netty5/codec-http2/src/main/java/io/netty/handler/codec/http2/WeightedFairQueueByteDistributor.java

/*
 * Copyright 2015 The Netty Project
 *
 * The Netty Project licenses this file to you under the Apache License, version 2.0 (the
 * "License"); you may not use this file except in compliance with the License. You may obtain a
 * copy of the License at:
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */
package io.netty.handler.codec.http2;

import io.netty.util.collection.IntCollections;
import io.netty.util.collection.IntObjectHashMap;
import io.netty.util.collection.IntObjectMap;
import io.netty.util.internal.DefaultPriorityQueue;
import io.netty.util.internal.EmptyPriorityQueue;
import io.netty.util.internal.MathUtil;
import io.netty.util.internal.PriorityQueue;
import io.netty.util.internal.PriorityQueueNode;
import io.netty.util.internal.SystemPropertyUtil;
import io.netty.util.internal.UnstableApi;

import java.io.Serializable;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.Iterator;
import java.util.List;

import static io.netty.handler.codec.http2.Http2CodecUtil.CONNECTION_STREAM_ID;
import static io.netty.handler.codec.http2.Http2CodecUtil.DEFAULT_MIN_ALLOCATION_CHUNK;
import static io.netty.handler.codec.http2.Http2CodecUtil.DEFAULT_PRIORITY_WEIGHT;
import static io.netty.handler.codec.http2.Http2CodecUtil.streamableBytes;
import static io.netty.handler.codec.http2.Http2Error.INTERNAL_ERROR;
import static io.netty.handler.codec.http2.Http2Exception.connectionError;
import static io.netty.util.internal.ObjectUtil.checkPositive;
import static io.netty.util.internal.ObjectUtil.checkPositiveOrZero;
import static java.lang.Integer.MAX_VALUE;
import static java.lang.Math.max;
import static java.lang.Math.min;

/**
 * A {@link StreamByteDistributor} that is sensitive to stream priority and uses
 * <a href="https://en.wikipedia.org/wiki/Weighted_fair_queueing">Weighted Fair Queueing</a> approach for distributing
 * bytes.
 * <p>
 * Inspiration for this distributor was taken from Linux's
 * <a href="https://www.kernel.org/doc/Documentation/scheduler/sched-design-CFS.txt">Completely Fair Scheduler</a>
 * to model the distribution of bytes to simulate an "ideal multi-tasking CPU", but in this case we are simulating
 * an "ideal multi-tasking NIC".
 * <p>
 * Each write operation will use the {@link #allocationQuantum(int)} to know how many more bytes should be allocated
 * relative to the next stream which wants to write. This is to balance fairness while also considering goodput.
 */
@UnstableApi
public final class WeightedFairQueueByteDistributor implements StreamByteDistributor {
    /**
     * The initial size of the children map is chosen to be conservative on initial memory allocations under
     * the assumption that most streams will have a small number of children. This choice may be
     * sub-optimal if when children are present there are many children (i.e. a web page which has many
     * dependencies to load).
     *
     * Visible only for testing!
     */
    static final int INITIAL_CHILDREN_MAP_SIZE =
            max(1, SystemPropertyUtil.getInt("io.netty.http2.childrenMapSize", 2));
    /**
     * FireFox currently uses 5 streams to establish QoS classes.
     */
    private static final int DEFAULT_MAX_STATE_ONLY_SIZE = 5;

    private final Http2Connection.PropertyKey stateKey;
    /**
     * If there is no Http2Stream object, but we still persist priority information then this is where the state will
     * reside.
     */
    private final IntObjectMap<State> stateOnlyMap;
    /**
     * This queue will hold streams that are not active and provides the capability to retain priority for streams which
     * have no {@link Http2Stream} object. See {@link StateOnlyComparator} for the priority comparator.
     */
    private final PriorityQueue<State> stateOnlyRemovalQueue;
    private final Http2Connection connection;
    private final State connectionState;
    /**
     * The minimum number of bytes that we will attempt to allocate to a stream. This is to
     * help improve goodput on a per-stream basis.
     */
    private int allocationQuantum = DEFAULT_MIN_ALLOCATION_CHUNK;
    private final int maxStateOnlySize;

