/*
    * Copyright 2016 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:
    *
    *   https://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.channel.kqueue;
  
  import io.netty.channel.Channel;
  import io.netty.channel.DefaultSelectStrategyFactory;
  import io.netty.channel.IoHandle;
  import io.netty.channel.IoHandler;
  import io.netty.channel.IoHandlerContext;
  import io.netty.channel.IoHandlerFactory;
  import io.netty.channel.IoOps;
  import io.netty.channel.IoRegistration;
  import io.netty.channel.SelectStrategy;
  import io.netty.channel.SelectStrategyFactory;
  import io.netty.channel.unix.FileDescriptor;
  import io.netty.util.IntSupplier;
  import io.netty.util.collection.LongObjectHashMap;
  import io.netty.util.collection.LongObjectMap;
  import io.netty.util.concurrent.ThreadAwareExecutor;
  import io.netty.util.internal.ObjectUtil;
  import io.netty.util.internal.StringUtil;
  import io.netty.util.internal.logging.InternalLogger;
  import io.netty.util.internal.logging.InternalLoggerFactory;
  
  import java.io.IOException;
  import java.util.ArrayDeque;
  import java.util.ArrayList;
  import java.util.Collections;
  import java.util.List;
  import java.util.Queue;
  import java.util.concurrent.atomic.AtomicBoolean;
  import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
  
  import static java.lang.Math.min;
  
  /**
   * {@link IoHandler} which uses kqueue under the covers. Only works on BSD!
   */
  public final class KQueueIoHandler implements IoHandler {
      private static final InternalLogger logger = InternalLoggerFactory.getInstance(KQueueIoHandler.class);
      private static final AtomicIntegerFieldUpdater<KQueueIoHandler> WAKEN_UP_UPDATER =
              AtomicIntegerFieldUpdater.newUpdater(KQueueIoHandler.class, "wakenUp");
      private static final int KQUEUE_WAKE_UP_IDENT = 0;
      // `kqueue()` may return EINVAL when a large number such as Integer.MAX_VALUE is specified as timeout.
      // 24 hours would be a large enough value.
      // https://man.freebsd.org/cgi/man.cgi?query=kevent&apropos=0&sektion=0&manpath=FreeBSD+6.1-RELEASE&format=html#end
      private static final int KQUEUE_MAX_TIMEOUT_SECONDS = 86399; // 24 hours - 1 second
  
      {
          KQueue.ensureAvailability();
      }
  
      private final boolean allowGrowing;
      private final FileDescriptor kqueueFd;
      private final KQueueEventArray changeList;
      private final KQueueEventArray eventList;
      private final SelectStrategy selectStrategy;
      private final NativeArrays nativeArrays;
      private final IntSupplier selectNowSupplier = new IntSupplier() {
          @Override
          public int get() throws Exception {
              return kqueueWaitNow();
          }
      };
      private final ThreadAwareExecutor executor;
      private final Queue<DefaultKqueueIoRegistration> cancelledRegistrations = new ArrayDeque<>();
      private final LongObjectMap<DefaultKqueueIoRegistration> registrations = new LongObjectHashMap<>(4096);
      private int numChannels;
      private long nextId;
  
      private volatile int wakenUp;
  
      private long generateNextId() {
          boolean reset = false;
          for (;;) {
              if (nextId == Long.MAX_VALUE) {
                  if (reset) {
                      throw new IllegalStateException("All possible ids in use");
                  }
                  reset = true;
              }
              nextId++;
              if (nextId == KQUEUE_WAKE_UP_IDENT) {
                  continue;
              }
              if (!registrations.containsKey(nextId)) {
                 return nextId;
             }
         }
     }
 
     /**
      * Returns a new {@link IoHandlerFactory} that creates {@link KQueueIoHandler} instances.
      */
     public static IoHandlerFactory newFactory() {
         return newFactory(0, DefaultSelectStrategyFactory.INSTANCE);
     }
 
     /**
      * Returns a new {@link IoHandlerFactory} that creates {@link KQueueIoHandler} instances.
      */
     public static IoHandlerFactory newFactory(final int maxEvents,
                                               final SelectStrategyFactory selectStrategyFactory) {
         KQueue.ensureAvailability();
         ObjectUtil.checkPositiveOrZero(maxEvents, "maxEvents");
         ObjectUtil.checkNotNull(selectStrategyFactory, "selectStrategyFactory");
         return new IoHandlerFactory() {
             @Override
             public IoHandler newHandler(ThreadAwareExecutor executor) {
                 return new KQueueIoHandler(executor, maxEvents, selectStrategyFactory.newSelectStrategy());
             }
 
