[#3848] Respect EPOLLERR event

Motivation:

Some glibc/kernel versions will trigger an EPOLLERR event to notify
about failed connect and not an EPOLLOUT. Also EPOLLERR may be triggered
when a connection is broke.

Modification:

React on EPOLLERR like if an EPOLLOUT / EPOLLIN was received, this will work in
all cases as we handle errors in EPOLLOUT / EPOLLIN anyway.

Result:

Correctly detect errors.

transport-native-epoll/src/main/c/io_netty_channel_epoll_Native.c

@@ -1615,6 +1615,10 @@ JNIEXPORT jint JNICALL Java_io_netty_channel_epoll_Native_epollrdhup(JNIEnv* env
     return EPOLLRDHUP;
 }
 
+JNIEXPORT jint JNICALL Java_io_netty_channel_epoll_Native_epollerr(JNIEnv* env, jclass clazz) {
+    return EPOLLERR;
+}
+
 JNIEXPORT jint JNICALL Java_io_netty_channel_epoll_Native_sizeofEpollEvent(JNIEnv* env, jclass clazz) {
     return sizeof(struct epoll_event);
 }

transport-native-epoll/src/main/c/io_netty_channel_epoll_Native.h

@@ -121,6 +121,7 @@ jint Java_io_netty_channel_epoll_Native_epollin(JNIEnv* env, jclass clazz);
 jint Java_io_netty_channel_epoll_Native_epollout(JNIEnv* env, jclass clazz);
 jint Java_io_netty_channel_epoll_Native_epollrdhup(JNIEnv* env, jclass clazz);
 jint Java_io_netty_channel_epoll_Native_epollet(JNIEnv* env, jclass clazz);
+jint Java_io_netty_channel_epoll_Native_epollerr(JNIEnv* env, jclass clazz);
 jint Java_io_netty_channel_epoll_Native_sizeofEpollEvent(JNIEnv* env, jclass clazz);
 jint Java_io_netty_channel_epoll_Native_offsetofEpollData(JNIEnv* env, jclass clazz);
 

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

@@ -317,10 +317,18 @@ final class EpollEventLoop extends SingleThreadEventLoop {
                     // past.
                     AbstractEpollUnsafe unsafe = (AbstractEpollUnsafe) ch.unsafe();
 
+                    // Check if an error was the cause of the wakeup.
+                    boolean err = (ev & Native.EPOLLERR) != 0;
+
                     // First check for EPOLLOUT as we may need to fail the connect ChannelPromise before try
                     // to read from the file descriptor.
                     // See https://github.com/netty/netty/issues/3785
-                    if ((ev & Native.EPOLLOUT) != 0 && ch.isOpen()) {
+                    //
+                    // It is possible for an EPOLLOUT or EPOLLERR to be generated when a connection is refused.
+                    // In either case epollOutReady() will do the correct thing (finish connecting, or fail
+                    // the connection).
+                    // See https://github.com/netty/netty/issues/3848
+                    if (err || ((ev & Native.EPOLLOUT) != 0) && ch.isOpen()) {
                         // Force flush of data as the epoll is writable again
                         unsafe.epollOutReady();
                     }
@@ -331,7 +339,11 @@ final class EpollEventLoop extends SingleThreadEventLoop {
                     if ((ev & Native.EPOLLRDHUP) != 0) {
                         unsafe.epollRdHupReady();
                     }
-                    if ((ev & Native.EPOLLIN) != 0 && ch.isOpen()) {
+
+                    // If EPOLLOUT or EPOLLING was received and the channel is still open call epollInReady().
+                    // This will try to read from the underlying filedescriptor and so notify the user about the
+                    // error.
+                    if ((err || (ev & Native.EPOLLIN) != 0) && ch.isOpen()) {
                         // The Channel is still open and there is something to read. Do it now.
                         unsafe.epollInReady();
                     }

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

@@ -55,6 +55,7 @@ public final class Native {
     public static final int EPOLLOUT = epollout();
     public static final int EPOLLRDHUP = epollrdhup();
     public static final int EPOLLET = epollet();
+    public static final int EPOLLERR = epollerr();
 
     public static final int IOV_MAX = iovMax();
     public static final int UIO_MAX_IOV = uioMaxIov();
@@ -701,6 +702,7 @@ public final class Native {
     private static native int epollout();
     private static native int epollrdhup();
     private static native int epollet();
+    private static native int epollerr();
 
     private static native long ssizeMax();
     private Native() {