    public WeightedFairQueueByteDistributor(Http2Connection connection) {
        this(connection, DEFAULT_MAX_STATE_ONLY_SIZE);
    }

    public WeightedFairQueueByteDistributor(Http2Connection connection, int maxStateOnlySize) {
        checkPositiveOrZero(maxStateOnlySize, "maxStateOnlySize");
        if (maxStateOnlySize == 0) {
            stateOnlyMap = IntCollections.emptyMap();
            stateOnlyRemovalQueue = EmptyPriorityQueue.instance();
        } else {
            stateOnlyMap = new IntObjectHashMap<State>(maxStateOnlySize);
            // +2 because we may exceed the limit by 2 if a new dependency has no associated Http2Stream object. We need
            // to create the State objects to put them into the dependency tree, which then impacts priority.
            stateOnlyRemovalQueue = new DefaultPriorityQueue<State>(StateOnlyComparator.INSTANCE, maxStateOnlySize + 2);
        }
        this.maxStateOnlySize = maxStateOnlySize;

        this.connection = connection;
        stateKey = connection.newKey();
        final Http2Stream connectionStream = connection.connectionStream();
        connectionStream.setProperty(stateKey, connectionState = new State(connectionStream, 16));

        // Register for notification of new streams.
        connection.addListener(new Http2ConnectionAdapter() {
            @Override
            public void onStreamAdded(Http2Stream stream) {
                State state = stateOnlyMap.remove(stream.id());
                if (state == null) {
                    state = new State(stream);
                    // Only the stream which was just added will change parents. So we only need an array of size 1.
                    List<ParentChangedEvent> events = new ArrayList<ParentChangedEvent>(1);
                    connectionState.takeChild(state, false, events);
                    notifyParentChanged(events);
                } else {
                    stateOnlyRemovalQueue.removeTyped(state);
                    state.stream = stream;
                }
                switch (stream.state()) {
                    case RESERVED_REMOTE:
                    case RESERVED_LOCAL:
                        state.setStreamReservedOrActivated();
                        // wasStreamReservedOrActivated is part of the comparator for stateOnlyRemovalQueue there is no
                        // need to reprioritize here because it will not be in stateOnlyRemovalQueue.
                        break;
                    default:
                        break;
                }
                stream.setProperty(stateKey, state);
            }

            @Override
            public void onStreamActive(Http2Stream stream) {
                state(stream).setStreamReservedOrActivated();
                // wasStreamReservedOrActivated is part of the comparator for stateOnlyRemovalQueue there is no need to
                // reprioritize here because it will not be in stateOnlyRemovalQueue.
            }

            @Override
            public void onStreamClosed(Http2Stream stream) {
                state(stream).close();
            }

            @Override
            public void onStreamRemoved(Http2Stream stream) {
                // The stream has been removed from the connection. We can no longer rely on the stream's property
                // storage to track the State. If we have room, and the precedence of the stream is sufficient, we
                // should retain the State in the stateOnlyMap.
                State state = state(stream);

                // Typically the stream is set to null when the stream is closed because it is no longer needed to write
                // data. However if the stream was not activated it may not be closed (reserved streams) so we ensure
                // the stream reference is set to null to avoid retaining a reference longer than necessary.
                state.stream = null;

                if (WeightedFairQueueByteDistributor.this.maxStateOnlySize == 0) {
                    state.parent.removeChild(state);
                    return;
                }
                if (stateOnlyRemovalQueue.size() == WeightedFairQueueByteDistributor.this.maxStateOnlySize) {
                    State stateToRemove = stateOnlyRemovalQueue.peek();
                    if (StateOnlyComparator.INSTANCE.compare(stateToRemove, state) >= 0) {
                        // The "lowest priority" stream is a "higher priority" than the stream being removed, so we
                        // just discard the state.
                        state.parent.removeChild(state);
                        return;
                    }
                    stateOnlyRemovalQueue.poll();
                    stateToRemove.parent.removeChild(stateToRemove);
                    stateOnlyMap.remove(stateToRemove.streamId);
                }
                stateOnlyRemovalQueue.add(state);
                stateOnlyMap.put(state.streamId, state);
            }
        });
    }