             @Override
             public boolean isChangingThreadSupported() {
                 return true;
             }
         };
     }
 
     private KQueueIoHandler(ThreadAwareExecutor executor, int maxEvents, SelectStrategy strategy) {
         this.executor = ObjectUtil.checkNotNull(executor, "executor");
         this.selectStrategy = ObjectUtil.checkNotNull(strategy, "strategy");
         this.kqueueFd = Native.newKQueue();
         if (maxEvents == 0) {
             allowGrowing = true;
             maxEvents = 4096;
         } else {
             allowGrowing = false;
         }
         this.changeList = new KQueueEventArray(maxEvents);
         this.eventList = new KQueueEventArray(maxEvents);
         nativeArrays = new NativeArrays();
         int result = Native.keventAddUserEvent(kqueueFd.intValue(), KQUEUE_WAKE_UP_IDENT);
         if (result < 0) {
             destroy();
             throw new IllegalStateException("kevent failed to add user event with errno: " + (-result));
         }
     }
 
     @Override
     public void wakeup() {
         if (!executor.isExecutorThread(Thread.currentThread())
                 && WAKEN_UP_UPDATER.compareAndSet(this, 0, 1)) {
             wakeup0();
         }
     }
 
     private void wakeup0() {
         Native.keventTriggerUserEvent(kqueueFd.intValue(), KQUEUE_WAKE_UP_IDENT);
         // Note that the result may return an error (e.g. errno = EBADF after the event loop has been shutdown).
         // So it is not very practical to assert the return value is always >= 0.
     }
 
     private int kqueueWait(IoHandlerContext context, boolean oldWakeup) throws IOException {
         // If a task was submitted when wakenUp value was 1, the task didn't get a chance to produce wakeup event.
         // So we need to check task queue again before calling kqueueWait. If we don't, the task might be pended
         // until kqueueWait was timed out. It might be pended until idle timeout if IdleStateHandler existed
         // in pipeline.
         if (oldWakeup && !context.canBlock()) {
             return kqueueWaitNow();
         }
 
         long totalDelay = context.delayNanos(System.nanoTime());
         int delaySeconds = (int) min(totalDelay / 1000000000L, KQUEUE_MAX_TIMEOUT_SECONDS);
         int delayNanos = (int) (totalDelay % 1000000000L);
         return kqueueWait(delaySeconds, delayNanos);
     }
 
     private int kqueueWaitNow() throws IOException {
         return kqueueWait(0, 0);
     }
 
     private int kqueueWait(int timeoutSec, int timeoutNs) throws IOException {
         int numEvents = Native.keventWait(kqueueFd.intValue(), changeList, eventList, timeoutSec, timeoutNs);
         changeList.clear();
         return numEvents;
     }
 
     private int processReady(int ready) {
         int ioCount = 0;
         for (int i = 0; i < ready; ++i) {
             final short filter = eventList.filter(i);
             final short flags = eventList.flags(i);
             final int ident = eventList.ident(i);
             if (filter == Native.EVFILT_USER || (flags & Native.EV_ERROR) != 0) {
                 // EV_ERROR is returned if the FD is closed synchronously (which removes from kqueue) and then
                 // we later attempt to delete the filters from kqueue.
                 assert filter != Native.EVFILT_USER ||
                         (filter == Native.EVFILT_USER && ident == KQUEUE_WAKE_UP_IDENT);
                 continue;
             }
 
             ioCount++;
             long id = eventList.udata(i);
             DefaultKqueueIoRegistration registration = registrations.get(id);
             if (registration == null) {
                 // This may happen if the channel has already been closed, and it will be removed from kqueue anyways.
                 // We also handle EV_ERROR above to skip this even early if it is a result of a referencing a closed and
                 // thus removed from kqueue FD.
                 logger.warn("events[{}]=[{}, {}, {}] had no registration!", i, ident, id, filter);
                 continue;
             }
             registration.handle(ident, filter, flags, eventList.fflags(i), eventList.data(i), id);
         }
         return ioCount;
     }
 
     @Override
     public int run(IoHandlerContext context) {
         int handled = 0;
         try {
             int strategy = selectStrategy.calculateStrategy(selectNowSupplier, !context.canBlock());
             switch (strategy) {
                 case SelectStrategy.CONTINUE:
                     if (context.shouldReportActiveIoTime()) {
                         context.reportActiveIoTime(0); // Report zero as we did no I/O.
                     }
                     return 0;
 
                 case SelectStrategy.BUSY_WAIT:
                     // fall-through to SELECT since the busy-wait is not supported with kqueue
 
                 case SelectStrategy.SELECT:
                     strategy = kqueueWait(context, WAKEN_UP_UPDATER.getAndSet(this, 0) == 1);
 
