Add generic socket fallback for externally passed socket FileDescriptors

When passing socket file descriptors from other processes, provide a
fallback to allow communicating with otherwise unsupported socket types.

This allows using FileDescriptorCast to Socket. Previously, only
ByteChannel/InputStream/OutputStream was supported.

Note that the socket addresses returned are not guaranteed to be
compatible across operating systems.

#151

junixsocket-common/src/main/java/org/newsclub/net/unix/AFGenericDatagramChannel.java

@@ -0,0 +1,32 @@
+/*
+ * junixsocket
+ *
+ * Copyright 2009-2023 Christian Kohlschütter
+ *
+ * Licensed 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 org.newsclub.net.unix;
+
+import java.nio.channels.DatagramChannel;
+
+/**
+ * A {@link DatagramChannel} implementation that works with {@code AF_SYSTEM} sockets.
+ *
+ * @author Christian Kohlschütter
+ */
+final class AFGenericDatagramChannel extends AFDatagramChannel<AFGenericSocketAddress> implements
+    AFGenericSocketExtensions {
+  AFGenericDatagramChannel(AFGenericDatagramSocket socket) {
+    super(AFGenericSelectorProvider.getInstance(), socket);
+  }
+}

junixsocket-common/src/main/java/org/newsclub/net/unix/AFGenericDatagramSocket.java

@@ -0,0 +1,84 @@
+/*
+ * junixsocket
+ *
+ * Copyright 2009-2023 Christian Kohlschütter
+ *
+ * Licensed 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 org.newsclub.net.unix;
+
+import java.io.FileDescriptor;
+import java.io.IOException;
+import java.net.DatagramSocket;
+
+/**
+ * A {@link DatagramSocket} implementation that works with {@code AF_SYSTEM} sockets.
+ *
+ * @author Christian Kohlschütter
+ */
+final class AFGenericDatagramSocket extends AFDatagramSocket<AFGenericSocketAddress> implements
+    AFGenericSocketExtensions {
+
+  AFGenericDatagramSocket(FileDescriptor fd) throws IOException {
+    this(fd, AFSocketType.SOCK_DGRAM);
+  }
+
+  AFGenericDatagramSocket(FileDescriptor fd, AFSocketType socketType) throws IOException {
+    super(new AFGenericDatagramSocketImpl(fd, socketType));
+  }
+
+  @Override
+  protected AFGenericDatagramChannel newChannel() {
+    return new AFGenericDatagramChannel(this);
+  }
+
+  /**
+   * Returns a new {@link AFGenericDatagramSocket} instance, using the default
+   * {@link AFSocketType#SOCK_DGRAM} socket type.
+   *
+   * @return The new instance.
+   * @throws IOException on error.
+   */
+  public static AFGenericDatagramSocket newInstance() throws IOException {
+    return (AFGenericDatagramSocket) newInstance(AFGenericDatagramSocket::new);
+  }
+
+  /**
+   * Returns a new {@link AFGenericDatagramSocket} instance for the given socket type.
+   *
+   * @param socketType The socket type.
+   * @return The new instance.
+   * @throws IOException on error.
+   */
+  public static AFGenericDatagramSocket newInstance(AFSocketType socketType) throws IOException {
+    return (AFGenericDatagramSocket) newInstance((fd) -> {
+      return new AFGenericDatagramSocket(fd, socketType);
+    });
+  }
+
+  @Override
+  public AFGenericDatagramChannel getChannel() {
+    return (AFGenericDatagramChannel) super.getChannel();
+  }
+
+  @Override
+  protected AFGenericSocketImplExtensions getImplExtensions() {
+    return (AFGenericSocketImplExtensions) super.getImplExtensions();
+  }
+
+  @Override
+  protected AFDatagramSocket<AFGenericSocketAddress> newDatagramSocketInstance()
+      throws IOException {
+    return new AFGenericDatagramSocket(null);
+  }
+}

junixsocket-common/src/main/java/org/newsclub/net/unix/AFGenericDatagramSocketImpl.java