    @Override
    public void updateStreamableBytes(StreamState state) {
        state(state.stream()).updateStreamableBytes(streamableBytes(state),
                                                    state.hasFrame() && state.windowSize() >= 0);
    }

    @Override
    public void updateDependencyTree(int childStreamId, int parentStreamId, short weight, boolean exclusive) {
        State state = state(childStreamId);
        if (state == null) {
            // If there is no State object that means there is no Http2Stream object and we would have to keep the
            // State object in the stateOnlyMap and stateOnlyRemovalQueue. However if maxStateOnlySize is 0 this means
            // stateOnlyMap and stateOnlyRemovalQueue are empty collections and cannot be modified so we drop the State.
            if (maxStateOnlySize == 0) {
                return;
            }
            state = new State(childStreamId);
            stateOnlyRemovalQueue.add(state);
            stateOnlyMap.put(childStreamId, state);
        }

        State newParent = state(parentStreamId);
        if (newParent == null) {
            // If there is no State object that means there is no Http2Stream object and we would have to keep the
            // State object in the stateOnlyMap and stateOnlyRemovalQueue. However if maxStateOnlySize is 0 this means
            // stateOnlyMap and stateOnlyRemovalQueue are empty collections and cannot be modified so we drop the State.
            if (maxStateOnlySize == 0) {
                return;
            }
            newParent = new State(parentStreamId);
            stateOnlyRemovalQueue.add(newParent);
            stateOnlyMap.put(parentStreamId, newParent);
            // Only the stream which was just added will change parents. So we only need an array of size 1.
            List<ParentChangedEvent> events = new ArrayList<ParentChangedEvent>(1);
            connectionState.takeChild(newParent, false, events);
            notifyParentChanged(events);
        }

        // if activeCountForTree == 0 then it will not be in its parent's pseudoTimeQueue and thus should not be counted
        // toward parent.totalQueuedWeights.
        if (state.activeCountForTree != 0 && state.parent != null) {
            state.parent.totalQueuedWeights += weight - state.weight;
        }
        state.weight = weight;

        if (newParent != state.parent || (exclusive && newParent.children.size() != 1)) {
            final List<ParentChangedEvent> events;
            if (newParent.isDescendantOf(state)) {
                events = new ArrayList<ParentChangedEvent>(2 + (exclusive ? newParent.children.size() : 0));
                state.parent.takeChild(newParent, false, events);
            } else {
                events = new ArrayList<ParentChangedEvent>(1 + (exclusive ? newParent.children.size() : 0));
            }
            newParent.takeChild(state, exclusive, events);
            notifyParentChanged(events);
        }

        // The location in the dependency tree impacts the priority in the stateOnlyRemovalQueue map. If we created new
        // State objects we must check if we exceeded the limit after we insert into the dependency tree to ensure the
        // stateOnlyRemovalQueue has been updated.
        while (stateOnlyRemovalQueue.size() > maxStateOnlySize) {
            State stateToRemove = stateOnlyRemovalQueue.poll();
            stateToRemove.parent.removeChild(stateToRemove);
            stateOnlyMap.remove(stateToRemove.streamId);
        }
    }

    @Override
    public boolean distribute(int maxBytes, Writer writer) throws Http2Exception {
        // As long as there is some active frame we should write at least 1 time.
        if (connectionState.activeCountForTree == 0) {
            return false;
        }