                     // 'wakenUp.compareAndSet(false, true)' is always evaluated
                     // before calling 'selector.wakeup()' to reduce the wake-up
                     // overhead. (Selector.wakeup() is an expensive operation.)
                     //
                     // However, there is a race condition in this approach.
                     // The race condition is triggered when 'wakenUp' is set to
                     // true too early.
                     //
                     // 'wakenUp' is set to true too early if:
                     // 1) Selector is waken up between 'wakenUp.set(false)' and
                     //    'selector.select(...)'. (BAD)
                     // 2) Selector is waken up between 'selector.select(...)' and
                     //    'if (wakenUp.get()) { ... }'. (OK)
                     //
                     // In the first case, 'wakenUp' is set to true and the
                     // following 'selector.select(...)' will wake up immediately.
                     // Until 'wakenUp' is set to false again in the next round,
                     // 'wakenUp.compareAndSet(false, true)' will fail, and therefore
                     // any attempt to wake up the Selector will fail, too, causing
                     // the following 'selector.select(...)' call to block
                     // unnecessarily.
                     //
                     // To fix this problem, we wake up the selector again if wakenUp
                     // is true immediately after selector.select(...).
                     // It is inefficient in that it wakes up the selector for both
                     // the first case (BAD - wake-up required) and the second case
                     // (OK - no wake-up required).
 
                     if (wakenUp == 1) {
                         wakeup0();
                     }
                     // fall-through
                 default:
             }
 
             if (strategy > 0) {
                 if (context.shouldReportActiveIoTime()) {
                     long activeIoStartTimeNanos = System.nanoTime();
                     handled = processReady(strategy);
                     long activeIoEndTimeNanos = System.nanoTime();
                     context.reportActiveIoTime(activeIoEndTimeNanos - activeIoStartTimeNanos);
                 } else {
                     handled = processReady(strategy);
                 }
             } else if (context.shouldReportActiveIoTime()) {
                 context.reportActiveIoTime(0);
             }
 
             if (allowGrowing && strategy == eventList.capacity()) {
                 //increase the size of the array as we needed the whole space for the events
                 eventList.realloc(false);
             }
         } catch (Error e) {
             throw e;
         } catch (Throwable t) {
             handleLoopException(t);
         } finally {
             processCancelledRegistrations();
         }
         return handled;
     }
 
     // Process all previous cannceld registrations and remove them from the registration map.
     private void processCancelledRegistrations() {
         for (;;) {
             DefaultKqueueIoRegistration cancelledRegistration = cancelledRegistrations.poll();
             if (cancelledRegistration == null) {
                 return;
             }
             DefaultKqueueIoRegistration removed = registrations.remove(cancelledRegistration.id);
             assert removed == cancelledRegistration;
             if (removed.isHandleForChannel()) {
                 numChannels--;
             }
             removed.handle.unregistered();
         }
     }
 
     int numRegisteredChannels() {
         return numChannels;
     }
 
     List<Channel> registeredChannelsList() {
         LongObjectMap<DefaultKqueueIoRegistration> ch = registrations;
         if (ch.isEmpty()) {
             return Collections.emptyList();
         }
 
         List<Channel> channels = new ArrayList<>(ch.size());
 
         for (DefaultKqueueIoRegistration registration : ch.values()) {
             if (registration.handle instanceof AbstractKQueueChannel.AbstractKQueueUnsafe) {
                 channels.add(((AbstractKQueueChannel.AbstractKQueueUnsafe) registration.handle).channel());
             }
         }
         return Collections.unmodifiableList(channels);
     }
 
     private static void handleLoopException(Throwable t) {
         logger.warn("Unexpected exception in the selector loop.", t);
 
         // Prevent possible consecutive immediate failures that lead to
         // excessive CPU consumption.
         try {
             Thread.sleep(1000);
         } catch (InterruptedException e) {
             // Ignore.
         }
     }
 
     @Override
     public void prepareToDestroy() {
         try {
             kqueueWaitNow();
         } catch (IOException e) {
             // ignore on close
         }
 
         // Using the intermediate collection to prevent ConcurrentModificationException.
         // In the `close()` method, the channel is deleted from `channels` map.
         DefaultKqueueIoRegistration[] copy = registrations.values().toArray(new DefaultKqueueIoRegistration[0]);
 
         for (DefaultKqueueIoRegistration reg: copy) {
             reg.close();
         }
 
         processCancelledRegistrations();
     }
 
     @Override
     public void destroy() {
         try {
             try {
                 kqueueFd.close();
             } catch (IOException e) {
                 logger.warn("Failed to close the kqueue fd.", e);
             }
         } finally {
             // Cleanup all native memory!
             nativeArrays.free();
             changeList.free();
             eventList.free();
         }
     }
 
