/*
    * Copyright 2017 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.netty5.resolver.dns;
  
  import io.netty5.util.internal.PlatformDependent;
  import io.netty5.util.internal.logging.InternalLogger;
  import io.netty5.util.internal.logging.InternalLoggerFactory;
  
  import java.lang.invoke.MethodHandle;
  import java.lang.invoke.MethodHandles;
  import java.lang.invoke.MethodType;
  import java.lang.reflect.InvocationTargetException;
  import java.security.AccessController;
  import java.security.PrivilegedAction;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicLong;
  
  /**
   * Utility methods related to {@link DnsServerAddressStreamProvider}.
   */
  public final class DnsServerAddressStreamProviders {
  
      private static final InternalLogger LOGGER =
              InternalLoggerFactory.getInstance(DnsServerAddressStreamProviders.class);
      private static final MethodHandle STREAM_PROVIDER_CONSTRUCTOR_HANDLE;
      private static final String MACOS_PROVIDER_CLASS_NAME =
              "io.netty5.resolver.dns.macos.MacOSDnsServerAddressStreamProvider";
  
      static {
          MethodHandle constructorHandle = null;
          if (PlatformDependent.isOsx()) {
              try {
                  // As MacOSDnsServerAddressStreamProvider is contained in another jar which depends on this jar
                  // we use reflection to use it if its on the classpath.
                  Object maybeProvider = AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
                      try {
                          return Class.forName(
                                  MACOS_PROVIDER_CLASS_NAME,
                                  true,
                                  DnsServerAddressStreamProviders.class.getClassLoader());
                      } catch (Throwable cause) {
                          return cause;
                      }
                  });
                  if (maybeProvider instanceof Class) {
                      @SuppressWarnings("unchecked")
                      Class<? extends DnsServerAddressStreamProvider> providerClass =
                              (Class<? extends DnsServerAddressStreamProvider>) maybeProvider;
                      MethodHandles.Lookup lookup = MethodHandles.lookup();
                      constructorHandle = lookup.findConstructor(providerClass, MethodType.methodType(void.class));
                      constructorHandle.invoke(); // ctor ensures availability
                      LOGGER.debug("{}: available", MACOS_PROVIDER_CLASS_NAME);
                  } else {
                      throw (Throwable) maybeProvider;
                  }
              } catch (ClassNotFoundException cause) {
                  LOGGER.warn("Can not find {} in the classpath, fallback to system defaults. This may result in "
                          + "incorrect DNS resolutions on MacOS.", MACOS_PROVIDER_CLASS_NAME);
              } catch (Throwable cause) {
                  LOGGER.error("Unable to load {}, fallback to system defaults. This may result in "
                          + "incorrect DNS resolutions on MacOS.", MACOS_PROVIDER_CLASS_NAME, cause);
                  constructorHandle = null;
              }
          }
          STREAM_PROVIDER_CONSTRUCTOR_HANDLE = constructorHandle;
      }
  
      private DnsServerAddressStreamProviders() {
      }
  
      /**
       * A {@link DnsServerAddressStreamProvider} which inherits the DNS servers from your local host's configuration.
       * <p>
       * Note that only macOS and Linux are currently supported.
       * @return A {@link DnsServerAddressStreamProvider} which inherits the DNS servers from your local host's
       * configuration.
       */
      public static DnsServerAddressStreamProvider platformDefault() {
          if (STREAM_PROVIDER_CONSTRUCTOR_HANDLE != null) {
              try {
                  return (DnsServerAddressStreamProvider) STREAM_PROVIDER_CONSTRUCTOR_HANDLE.invoke();
              } catch (IllegalAccessException | InstantiationException | InvocationTargetException e) {
                  // ignore
              } catch (Throwable cause) {
                  PlatformDependent.throwException(cause);
              }
         }
         return unixDefault();
     }
 
     public static DnsServerAddressStreamProvider unixDefault() {
         return DefaultProviderHolder.DEFAULT_DNS_SERVER_ADDRESS_STREAM_PROVIDER;
     }
 
     // We use a Holder class to only initialize DEFAULT_DNS_SERVER_ADDRESS_STREAM_PROVIDER if we really
     // need it.
     private static final class DefaultProviderHolder {
         // We use 5 minutes which is the same as what OpenJDK is using in sun.net.dns.ResolverConfigurationImpl.
         private static final long REFRESH_INTERVAL = TimeUnit.MINUTES.toNanos(5);
 
         // TODO(scott): how is this done on Windows? This may require a JNI call to GetNetworkParams
         // https://msdn.microsoft.com/en-us/library/aa365968(VS.85).aspx.
         static final DnsServerAddressStreamProvider DEFAULT_DNS_SERVER_ADDRESS_STREAM_PROVIDER =
                 new DnsServerAddressStreamProvider() {
                     private volatile DnsServerAddressStreamProvider currentProvider = provider();
                     private final AtomicLong lastRefresh = new AtomicLong(System.nanoTime());
 
                     @Override
                     public DnsServerAddressStream nameServerAddressStream(String hostname) {
                         long last = lastRefresh.get();
                         DnsServerAddressStreamProvider current = currentProvider;
                         if (System.nanoTime() - last > REFRESH_INTERVAL) {
                             // This is slightly racy which means it will be possible still use the old configuration
                             // for a small amount of time, but that's ok.
                             if (lastRefresh.compareAndSet(last, System.nanoTime())) {
                                 current = currentProvider = provider();
                             }
                         }
                         return current.nameServerAddressStream(hostname);
                     }
 
                     private DnsServerAddressStreamProvider provider() {
                         // If on windows just use the DefaultDnsServerAddressStreamProvider.INSTANCE as otherwise
                         // we will log some error which may be confusing.
                         return PlatformDependent.isWindows() ? DefaultDnsServerAddressStreamProvider.INSTANCE :
                                 UnixResolverDnsServerAddressStreamProvider.parseSilently();
                     }
                 };
     }
 }