[#5028] Fix re-entrance issue with channelWritabilityChanged(...) and write(...)

Motivation:

When always triggered fireChannelWritabilityChanged() directly when the update the pending bytes in the ChannelOutboundBuffer was made from within the EventLoop. This is problematic as this can cause some re-entrance issue if the user has a custom ChannelOutboundHandler that does multiple writes from within the write(...) method and also has a handler that will intercept the channelWritabilityChanged event and trigger another write when the Channel is writable. This can also easily happen if the user just use a MessageToMessageEncoder subclass and triggers a write from channelWritabilityChanged().

Modification:

- Always invoke the fireChannelWritabilityChanged() later on the EventLoop.
- Only trigger the fireChannelWritabilityChanged() if the channel is still active.
- when write(...) is called from outside the EventLoop we only increase the pending bytes in the outbound buffer (so that Channel.isWritable() is updated directly) but not cause a fireChannelWritabilityChanged(). The fireChannelWritabilityChanged() is then triggered once the task is picked up by the EventLoop as usual.

Result:

No more re-entrance possible because of writes from within channelWritabilityChanged(...) method.

transport/src/main/java/io/netty/channel/ChannelOutboundBuffer.java

@@ -96,7 +96,7 @@ public final class ChannelOutboundBuffer {
     @SuppressWarnings("UnusedDeclaration")
     private volatile int unwritable;
 
-    private volatile Runnable fireChannelWritabilityChangedTask;
+    private final Runnable fireChannelWritabilityChangedTask;
 
     static {
         AtomicIntegerFieldUpdater<ChannelOutboundBuffer> unwritableUpdater =
@@ -116,6 +116,7 @@ public final class ChannelOutboundBuffer {
 
     ChannelOutboundBuffer(AbstractChannel channel) {
         this.channel = channel;
+        fireChannelWritabilityChangedTask = new ChannelWritabilityChangedTask(channel);
     }
 
     /**
@@ -138,7 +139,7 @@ public final class ChannelOutboundBuffer {
 
         // increment pending bytes after adding message to the unflushed arrays.
         // See https://github.com/netty/netty/issues/1619
-        incrementPendingOutboundBytes(entry.pendingSize, false);
+        incrementPendingOutboundBytes(entry.pendingSize);
     }
 
     /**
@@ -161,7 +162,7 @@ public final class ChannelOutboundBuffer {
                 if (!entry.promise.setUncancellable()) {
                     // Was cancelled so make sure we free up memory and notify about the freed bytes
                     int pending = entry.cancel();
-                    decrementPendingOutboundBytes(pending, false, true);
+                    decrementPendingOutboundBytes(pending);
                 }
                 entry = entry.next;
             } while (entry != null);
@@ -176,17 +177,13 @@ public final class ChannelOutboundBuffer {
      * This method is thread-safe!
      */
     void incrementPendingOutboundBytes(long size) {
-        incrementPendingOutboundBytes(size, true);
-    }
-
-    private void incrementPendingOutboundBytes(long size, boolean invokeLater) {
         if (size == 0) {
             return;
         }
 
         long newWriteBufferSize = TOTAL_PENDING_SIZE_UPDATER.addAndGet(this, size);
-        if (newWriteBufferSize > channel.config().getWriteBufferHighWaterMark()) {
+        if (newWriteBufferSize >= channel.config().getWriteBufferHighWaterMark()) {
-            setUnwritable(invokeLater);
+            setUnwritable();
         }
     }
 
@@ -195,17 +192,17 @@ public final class ChannelOutboundBuffer {
      * This method is thread-safe!
      */
     void decrementPendingOutboundBytes(long size) {
-        decrementPendingOutboundBytes(size, true, true);
+        decrementPendingOutboundBytes(size, true);
     }
 
-    private void decrementPendingOutboundBytes(long size, boolean invokeLater, boolean notifyWritability) {
+    private void decrementPendingOutboundBytes(long size, boolean notifyWritability) {
         if (size == 0) {
             return;
         }
 
         long newWriteBufferSize = TOTAL_PENDING_SIZE_UPDATER.addAndGet(this, -size);
         if (notifyWritability && newWriteBufferSize < channel.config().getWriteBufferLowWaterMark()) {
-            setWritable(invokeLater);
+            setWritable(notifyWritability);
         }
     }
 
@@ -270,7 +267,7 @@ public final class ChannelOutboundBuffer {
             // only release message, notify and decrement if it was not canceled before.
             ReferenceCountUtil.safeRelease(msg);
             safeSuccess(promise);
-            decrementPendingOutboundBytes(size, false, true);
+            decrementPendingOutboundBytes(size);
         }
 
         // recycle the entry
@@ -306,7 +303,7 @@ public final class ChannelOutboundBuffer {
             ReferenceCountUtil.safeRelease(msg);
 
             safeFail(promise, cause);
-            decrementPendingOutboundBytes(size, false, notifyWritability);
+            decrementPendingOutboundBytes(size, notifyWritability);
         }
 