     @Override
     public IoRegistration register(IoHandle handle) {
         final KQueueIoHandle kqueueHandle = cast(handle);
         if (kqueueHandle.ident() == KQUEUE_WAKE_UP_IDENT) {
             throw new IllegalArgumentException("ident " + KQUEUE_WAKE_UP_IDENT + " is reserved for internal usage");
         }
 
         DefaultKqueueIoRegistration registration = new DefaultKqueueIoRegistration(
                 executor, kqueueHandle);
         DefaultKqueueIoRegistration old = registrations.put(registration.id, registration);
         if (old != null) {
             // This should never happen but just in case.
             registrations.put(old.id, old);
             throw new IllegalStateException();
         }
         if (registration.isHandleForChannel()) {
             numChannels++;
         }
         handle.registered();
         return registration;
     }
 
     private static KQueueIoHandle cast(IoHandle handle) {
         if (handle instanceof KQueueIoHandle) {
             return (KQueueIoHandle) handle;
         }
         throw new IllegalArgumentException("IoHandle of type " + StringUtil.simpleClassName(handle) + " not supported");
     }
 
     private static KQueueIoOps cast(IoOps ops) {
         if (ops instanceof KQueueIoOps) {
             return (KQueueIoOps) ops;
         }
         throw new IllegalArgumentException("IoOps of type " + StringUtil.simpleClassName(ops) + " not supported");
     }
 
     @Override
     public boolean isCompatible(Class<? extends IoHandle> handleType) {
         return KQueueIoHandle.class.isAssignableFrom(handleType);
     }
 
     private final class DefaultKqueueIoRegistration implements IoRegistration {
         private boolean cancellationPending;
         private final AtomicBoolean canceled = new AtomicBoolean();
         private final KQueueIoEvent event = new KQueueIoEvent();
 
         final KQueueIoHandle handle;
         final long id;
         private final ThreadAwareExecutor executor;
 
         DefaultKqueueIoRegistration(ThreadAwareExecutor executor, KQueueIoHandle handle) {
             this.executor = executor;
             this.handle = handle;
             id = generateNextId();
         }
 
         boolean isHandleForChannel() {
             return handle instanceof AbstractKQueueChannel.AbstractKQueueUnsafe;
         }
 
         @SuppressWarnings("unchecked")
         @Override
         public <T> T attachment() {
             return (T) nativeArrays;
         }
 
         @Override
         public long submit(IoOps ops) {
             KQueueIoOps kQueueIoOps = cast(ops);
             if (!isValid()) {
                 return -1;
             }
             short filter = kQueueIoOps.filter();
             short flags = kQueueIoOps.flags();
             int fflags = kQueueIoOps.fflags();
             long data = kQueueIoOps.data();
             if (executor.isExecutorThread(Thread.currentThread())) {
                 evSet(filter, flags, fflags, data);
             } else {
                 executor.execute(() -> evSet(filter, flags, fflags, data));
             }
             return 0;
         }
 
         void handle(int ident, short filter, short flags, int fflags, long data, long udata) {
             if (cancellationPending) {
                 // This registration was already cancelled but not removed from the map yet, just ignore.
                 return;
             }
             event.update(ident, filter, flags, fflags, data, udata);
             handle.handle(this, event);
         }
 
         private void evSet(short filter, short flags, int fflags, long data) {
             if (cancellationPending) {
                 // This registration was already cancelled but not removed from the map yet, just ignore.
                 return;
             }
             changeList.evSet(handle.ident(), filter, flags, fflags, data, id);
         }
 
         @Override
         public boolean isValid() {
             return !canceled.get();
         }
 
         @Override
         public boolean cancel() {
             if (!canceled.compareAndSet(false, true)) {
                 return false;
             }
             if (executor.isExecutorThread(Thread.currentThread())) {
                 cancel0();
             } else {
                 executor.execute(this::cancel0);
             }
             return true;
         }
 
         private void cancel0() {
             // Let's add the registration to our cancelledRegistrations queue so we will process it
             // after we processed all events. This is needed as otherwise we might end up removing it
             // from the registration map while we still have some unprocessed events.
             cancellationPending = true;
             cancelledRegistrations.offer(this);
         }
 
         void close() {
             cancel();
             try {
                 handle.close();
             } catch (Exception e) {
                 logger.debug("Exception during closing " + handle, e);
             }
         }
     }
 }