package org.eclipse.aether.named.support;
   
   /*
    * Licensed to the Apache Software Foundation (ASF) under one
    * or more contributor license agreements.  See the NOTICE file
    * distributed with this work for additional information
    * regarding copyright ownership.  The ASF 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.
   */
  
  import java.io.IOException;
  import java.io.UncheckedIOException;
  import java.nio.channels.FileChannel;
  import java.nio.channels.FileLock;
  import java.nio.channels.OverlappingFileLockException;
  import java.util.ArrayDeque;
  import java.util.Deque;
  import java.util.HashMap;
  import java.util.Map;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicReference;
  import java.util.concurrent.locks.ReentrantLock;
  
  import static org.eclipse.aether.named.support.Retry.retry;
  
  /**
   * Named lock that uses {@link FileLock}. An instance of this class is about ONE LOCK (one file)
   * and is possibly used by multiple threads. Each thread (if properly coded re boxing) will try to
   * obtain either shared or exclusive lock. As file locks are JVM-scoped (so one JVM can obtain
   * same file lock only once), the threads share file lock and synchronize according to it. Still,
   * as file lock obtain operation does not block (or in other words, the method that does block
   * cannot be controlled for how long it blocks), we are "simulating" thread blocking using
   * {@link Retry} utility.
   * This implementation performs coordination not only on thread (JVM-local) level, but also on
   * process level, as long as other parties are using this same "advisory" locking mechanism.
   *
   * @since 1.7.3
   */
  public final class FileLockNamedLock
      extends NamedLockSupport
  {
      private static final long RETRY_SLEEP_MILLIS = 100L;
  
      private static final long LOCK_POSITION = 0L;
  
      private static final long LOCK_SIZE = 1L;
  
      /**
       * Thread -> steps stack (true = shared, false = exclusive)
       */
      private final Map<Thread, Deque<Boolean>> threadSteps;
  
      /**
       * The {@link FileChannel} this instance is about.
       */
      private final FileChannel fileChannel;
  
      /**
       * The reference of {@link FileLock}, if obtained.
       */
      private final AtomicReference<FileLock> fileLockRef;
  
      /**
       * Lock protecting "critical region": this is where threads are allowed to perform locking but should leave this
       * region as quick as possible.
       */
      private final ReentrantLock criticalRegion;
  
      public FileLockNamedLock( final String name,
                                final FileChannel fileChannel,
                                final NamedLockFactorySupport factory )
      {
          super( name, factory );
          this.threadSteps = new HashMap<>();
          this.fileChannel = fileChannel;
          this.fileLockRef = new AtomicReference<>( null );
          this.criticalRegion = new ReentrantLock();
      }
  
      @Override
      public boolean lockShared( final long time, final TimeUnit unit ) throws InterruptedException
      {
          return retry( time, unit, RETRY_SLEEP_MILLIS, this::doLockShared, null, false );
      }
  
      @Override
      public boolean lockExclusively( final long time, final TimeUnit unit ) throws InterruptedException
      {
         return retry( time, unit, RETRY_SLEEP_MILLIS, this::doLockExclusively, null, false );
     }
 
     private Boolean doLockShared()
     {
         if ( criticalRegion.tryLock() )
         {
             try
             {
                 Deque<Boolean> steps = threadSteps.computeIfAbsent( Thread.currentThread(), k -> new ArrayDeque<>() );
                 FileLock obtainedLock = fileLockRef.get();
                 if ( obtainedLock != null )
                 {
                     if ( obtainedLock.isShared() )
                     {
                         steps.push( Boolean.TRUE );
                         return true;
                     }
                     else
                     {
                         // if we own exclusive, that's still fine
                         boolean weOwnExclusive = steps.contains( Boolean.FALSE );
                         if ( weOwnExclusive )
                         {
                             steps.push( Boolean.TRUE );
                             return true;
                         }
                         else
                         {
                             // someone else owns exclusive, let's wait
                             return null;
                         }
                     }
                 }
 
                 FileLock fileLock = obtainFileLock( true );
                 if ( fileLock != null )
                 {
                     fileLockRef.set( fileLock );
                     steps.push( Boolean.TRUE );
                     return true;
                 }
             }
             finally
             {
                 criticalRegion.unlock();
             }
         }
         return null;
     }
 
     private Boolean doLockExclusively()
     {
         if ( criticalRegion.tryLock() )
         {
             try
             {
                 Deque<Boolean> steps = threadSteps.computeIfAbsent( Thread.currentThread(), k -> new ArrayDeque<>() );
                 FileLock obtainedLock = fileLockRef.get();
                 if ( obtainedLock != null )
                 {
                     if ( obtainedLock.isShared() )
                     {
                         // if we own shared, that's attempted upgrade
                         boolean weOwnShared = steps.contains( Boolean.TRUE );
                         if ( weOwnShared )
                         {
                             return false; // Lock upgrade not supported
                         }
                         else
                         {
                             // someone else owns shared, let's wait
                             return null;
                         }
                     }
                     else
                     {
                         // if we own exclusive, that's fine
                         boolean weOwnExclusive = steps.contains( Boolean.FALSE );
                         if ( weOwnExclusive )
                         {
                             steps.push( Boolean.FALSE );
                             return true;
                         }
                         else
                         {
                             // someone else owns exclusive, let's wait
                             return null;
                         }
                     }
                 }
 
                 FileLock fileLock = obtainFileLock( false );
                 if ( fileLock != null )
                 {
                     fileLockRef.set( fileLock );
                     steps.push( Boolean.FALSE );
                     return true;
                 }
             }
             finally
             {
                 criticalRegion.unlock();
             }
         }
         return null;
     }
 
     @Override
     public void unlock()
     {
         criticalRegion.lock();
         try
         {
             Deque<Boolean> steps = threadSteps.computeIfAbsent( Thread.currentThread(), k -> new ArrayDeque<>() );
             if ( steps.isEmpty() )
             {
                 throw new IllegalStateException( "Wrong API usage: unlock without lock" );
             }
             steps.pop();
             if ( steps.isEmpty() && !anyOtherThreadHasSteps() )
             {
                 try
                 {
                     fileLockRef.getAndSet( null ).release();
                 }
                 catch ( IOException e )
                 {
                     throw new UncheckedIOException( e );
                 }
             }
         }
         finally
         {
             criticalRegion.unlock();
         }
     }
 
     /**
      * Returns {@code true} if any other than this thread using this instance has any step recorded.
      */
     private boolean anyOtherThreadHasSteps()
     {
         return threadSteps.entrySet().stream()
                 .filter( e -> !Thread.currentThread().equals( e.getKey() ) )
                 .map( Map.Entry::getValue )
                 .anyMatch( d -> !d.isEmpty() );
     }
 
     /**
      * Attempts to obtain real {@link FileLock}, returns non-null value is succeeds, or {@code null} if cannot.
      */
     private FileLock obtainFileLock( final boolean shared )
     {
         FileLock result;
         try
         {
             result = fileChannel.tryLock( LOCK_POSITION, LOCK_SIZE, shared );
         }
         catch ( OverlappingFileLockException e )
         {
             logger.trace( "File lock overlap on '{}'", name(), e );
             return null;
         }
         catch ( IOException e )
         {
             logger.trace( "Failure on acquire of file lock for '{}'", name(), e );
             throw new UncheckedIOException( "Failed to acquire lock file channel for '" + name() + "'", e );
         }
         return result;
     }
 }