        // The goal is to write until we write all the allocated bytes or are no longer making progress.
        // We still attempt to write even after the number of allocated bytes has been exhausted to allow empty frames
        // to be sent. Making progress means the active streams rooted at the connection stream has changed.
        int oldIsActiveCountForTree;
        do {
            oldIsActiveCountForTree = connectionState.activeCountForTree;
            // connectionState will never be active, so go right to its children.
            maxBytes -= distributeToChildren(maxBytes, writer, connectionState);
        } while (connectionState.activeCountForTree != 0 &&
                (maxBytes > 0 || oldIsActiveCountForTree != connectionState.activeCountForTree));

        return connectionState.activeCountForTree != 0;
    }

    /**
     * Sets the amount of bytes that will be allocated to each stream. Defaults to 1KiB.
     * @param allocationQuantum the amount of bytes that will be allocated to each stream. Must be &gt; 0.
     */
    public void allocationQuantum(int allocationQuantum) {
        checkPositive(allocationQuantum, "allocationQuantum");
        this.allocationQuantum = allocationQuantum;
    }

    private int distribute(int maxBytes, Writer writer, State state) throws Http2Exception {
        if (state.isActive()) {
            int nsent = min(maxBytes, state.streamableBytes);
            state.write(nsent, writer);
            if (nsent == 0 && maxBytes != 0) {
                // If a stream sends zero bytes, then we gave it a chance to write empty frames and it is now
                // considered inactive until the next call to updateStreamableBytes. This allows descendant streams to
                // be allocated bytes when the parent stream can't utilize them. This may be as a result of the
                // stream's flow control window being 0.
                state.updateStreamableBytes(state.streamableBytes, false);
            }
            return nsent;
        }

        return distributeToChildren(maxBytes, writer, state);
    }

    /**
     * It is a pre-condition that {@code state.poll()} returns a non-{@code null} value. This is a result of the way
     * the allocation algorithm is structured and can be explained in the following cases:
     * <h3>For the recursive case</h3>
     * If a stream has no children (in the allocation tree) than that node must be active or it will not be in the
     * allocation tree. If a node is active then it will not delegate to children and recursion ends.
     * <h3>For the initial case</h3>
     * We check connectionState.activeCountForTree == 0 before any allocation is done. So if the connection stream
     * has no active children we don't get into this method.
     */
    private int distributeToChildren(int maxBytes, Writer writer, State state) throws Http2Exception {
        long oldTotalQueuedWeights = state.totalQueuedWeights;
        State childState = state.pollPseudoTimeQueue();
        State nextChildState = state.peekPseudoTimeQueue();
        childState.setDistributing();
        try {
            assert nextChildState == null || nextChildState.pseudoTimeToWrite >= childState.pseudoTimeToWrite :
                "nextChildState[" + nextChildState.streamId + "].pseudoTime(" + nextChildState.pseudoTimeToWrite +
                ") < " + " childState[" + childState.streamId + "].pseudoTime(" + childState.pseudoTimeToWrite + ")";
            int nsent = distribute(nextChildState == null ? maxBytes :
                            min(maxBytes, (int) min((nextChildState.pseudoTimeToWrite - childState.pseudoTimeToWrite) *
                                               childState.weight / oldTotalQueuedWeights + allocationQuantum, MAX_VALUE)
                               ),
                               writer,
                               childState);
            state.pseudoTime += nsent;
            childState.updatePseudoTime(state, nsent, oldTotalQueuedWeights);
            return nsent;
        } finally {
            childState.unsetDistributing();
            // Do in finally to ensure the internal flags is not corrupted if an exception is thrown.
            // The offer operation is delayed until we unroll up the recursive stack, so we don't have to remove from
            // the priority pseudoTimeQueue due to a write operation.
            if (childState.activeCountForTree != 0) {
                state.offerPseudoTimeQueue(childState);
            }
        }
    }

    private State state(Http2Stream stream) {
        return stream.getProperty(stateKey);
    }

    private State state(int streamId) {
        Http2Stream stream = connection.stream(streamId);
        return stream != null ? state(stream) : stateOnlyMap.get(streamId);
    }

