Set readPending to false when ever a read is done

Motivation:
readPending is currently only set to false if data is delivered to the application, however this may result in duplicate events being received from the selector in the event that the socket was closed.

Modifications:
- We should set readPending to false before each read attempt for all
transports besides NIO.
- Based upon the Javadocs it is possible that NIO may have spurious
wakeups [1]. In this case we should be more cautious and only set
readPending to false if data was actually read.

[1] https://docs.oracle.com/javase/7/docs/api/java/nio/channels/SelectionKey.html
That a selection key's ready set indicates that its channel is ready for some operation category is a hint, but not a guarantee, that an operation in such a category may be performed by a thread without causing the thread to block.

Result:
Notification from the selector (or simulated events from kqueue/epoll ET) in the event of socket closure.
Fixes netty#7255

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

@@ -727,7 +727,6 @@ private void handleReadException(ChannelPipeline pipeline, ByteBuf byteBuf, Thro
                 EpollRecvByteAllocatorHandle allocHandle) {
             if (byteBuf != null) {
                 if (byteBuf.isReadable()) {
-                    readPending = false;
                     pipeline.fireChannelRead(byteBuf);
                 } else {
                     byteBuf.release();
@@ -784,6 +783,7 @@ void epollInReady() {
                     // we use a direct buffer here as the native implementations only be able
                     // to handle direct buffers.
                     byteBuf = allocHandle.allocate(allocator);
+                    readPending = false;
                     allocHandle.lastBytesRead(doReadBytes(byteBuf));
                     if (allocHandle.lastBytesRead() <= 0) {
                         // nothing was read, release the buffer.
@@ -793,7 +793,6 @@ void epollInReady() {
                         break;
                     }
                     allocHandle.incMessagesRead(1);
-                    readPending = false;
                     pipeline.fireChannelRead(byteBuf);
                     byteBuf = null;
 

transport-native-kqueue/src/main/java/io/netty/channel/kqueue/AbstractKQueueStreamChannel.java

@@ -526,6 +526,7 @@ void readReady(final KQueueRecvByteAllocatorHandle allocHandle) {
                     // we use a direct buffer here as the native implementations only be able
                     // to handle direct buffers.
                     byteBuf = allocHandle.allocate(allocator);
+                    readPending = false;
                     allocHandle.lastBytesRead(doReadBytes(byteBuf));
                     if (allocHandle.lastBytesRead() <= 0) {
                         // nothing was read, release the buffer.
@@ -535,7 +536,6 @@ void readReady(final KQueueRecvByteAllocatorHandle allocHandle) {
                         break;
                     }
                     allocHandle.incMessagesRead(1);
-                    readPending = false;
                     pipeline.fireChannelRead(byteBuf);
                     byteBuf = null;
 
@@ -572,7 +572,6 @@ private void handleReadException(ChannelPipeline pipeline, ByteBuf byteBuf, Thro
                                          KQueueRecvByteAllocatorHandle allocHandle) {
             if (byteBuf != null) {
                 if (byteBuf.isReadable()) {
-                    readPending = false;
                     pipeline.fireChannelRead(byteBuf);
                 } else {
                     byteBuf.release();

transport/src/main/java/io/netty/channel/nio/AbstractNioByteChannel.java

@@ -126,6 +126,16 @@ public final void read() {
                         byteBuf.release();
                         byteBuf = null;
                         close = allocHandle.lastBytesRead() < 0;
+                        if (close) {
+                            // Based upon the Javadocs it is possible that NIO may have spurious wake ups [1]. In this
+                            // case we should be more cautious and only set readPending to false if data was actually
+                            // read.
+                            // [1] https://docs.oracle.com/javase/7/docs/api/java/nio/channels/SelectionKey.html
+                            // That a selection key's ready set indicates that its channel is ready for some operation
+                            // category is a hint, but not a guarantee, that an operation in such a category may be
+                            // performed by a thread without causing the thread to block.
+                            readPending = false;
+                        }
                         break;
                     }
 

transport/src/main/java/io/netty/channel/oio/AbstractOioByteChannel.java

@@ -80,7 +80,6 @@ private void handleReadException(ChannelPipeline pipeline, ByteBuf byteBuf, Thro
             RecvByteBufAllocator.Handle allocHandle) {
         if (byteBuf != null) {
             if (byteBuf.isReadable()) {
-                readPending = false;
                 pipeline.fireChannelRead(byteBuf);
             } else {
                 byteBuf.release();
@@ -102,9 +101,6 @@ protected void doRead() {
             // during the same read loop readPending was set to false.
             return;
         }
-        // In OIO we should set readPending to false even if the read was not successful so we can schedule
-        // another read on the event loop if no reads are done.
-        readPending = false;
 
         final ChannelPipeline pipeline = pipeline();
         final ByteBufAllocator allocator = config.getAllocator();
@@ -117,6 +113,7 @@ protected void doRead() {
         try {
             byteBuf = allocHandle.allocate(allocator);
             do {
+                readPending = false;
                 allocHandle.lastBytesRead(doReadBytes(byteBuf));
                 if (allocHandle.lastBytesRead() <= 0) {
                     if (!byteBuf.isReadable()) { // nothing was read. release the buffer.
@@ -140,7 +137,6 @@ protected void doRead() {
                     final int maxCapacity = byteBuf.maxCapacity();
                     if (capacity == maxCapacity) {
                         allocHandle.incMessagesRead(1);
-                        readPending = false;
                         pipeline.fireChannelRead(byteBuf);
                         byteBuf = allocHandle.allocate(allocator);
                     } else {
@@ -158,7 +154,6 @@ protected void doRead() {
                 // It is possible we allocated a buffer because the previous one was not writable, but then didn't use
                 // it because allocHandle.continueReading() returned false.
                 if (byteBuf.isReadable()) {
-                    readPending = false;
                     pipeline.fireChannelRead(byteBuf);
                 } else {
                     byteBuf.release();