         // recycle the entry
@@ -528,7 +525,7 @@ public final class ChannelOutboundBuffer {
             final int newValue = oldValue & mask;
             if (UNWRITABLE_UPDATER.compareAndSet(this, oldValue, newValue)) {
                 if (oldValue != 0 && newValue == 0) {
-                    fireChannelWritabilityChanged(true);
+                    fireChannelWritabilityChanged();
                 }
                 break;
             }
@@ -542,7 +539,7 @@ public final class ChannelOutboundBuffer {
             final int newValue = oldValue | mask;
             if (UNWRITABLE_UPDATER.compareAndSet(this, oldValue, newValue)) {
                 if (oldValue == 0 && newValue != 0) {
-                    fireChannelWritabilityChanged(true);
+                    fireChannelWritabilityChanged();
                 }
                 break;
             }
@@ -556,48 +553,36 @@ public final class ChannelOutboundBuffer {
         return 1 << index;
     }
 
-    private void setWritable(boolean invokeLater) {
+    private void setWritable(boolean notify) {
         for (;;) {
             final int oldValue = unwritable;
             final int newValue = oldValue & ~1;
             if (UNWRITABLE_UPDATER.compareAndSet(this, oldValue, newValue)) {
-                if (oldValue != 0 && newValue == 0) {
+                if (notify && oldValue != 0 && newValue == 0) {
-                    fireChannelWritabilityChanged(invokeLater);
+                    fireChannelWritabilityChanged();
                 }
                 break;
             }
         }
     }
 
-    private void setUnwritable(boolean invokeLater) {
+    private void setUnwritable() {
         for (;;) {
             final int oldValue = unwritable;
             final int newValue = oldValue | 1;
             if (UNWRITABLE_UPDATER.compareAndSet(this, oldValue, newValue)) {
                 if (oldValue == 0 && newValue != 0) {
-                    fireChannelWritabilityChanged(invokeLater);
+                    fireChannelWritabilityChanged();
                 }
                 break;
             }
         }
     }
 
-    private void fireChannelWritabilityChanged(boolean invokeLater) {
+    private void fireChannelWritabilityChanged() {
-        final ChannelPipeline pipeline = channel.pipeline();
+        // Always invoke it later to prevent re-entrance bug.
-        if (invokeLater) {
+        // See https://github.com/netty/netty/issues/5028
-            Runnable task = fireChannelWritabilityChangedTask;
+        channel.eventLoop().execute(fireChannelWritabilityChangedTask);
-            if (task == null) {
-                fireChannelWritabilityChangedTask = task = new Runnable() {
-                    @Override
-                    public void run() {
-                        pipeline.fireChannelWritabilityChanged();
-                    }
-                };
-            }
-            channel.eventLoop().execute(task);
-        } else {
-            pipeline.fireChannelWritabilityChanged();
-        }
     }
 
     /**
@@ -838,4 +823,20 @@ public final class ChannelOutboundBuffer {
             return next;
         }
     }
+
+    private static final class ChannelWritabilityChangedTask implements Runnable {
+        private final Channel channel;
+
+        ChannelWritabilityChangedTask(Channel channel) {
+            this.channel = channel;
+        }
+
+        @Override
+        public void run() {
+            // Only need to fire if the Channel is still active otherwise it may be confusing to receive such an event.
+            if (channel.isActive()) {
+                channel.pipeline().fireChannelWritabilityChanged();
+            }
+        }
+    }
 }

transport/src/test/java/io/netty/channel/ChannelOutboundBufferTest.java

@@ -230,6 +230,9 @@ public class ChannelOutboundBufferTest {
 
         // Ensure exceeding the high watermark makes channel unwritable.
         ch.write(buffer().writeZero(127));
+        assertThat(buf.toString(), is(""));
+
+        ch.runPendingTasks();
         assertThat(buf.toString(), is("false "));
 
         // Ensure going down to the low watermark makes channel writable again by flushing the first write.
@@ -237,6 +240,9 @@ public class ChannelOutboundBufferTest {
         assertThat(ch.unsafe().outboundBuffer().remove(), is(true));
         assertThat(ch.unsafe().outboundBuffer().totalPendingWriteBytes(),
                 is(127L + ChannelOutboundBuffer.CHANNEL_OUTBOUND_BUFFER_ENTRY_OVERHEAD));
+        assertThat(buf.toString(), is("false "));
+
+        ch.runPendingTasks();
         assertThat(buf.toString(), is("false true "));
 
         safeClose(ch);
@@ -334,6 +340,9 @@ public class ChannelOutboundBufferTest {
 
         // Trigger channelWritabilityChanged() by writing a lot.
         ch.write(buffer().writeZero(257));
+        assertThat(buf.toString(), is(""));
+
+        ch.runPendingTasks();
         assertThat(buf.toString(), is("false "));
 
         // Ensure that setting a user-defined writability flag to false does not trigger channelWritabilityChanged()