293 lines
11 KiB
Java
293 lines
11 KiB
Java
/*
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* Copyright 2018 The Netty Project
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*
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* The Netty Project licenses this file to you under the Apache License,
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* version 2.0 (the "License"); you may not use this file except in compliance
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* with the License. You may obtain a copy of the License at:
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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* License for the specific language governing permissions and limitations
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* under the License.
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*/
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package io.netty.resolver.dns;
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import io.netty.channel.EventLoop;
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import java.util.ArrayList;
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import java.util.Collections;
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import java.util.Iterator;
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import java.util.List;
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import java.util.Map;
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import java.util.Map.Entry;
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import java.util.concurrent.ConcurrentHashMap;
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import java.util.concurrent.ConcurrentMap;
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import java.util.concurrent.Delayed;
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import java.util.concurrent.ScheduledFuture;
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import java.util.concurrent.TimeUnit;
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import java.util.concurrent.atomic.AtomicReference;
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import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
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import static java.util.Collections.singletonList;
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/**
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* Abstract cache that automatically removes entries for a hostname once the TTL for an entry is reached.
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*
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* @param <E>
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*/
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abstract class Cache<E> {
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private static final AtomicReferenceFieldUpdater<Cache.Entries, ScheduledFuture> FUTURE_UPDATER =
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AtomicReferenceFieldUpdater.newUpdater(Cache.Entries.class, ScheduledFuture.class, "expirationFuture");
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private static final ScheduledFuture<?> CANCELLED = new ScheduledFuture<Object>() {
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@Override
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public boolean cancel(boolean mayInterruptIfRunning) {
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return false;
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}
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@Override
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public long getDelay(TimeUnit unit) {
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// We ignore unit and always return the minimum value to ensure the TTL of the cancelled marker is
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// the smallest.
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return Long.MIN_VALUE;
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}
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@Override
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public int compareTo(Delayed o) {
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throw new UnsupportedOperationException();
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}
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@Override
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public boolean isCancelled() {
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return true;
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}
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@Override
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public boolean isDone() {
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return true;
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}
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@Override
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public Object get() {
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throw new UnsupportedOperationException();
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}
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@Override
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public Object get(long timeout, TimeUnit unit) {
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throw new UnsupportedOperationException();
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}
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};
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// Two years are supported by all our EventLoop implementations and so safe to use as maximum.
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// See also: https://github.com/netty/netty/commit/b47fb817991b42ec8808c7d26538f3f2464e1fa6
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static final int MAX_SUPPORTED_TTL_SECS = (int) TimeUnit.DAYS.toSeconds(365 * 2);
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private final ConcurrentMap<String, Entries> resolveCache = new ConcurrentHashMap<>();
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/**
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* Remove everything from the cache.
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*/
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final void clear() {
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while (!resolveCache.isEmpty()) {
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for (Iterator<Entry<String, Entries>> i = resolveCache.entrySet().iterator(); i.hasNext();) {
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Map.Entry<String, Entries> e = i.next();
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i.remove();
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e.getValue().clearAndCancel();
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}
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}
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}
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/**
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* Clear all entries (if anything exists) for the given hostname and return {@code true} if anything was removed.
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*/
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final boolean clear(String hostname) {
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Entries entries = resolveCache.remove(hostname);
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return entries != null && entries.clearAndCancel();
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}
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/**
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* Returns all caches entries for the given hostname.
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*/
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final List<? extends E> get(String hostname) {
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Entries entries = resolveCache.get(hostname);
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return entries == null ? null : entries.get();
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}
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/**
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* Cache a value for the given hostname that will automatically expire once the TTL is reached.
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*/
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final void cache(String hostname, E value, int ttl, EventLoop loop) {
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Entries entries = resolveCache.get(hostname);
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if (entries == null) {
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entries = new Entries(hostname);
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Entries oldEntries = resolveCache.putIfAbsent(hostname, entries);
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if (oldEntries != null) {
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entries = oldEntries;
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}
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}
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entries.add(value, ttl, loop);
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}
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/**
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* Return the number of hostames for which we have cached something.
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*/
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final int size() {
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return resolveCache.size();
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}
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/**
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* Returns {@code true} if this entry should replace all other entries that are already cached for the hostname.
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*/
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protected abstract boolean shouldReplaceAll(E entry);
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/**
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* Sort the {@link List} for a {@code hostname} before caching these.
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*/
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protected void sortEntries(
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@SuppressWarnings("unused") String hostname, @SuppressWarnings("unused") List<E> entries) {
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// NOOP.
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}
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/**
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* Returns {@code true} if both entries are equal.