    /**
     * For testing only!
     */
    boolean isChild(int childId, int parentId, short weight) {
        State parent = state(parentId);
        State child;
        return parent.children.containsKey(childId) &&
                (child = state(childId)).parent == parent && child.weight == weight;
    }

    /**
     * For testing only!
     */
    int numChildren(int streamId) {
        State state = state(streamId);
        return state == null ? 0 : state.children.size();
    }

    /**
     * Notify all listeners of the priority tree change events (in ascending order)
     * @param events The events (top down order) which have changed
     */
    void notifyParentChanged(List<ParentChangedEvent> events) {
        for (int i = 0; i < events.size(); ++i) {
            ParentChangedEvent event = events.get(i);
            stateOnlyRemovalQueue.priorityChanged(event.state);
            if (event.state.parent != null && event.state.activeCountForTree != 0) {
                event.state.parent.offerAndInitializePseudoTime(event.state);
                event.state.parent.activeCountChangeForTree(event.state.activeCountForTree);
            }
        }
    }

    /**
     * A comparator for {@link State} which has no associated {@link Http2Stream} object. The general precedence is:
     * <ul>
     *     <li>Was a stream activated or reserved (streams only used for priority are higher priority)</li>
     *     <li>Depth in the priority tree (closer to root is higher priority></li>
     *     <li>Stream ID (higher stream ID is higher priority - used for tie breaker)</li>
     * </ul>
     */
    private static final class StateOnlyComparator implements Comparator<State>, Serializable {
        private static final long serialVersionUID = -4806936913002105966L;

        static final StateOnlyComparator INSTANCE = new StateOnlyComparator();

        private StateOnlyComparator() {
        }

        @Override
        public int compare(State o1, State o2) {
            // "priority only streams" (which have not been activated) are higher priority than streams used for data.
            boolean o1Actived = o1.wasStreamReservedOrActivated();
            if (o1Actived != o2.wasStreamReservedOrActivated()) {
                return o1Actived ? -1 : 1;
            }
            // Numerically greater depth is higher priority.
            int x = o2.dependencyTreeDepth - o1.dependencyTreeDepth;

            // I also considered tracking the number of streams which are "activated" (eligible transfer data) at each
            // subtree. This would require a traversal from each node to the root on dependency tree structural changes,
            // and then it would require a re-prioritization at each of these nodes (instead of just the nodes where the
            // direct parent changed). The costs of this are judged to be relatively high compared to the nominal
            // benefit it provides to the heuristic. Instead folks should just increase maxStateOnlySize.

            // Last resort is to give larger stream ids more priority.
            return x != 0 ? x : o1.streamId - o2.streamId;
        }
    }

    private static final class StatePseudoTimeComparator implements Comparator<State>, Serializable {
        private static final long serialVersionUID = -1437548640227161828L;

        static final StatePseudoTimeComparator INSTANCE = new StatePseudoTimeComparator();

        private StatePseudoTimeComparator() {
        }

        @Override
        public int compare(State o1, State o2) {
            return MathUtil.compare(o1.pseudoTimeToWrite, o2.pseudoTimeToWrite);
        }
    }

    /**
     * The remote flow control state for a single stream.
     */
    private final class State implements PriorityQueueNode {
        private static final byte STATE_IS_ACTIVE = 0x1;
        private static final byte STATE_IS_DISTRIBUTING = 0x2;
        private static final byte STATE_STREAM_ACTIVATED = 0x4;

        /**
         * Maybe {@code null} if the stream if the stream is not active.
         */
        Http2Stream stream;
        State parent;
        IntObjectMap<State> children = IntCollections.emptyMap();
        private final PriorityQueue<State> pseudoTimeQueue;
        final int streamId;
        int streamableBytes;
        int dependencyTreeDepth;
        /**
         * Count of nodes rooted at this sub tree with {@link #isActive()} equal to {@code true}.
         */
        int activeCountForTree;
        private int pseudoTimeQueueIndex = INDEX_NOT_IN_QUEUE;
        private int stateOnlyQueueIndex = INDEX_NOT_IN_QUEUE;
        /**
         * An estimate of when this node should be given the opportunity to write data.
         */
        long pseudoTimeToWrite;
        /**
         * A pseudo time maintained for immediate children to base their {@link #pseudoTimeToWrite} off of.
         */
        long pseudoTime;
        long totalQueuedWeights;
        private byte flags;
        short weight = DEFAULT_PRIORITY_WEIGHT;