@@ -0,0 +1,27 @@
+/*
+ * junixsocket
+ *
+ * Copyright 2009-2023 Christian Kohlschütter
+ *
+ * Licensed 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 org.newsclub.net.unix;
+
+import java.io.FileDescriptor;
+import java.io.IOException;
+
+final class AFGenericDatagramSocketImpl extends AFDatagramSocketImpl<AFGenericSocketAddress> {
+  AFGenericDatagramSocketImpl(FileDescriptor fd, AFSocketType socketType) throws IOException {
+    super(AFGenericSelectorProvider.AF_GENERIC, fd, socketType);
+  }
+}

junixsocket-common/src/main/java/org/newsclub/net/unix/AFGenericProtocolFamily.java

@@ -0,0 +1,32 @@
+/*
+ * junixsocket
+ *
+ * Copyright 2009-2023 Christian Kohlschütter
+ *
+ * Licensed 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 org.newsclub.net.unix;
+
+import java.net.ProtocolFamily;
+
+/**
+ * Describes the protocol families supported by junixsocket-afsystem.
+ *
+ * @author Christian Kohlschütter
+ */
+enum AFGenericProtocolFamily implements ProtocolFamily {
+  /**
+   * generic sockets.
+   */
+  GENERIC;
+}

junixsocket-common/src/main/java/org/newsclub/net/unix/AFGenericSelectorProvider.java