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*/
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protected abstract boolean equals(E entry, E otherEntry);
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// Directly extend AtomicReference for intrinsics and also to keep memory overhead low.
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private final class Entries extends AtomicReference<List<E>> implements Runnable {
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private final String hostname;
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// Needs to be package-private to be able to access it via the AtomicReferenceFieldUpdater
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volatile ScheduledFuture<?> expirationFuture;
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Entries(String hostname) {
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super(Collections.emptyList());
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this.hostname = hostname;
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}
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void add(E e, int ttl, EventLoop loop) {
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if (!shouldReplaceAll(e)) {
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for (;;) {
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List<E> entries = get();
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if (!entries.isEmpty()) {
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final E firstEntry = entries.get(0);
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if (shouldReplaceAll(firstEntry)) {
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assert entries.size() == 1;
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if (compareAndSet(entries, singletonList(e))) {
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scheduleCacheExpirationIfNeeded(ttl, loop);
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return;
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} else {
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// Need to try again as CAS failed
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continue;
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}
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}
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// Create a new List for COW semantics
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List<E> newEntries = new ArrayList<>(entries.size() + 1);
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int i = 0;
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E replacedEntry = null;
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do {
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E entry = entries.get(i);
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// Only add old entry if the address is not the same as the one we try to add as well.
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// In this case we will skip it and just add the new entry as this may have
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// more up-to-date data and cancel the old after we were able to update the cache.
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if (!Cache.this.equals(e, entry)) {
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newEntries.add(entry);
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} else {
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replacedEntry = entry;
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newEntries.add(e);
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++i;
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for (; i < entries.size(); ++i) {
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newEntries.add(entries.get(i));
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}
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break;
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}
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} while (++i < entries.size());
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if (replacedEntry == null) {
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newEntries.add(e);
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}
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sortEntries(hostname, newEntries);
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if (compareAndSet(entries, Collections.unmodifiableList(newEntries))) {
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scheduleCacheExpirationIfNeeded(ttl, loop);
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return;
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}
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} else if (compareAndSet(entries, singletonList(e))) {
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scheduleCacheExpirationIfNeeded(ttl, loop);
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return;
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}
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}
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} else {
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set(singletonList(e));
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scheduleCacheExpirationIfNeeded(ttl, loop);
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}
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}
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private void scheduleCacheExpirationIfNeeded(int ttl, EventLoop loop) {
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for (;;) {
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// We currently don't calculate a new TTL when we need to retry the CAS as we don't expect this to
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// be invoked very concurrently and also we use SECONDS anyway. If this ever becomes a problem
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// we can reconsider.
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ScheduledFuture<?> oldFuture = FUTURE_UPDATER.get(this);
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if (oldFuture == null || oldFuture.getDelay(TimeUnit.SECONDS) > ttl) {
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ScheduledFuture<?> newFuture = loop.schedule(this, ttl, TimeUnit.SECONDS);
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// It is possible that
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// 1. task will fire in between this line, or
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// 2. multiple timers may be set if there is concurrency
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// (1) Shouldn't be a problem because we will fail the CAS and then the next loop will see CANCELLED
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// so the ttl will not be less, and we will bail out of the loop.
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// (2) This is a trade-off to avoid concurrency resulting in contention on a synchronized block.
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if (FUTURE_UPDATER.compareAndSet(this, oldFuture, newFuture)) {
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if (oldFuture != null) {
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oldFuture.cancel(true);
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}
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break;
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} else {
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// There was something else scheduled in the meantime... Cancel and try again.
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newFuture.cancel(true);
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}
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} else {
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break;
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}
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}
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}
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boolean clearAndCancel() {
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List<E> entries = getAndSet(Collections.emptyList());
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if (entries.isEmpty()) {
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return false;
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}
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ScheduledFuture<?> expirationFuture = FUTURE_UPDATER.getAndSet(this, CANCELLED);
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if (expirationFuture != null) {
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expirationFuture.cancel(false);
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}
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return true;
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}
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@Override
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public void run() {
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// We always remove all entries for a hostname once one entry expire. This is not the
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// most efficient to do but this way we can guarantee that if a DnsResolver
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// be configured to prefer one ip family over the other we will not return unexpected
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// results to the enduser if one of the A or AAAA records has different TTL settings.
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//
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// As a TTL is just a hint of the maximum time a cache is allowed to cache stuff it's
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// completely fine to remove the entry even if the TTL is not reached yet.
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//
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// See https://github.com/netty/netty/issues/7329
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resolveCache.remove(hostname, this);
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clearAndCancel();
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}
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}
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}
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