        State(int streamId) {
            this(streamId, null, 0);
        }

        State(Http2Stream stream) {
            this(stream, 0);
        }

        State(Http2Stream stream, int initialSize) {
            this(stream.id(), stream, initialSize);
        }

        State(int streamId, Http2Stream stream, int initialSize) {
            this.stream = stream;
            this.streamId = streamId;
            pseudoTimeQueue = new DefaultPriorityQueue<State>(StatePseudoTimeComparator.INSTANCE, initialSize);
        }

        boolean isDescendantOf(State state) {
            State next = parent;
            while (next != null) {
                if (next == state) {
                    return true;
                }
                next = next.parent;
            }
            return false;
        }

        void takeChild(State child, boolean exclusive, List<ParentChangedEvent> events) {
            takeChild(null, child, exclusive, events);
        }

        /**
         * Adds a child to this priority. If exclusive is set, any children of this node are moved to being dependent on
         * the child.
         */
        void takeChild(Iterator<IntObjectMap.PrimitiveEntry<State>> childItr, State child, boolean exclusive,
                       List<ParentChangedEvent> events) {
            State oldParent = child.parent;

            if (oldParent != this) {
                events.add(new ParentChangedEvent(child, oldParent));
                child.setParent(this);
                // If the childItr is not null we are iterating over the oldParent.children collection and should
                // use the iterator to remove from the collection to avoid concurrent modification. Otherwise it is
                // assumed we are not iterating over this collection and it is safe to call remove directly.
                if (childItr != null) {
                    childItr.remove();
                } else if (oldParent != null) {
                    oldParent.children.remove(child.streamId);
                }

                // Lazily initialize the children to save object allocations.
                initChildrenIfEmpty();

                final State oldChild = children.put(child.streamId, child);
                assert oldChild == null : "A stream with the same stream ID was already in the child map.";
            }

            if (exclusive && !children.isEmpty()) {
                // If it was requested that this child be the exclusive dependency of this node,
                // move any previous children to the child node, becoming grand children of this node.
                Iterator<IntObjectMap.PrimitiveEntry<State>> itr = removeAllChildrenExcept(child).entries().iterator();
                while (itr.hasNext()) {
                    child.takeChild(itr, itr.next().value(), false, events);
                }
            }
        }

        /**
         * Removes the child priority and moves any of its dependencies to being direct dependencies on this node.
         */
        void removeChild(State child) {
            if (children.remove(child.streamId) != null) {
                List<ParentChangedEvent> events = new ArrayList<ParentChangedEvent>(1 + child.children.size());
                events.add(new ParentChangedEvent(child, child.parent));
                child.setParent(null);

                // Move up any grand children to be directly dependent on this node.
                Iterator<IntObjectMap.PrimitiveEntry<State>> itr = child.children.entries().iterator();
                while (itr.hasNext()) {
                    takeChild(itr, itr.next().value(), false, events);
                }

                notifyParentChanged(events);
            }
        }

        /**
         * Remove all children with the exception of {@code streamToRetain}.
         * This method is intended to be used to support an exclusive priority dependency operation.
         * @return The map of children prior to this operation, excluding {@code streamToRetain} if present.
         */
        private IntObjectMap<State> removeAllChildrenExcept(State stateToRetain) {
            stateToRetain = children.remove(stateToRetain.streamId);
            IntObjectMap<State> prevChildren = children;
            // This map should be re-initialized in anticipation for the 1 exclusive child which will be added.
            // It will either be added directly in this method, or after this method is called...but it will be added.
            initChildren();
            if (stateToRetain != null) {
                children.put(stateToRetain.streamId, stateToRetain);
            }
            return prevChildren;
        }

