[#3457] Proper fix for IllegalStateException caused by closed file descriptor / channel

Motivation:

During 6b941e9bdb I introduced a regression that could cause an IllegalStateException.
A non-proper fix was commited as part of #3443. This commit add a proper fix.

Modifications:

Remove FileDescriptor.INVALID and add FileDescriptor.isOpen() as replacement. Once FileDescriptor.close() is called isOpen() will return false.

Result:

No more IllegalStateException caused by a close channel.

transport-native-epoll/src/main/java/io/netty/channel/epoll/AbstractEpollChannel.java

@@ -35,7 +35,7 @@ import java.nio.channels.UnresolvedAddressException;
 abstract class AbstractEpollChannel extends AbstractChannel implements UnixChannel {
     private static final ChannelMetadata DATA = new ChannelMetadata(false);
     private final int readFlag;
-    private volatile FileDescriptor fileDescriptor;
+    private final FileDescriptor fileDescriptor;
     protected int flags = Native.EPOLLET;
 
     protected volatile boolean active;
@@ -103,8 +103,7 @@ abstract class AbstractEpollChannel extends AbstractChannel implements UnixChann
         doDeregister();
 
         FileDescriptor fd = fileDescriptor;
-        fileDescriptor = FileDescriptor.INVALID;
-        Native.close(fd.intValue());
+        fd.close();
     }
 
     @Override
@@ -119,7 +118,7 @@ abstract class AbstractEpollChannel extends AbstractChannel implements UnixChann
 
     @Override
     public boolean isOpen() {
-        return fileDescriptor != FileDescriptor.INVALID;
+        return fileDescriptor.isOpen();
     }
 
     @Override

transport-native-epoll/src/main/java/io/netty/channel/epoll/EpollDatagramChannel.java

@@ -97,7 +97,7 @@ public final class EpollDatagramChannel extends AbstractEpollChannel implements
     @Override
     @SuppressWarnings("deprecation")
     public boolean isActive() {
-        return fd() != FileDescriptor.INVALID &&
+        return fd().isOpen() &&
                 (config.getOption(ChannelOption.DATAGRAM_CHANNEL_ACTIVE_ON_REGISTRATION) && isRegistered()
                         || active);
     }

transport-native-epoll/src/main/java/io/netty/channel/unix/FileDescriptor.java

@@ -24,6 +24,7 @@ import java.io.IOException;
 public class FileDescriptor {
 
     private final int fd;
+    private volatile boolean open = true;
 
     public FileDescriptor(int fd) {
         if (fd < 0) {
@@ -32,21 +33,6 @@ public class FileDescriptor {
         this.fd = fd;
     }
 
-    /**
-     * An invalid file descriptor which was closed before.
-     */
-    public static final FileDescriptor INVALID = new FileDescriptor(0) {
-        @Override
-        public int intValue() {
-            throw new IllegalStateException("invalid file descriptor");
-        }
-
-        @Override
-        public void close() {
-            // NOOP
-        }
-    };
-
     /**
      * Return the int value of the filedescriptor.
      */
@@ -58,9 +44,17 @@ public class FileDescriptor {
      * Close the file descriptor.
      */
     public void close() throws IOException {
+        open = false;
         close(fd);
     }
 
+    /**
+     * Returns {@code true} if the file descriptor is open.
+     */
+    public boolean isOpen() {
+        return open;
+    }
+
     @Override
     public String toString() {
         return "FileDescriptor{" +

transport-native-epoll/src/test/java/io/netty/channel/epoll/EpollDomainSocketFdTest.java

@@ -93,8 +93,10 @@ public class EpollDomainSocketFdTest extends AbstractSocketTest {
         sc.close().sync();
 
         if (received instanceof FileDescriptor) {
-            Assert.assertNotSame(FileDescriptor.INVALID, received);
-            ((FileDescriptor) received).close();
+            FileDescriptor fd = (FileDescriptor) received;
+            Assert.assertTrue(fd.isOpen());
+            fd.close();
+            Assert.assertFalse(fd.isOpen());
             Assert.assertNull(queue.poll());
         } else {
             throw (Throwable) received;