(long timestamp, DispatcherHandle handle) {
+        /**
+         * Cancel the engine invocation.
+         */
+        void cancel() {
+            handle.cancel();
+        }
+    }
+}