        private void setParent(State newParent) {
            // if activeCountForTree == 0 then it will not be in its parent's pseudoTimeQueue.
            if (activeCountForTree != 0 && parent != null) {
                parent.removePseudoTimeQueue(this);
                parent.activeCountChangeForTree(-activeCountForTree);
            }
            parent = newParent;
            // Use MAX_VALUE if no parent because lower depth is considered higher priority by StateOnlyComparator.
            dependencyTreeDepth = newParent == null ? MAX_VALUE : newParent.dependencyTreeDepth + 1;
        }

        private void initChildrenIfEmpty() {
            if (children == IntCollections.<State>emptyMap()) {
                initChildren();
            }
        }

        private void initChildren() {
            children = new IntObjectHashMap<State>(INITIAL_CHILDREN_MAP_SIZE);
        }

        void write(int numBytes, Writer writer) throws Http2Exception {
            assert stream != null;
            try {
                writer.write(stream, numBytes);
            } catch (Throwable t) {
                throw connectionError(INTERNAL_ERROR, t, "byte distribution write error");
            }
        }

        void activeCountChangeForTree(int increment) {
            assert activeCountForTree + increment >= 0;
            activeCountForTree += increment;
            if (parent != null) {
                assert activeCountForTree != increment ||
                       pseudoTimeQueueIndex == INDEX_NOT_IN_QUEUE ||
                       parent.pseudoTimeQueue.containsTyped(this) :
                     "State[" + streamId + "].activeCountForTree changed from 0 to " + increment + " is in a " +
                     "pseudoTimeQueue, but not in parent[ " + parent.streamId + "]'s pseudoTimeQueue";
                if (activeCountForTree == 0) {
                    parent.removePseudoTimeQueue(this);
                } else if (activeCountForTree == increment && !isDistributing()) {
                    // If frame count was 0 but is now not, and this node is not already in a pseudoTimeQueue (assumed
                    // to be pState's pseudoTimeQueue) then enqueue it. If this State object is being processed the
                    // pseudoTime for this node should not be adjusted, and the node will be added back to the
                    // pseudoTimeQueue/tree structure after it is done being processed. This may happen if the
                    // activeCountForTree == 0 (a node which can't stream anything and is blocked) is at/near root of
                    // the tree, and is popped off the pseudoTimeQueue during processing, and then put back on the
                    // pseudoTimeQueue because a child changes position in the priority tree (or is closed because it is
                    // not blocked and finished writing all data).
                    parent.offerAndInitializePseudoTime(this);
                }
                parent.activeCountChangeForTree(increment);
            }
        }

        void updateStreamableBytes(int newStreamableBytes, boolean isActive) {
            if (isActive() != isActive) {
                if (isActive) {
                    activeCountChangeForTree(1);
                    setActive();
                } else {
                    activeCountChangeForTree(-1);
                    unsetActive();
                }
            }

            streamableBytes = newStreamableBytes;
        }

        /**
         * Assumes the parents {@link #totalQueuedWeights} includes this node's weight.
         */
        void updatePseudoTime(State parentState, int nsent, long totalQueuedWeights) {
            assert streamId != CONNECTION_STREAM_ID && nsent >= 0;
            // If the current pseudoTimeToSend is greater than parentState.pseudoTime then we previously over accounted
            // and should use parentState.pseudoTime.
            pseudoTimeToWrite = min(pseudoTimeToWrite, parentState.pseudoTime) + nsent * totalQueuedWeights / weight;
        }

        /**
         * The concept of pseudoTime can be influenced by priority tree manipulations or if a stream goes from "active"
         * to "non-active". This method accounts for that by initializing the {@link #pseudoTimeToWrite} for
         * {@code state} to {@link #pseudoTime} of this node and then calls {@link #offerPseudoTimeQueue(State)}.
         */
        void offerAndInitializePseudoTime(State state) {
            state.pseudoTimeToWrite = pseudoTime;
            offerPseudoTimeQueue(state);
        }

        void offerPseudoTimeQueue(State state) {
            pseudoTimeQueue.offer(state);
            totalQueuedWeights += state.weight;
        }