@@ -0,0 +1,189 @@
+/*
+ * junixsocket
+ *
+ * Copyright 2009-2023 Christian Kohlschütter
+ *
+ * Licensed 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 org.newsclub.net.unix;
+
+import java.io.IOException;
+import java.net.ProtocolFamily;
+import java.net.SocketAddress;
+
+import org.eclipse.jdt.annotation.NonNull;
+
+import com.kohlschutter.annotations.compiletime.SuppressFBWarnings;
+
+/**
+ * Service-provider class for junixsocket selectors and selectable channels.
+ */
+final class AFGenericSelectorProvider extends AFSelectorProvider<AFGenericSocketAddress> {
+  private static final AFGenericSelectorProvider INSTANCE = new AFGenericSelectorProvider();
+
+  @SuppressWarnings("null")
+  static final AFAddressFamily<@NonNull AFGenericSocketAddress> AF_GENERIC = AFAddressFamily
+      .registerAddressFamilyImpl("generic", AFGenericSocketAddress.addressFamily(),
+          new AFAddressFamilyConfig<AFGenericSocketAddress>() {
+
+            @Override
+            protected Class<? extends AFSocket<AFGenericSocketAddress>> socketClass() {
+              return AFGenericSocket.class;
+            }
+
+            @Override
+            protected AFSocket.Constructor<AFGenericSocketAddress> socketConstructor() {
+              return AFGenericSocket::new;
+            }
+
+            @Override
+            protected Class<? extends AFServerSocket<AFGenericSocketAddress>> serverSocketClass() {
+              return AFGenericServerSocket.class;
+            }
+
+            @Override
+            protected AFServerSocket.Constructor<AFGenericSocketAddress> serverSocketConstructor() {
+              return AFGenericServerSocket::new;
+            }
+
+            @Override
+            protected Class<? extends AFSocketChannel<AFGenericSocketAddress>> socketChannelClass() {
+              return AFGenericSocketChannel.class;
+            }
+
+            @Override
+            protected Class<? extends AFServerSocketChannel<AFGenericSocketAddress>> serverSocketChannelClass() {
+              return AFGenericServerSocketChannel.class;
+            }
+
+            @Override
+            protected Class<? extends AFDatagramSocket<AFGenericSocketAddress>> datagramSocketClass() {
+              return AFGenericDatagramSocket.class;
+            }
+
+            @Override
+            protected AFDatagramSocket.Constructor<AFGenericSocketAddress> datagramSocketConstructor() {
+              return AFGenericDatagramSocket::new;
+            }
+
+            @Override
+            protected Class<? extends AFDatagramChannel<AFGenericSocketAddress>> datagramChannelClass() {
+              return AFGenericDatagramChannel.class;
+            }
+          });
+
+  private AFGenericSelectorProvider() {
+    super();
+  }
+
+  /**
+   * Returns the singleton instance.
+   *
+   * @return The instance.
+   */
+  @SuppressFBWarnings("MS_EXPOSE_REP")
+  public static AFGenericSelectorProvider getInstance() {
+    return INSTANCE;
+  }
+
+  /**
+   * Returns the singleton instance.
+   *
+   * @return The instance.
+   */
+  public static AFGenericSelectorProvider provider() {
+    return getInstance();
+  }
+
+  /**
+   * Constructs a new socket pair from two sockets.
+   *
+   * @param s1 Some socket, the first one.
+   * @param s2 Some socket, the second one.
+   * @return The pair.
+   */
+  @Override
+  protected <P extends AFSomeSocket> AFSocketPair<P> newSocketPair(P s1, P s2) {
+    return new AFGenericSocketPair<>(s1, s2);
+  }
+
+  @SuppressWarnings("unchecked")
+  @Override
+  public AFGenericSocketPair<AFGenericSocketChannel> openSocketChannelPair() throws IOException {
+    return (AFGenericSocketPair<AFGenericSocketChannel>) super.openSocketChannelPair();
+  }
+
+  @SuppressWarnings("unchecked")
+  @Override
+  public AFGenericSocketPair<AFGenericDatagramChannel> openDatagramChannelPair()
+      throws IOException {
+    return (AFGenericSocketPair<AFGenericDatagramChannel>) super.openDatagramChannelPair();
+  }
+
+  @SuppressWarnings("unchecked")
+  @Override
+  public AFGenericSocketPair<AFGenericDatagramChannel> openDatagramChannelPair(AFSocketType type)
+      throws IOException {
+    return (AFGenericSocketPair<AFGenericDatagramChannel>) super.openDatagramChannelPair(type);
+  }
+
+  @Override
+  protected AFGenericSocket newSocket() throws IOException {
+    return AFGenericSocket.newInstance();
+  }
+
+  @Override
+  public AFGenericDatagramChannel openDatagramChannel() throws IOException {
+    return AFGenericDatagramSocket.newInstance().getChannel();
+  }
+
+  @Override
+  public AFGenericDatagramChannel openDatagramChannel(AFSocketType type) throws IOException {
+    return AFGenericDatagramSocket.newInstance(type).getChannel();
+  }
+
+  @Override
+  public AFGenericDatagramChannel openDatagramChannel(ProtocolFamily family) throws IOException {
+    return (AFGenericDatagramChannel) super.openDatagramChannel(family);
+  }
+
+  @Override
+  public AFGenericServerSocketChannel openServerSocketChannel() throws IOException {
+    return AFGenericServerSocket.newInstance().getChannel();
+  }
+
+  @Override
+  public AFGenericServerSocketChannel openServerSocketChannel(SocketAddress sa) throws IOException {
+    return AFGenericServerSocket.bindOn(AFGenericSocketAddress.unwrap(sa)).getChannel();
+  }
+
+  @Override
+  public AFGenericSocketChannel openSocketChannel() throws IOException {
+    return (AFGenericSocketChannel) super.openSocketChannel();
+  }
+
+  @Override
+  public AFGenericSocketChannel openSocketChannel(SocketAddress sa) throws IOException {
+    return AFGenericSocket.connectTo(AFGenericSocketAddress.unwrap(sa)).getChannel();
+  }
+
+  @Override
+  protected ProtocolFamily protocolFamily() {
+    return AFGenericProtocolFamily.GENERIC;
+  }
+
+  @Override
+  protected AFAddressFamily<@NonNull AFGenericSocketAddress> addressFamily() {
+    return AF_GENERIC;
+  }
+}