        /**
         * Must only be called if the pseudoTimeQueue is non-empty!
         */
        State pollPseudoTimeQueue() {
            State state = pseudoTimeQueue.poll();
            // This method is only ever called if the pseudoTimeQueue is non-empty.
            totalQueuedWeights -= state.weight;
            return state;
        }

        void removePseudoTimeQueue(State state) {
            if (pseudoTimeQueue.removeTyped(state)) {
                totalQueuedWeights -= state.weight;
            }
        }

        State peekPseudoTimeQueue() {
            return pseudoTimeQueue.peek();
        }

        void close() {
            updateStreamableBytes(0, false);
            stream = null;
        }

        boolean wasStreamReservedOrActivated() {
            return (flags & STATE_STREAM_ACTIVATED) != 0;
        }

        void setStreamReservedOrActivated() {
            flags |= STATE_STREAM_ACTIVATED;
        }

        boolean isActive() {
            return (flags & STATE_IS_ACTIVE) != 0;
        }

        private void setActive() {
            flags |= STATE_IS_ACTIVE;
        }

        private void unsetActive() {
            flags &= ~STATE_IS_ACTIVE;
        }

        boolean isDistributing() {
            return (flags & STATE_IS_DISTRIBUTING) != 0;
        }

        void setDistributing() {
            flags |= STATE_IS_DISTRIBUTING;
        }

        void unsetDistributing() {
            flags &= ~STATE_IS_DISTRIBUTING;
        }

        @Override
        public int priorityQueueIndex(DefaultPriorityQueue<?> queue) {
            return queue == stateOnlyRemovalQueue ? stateOnlyQueueIndex : pseudoTimeQueueIndex;
        }

        @Override
        public void priorityQueueIndex(DefaultPriorityQueue<?> queue, int i) {
            if (queue == stateOnlyRemovalQueue) {
                stateOnlyQueueIndex = i;
            } else {
                pseudoTimeQueueIndex = i;
            }
        }

        @Override
        public String toString() {
            // Use activeCountForTree as a rough estimate for how many nodes are in this subtree.
            StringBuilder sb = new StringBuilder(256 * (activeCountForTree > 0 ? activeCountForTree : 1));
            toString(sb);
            return sb.toString();
        }

        private void toString(StringBuilder sb) {
            sb.append("{streamId ").append(streamId)
                    .append(" streamableBytes ").append(streamableBytes)
                    .append(" activeCountForTree ").append(activeCountForTree)
                    .append(" pseudoTimeQueueIndex ").append(pseudoTimeQueueIndex)
                    .append(" pseudoTimeToWrite ").append(pseudoTimeToWrite)
                    .append(" pseudoTime ").append(pseudoTime)
                    .append(" flags ").append(flags)
                    .append(" pseudoTimeQueue.size() ").append(pseudoTimeQueue.size())
                    .append(" stateOnlyQueueIndex ").append(stateOnlyQueueIndex)
                    .append(" parent.streamId ").append(parent == null ? -1 : parent.streamId).append("} [");

            if (!pseudoTimeQueue.isEmpty()) {
                for (State s : pseudoTimeQueue) {
                    s.toString(sb);
                    sb.append(", ");
                }
                // Remove the last ", "
                sb.setLength(sb.length() - 2);
            }
            sb.append(']');
        }
    }

    /**
     * Allows a correlation to be made between a stream and its old parent before a parent change occurs.
     */
    private static final class ParentChangedEvent {
        final State state;
        final State oldParent;

        /**
         * Create a new instance.
         * @param state The state who has had a parent change.
         * @param oldParent The previous parent.
         */
        ParentChangedEvent(State state, State oldParent) {
            this.state = state;
            this.oldParent = oldParent;
        }
    }
}