Channel#bytesBefore[un]writable off by 1 (#13389)

Motivation:
Channel#bytesBefore[un]writable methods are described as returning the
number of bytes until writability state changes. The ChannelConfig
buffer high/low water marks are described as the thresholds must be
exceeded before writability changes. The implementation of
bytesBefore[un]writable methods return the number of bytes until the
threshold is meet but not exceeded. If implementations depend upon this
to drive readability they may hang.

Modifications:
- Channel#bytesBefore[un]writable implementations in http/2 and
ChannelOutboundBuffer increment the value by 1 to return how much is
required to cross the water mark therefore trigger a change in
writability.

Result:

No more stales when implementations use `bytesBeforeWritable` / `bytesBeforeUnwritable`

codec-http2/src/main/java/io/netty/handler/codec/http2/AbstractHttp2StreamChannel.java

@@ -361,26 +361,22 @@ public ChannelFuture closeFuture() {
 
     @Override
     public long bytesBeforeUnwritable() {
-        long bytes = config().getWriteBufferHighWaterMark() - totalPendingSize;
+        // +1 because writability doesn't change until the threshold is crossed (not equal to).
+        long bytes = config().getWriteBufferHighWaterMark() - totalPendingSize + 1;
         // If bytes is negative we know we are not writable, but if bytes is non-negative we have to check
         // writability. Note that totalPendingSize and isWritable() use different volatile variables that are not
         // synchronized together. totalPendingSize will be updated before isWritable().
-        if (bytes > 0) {
-            return isWritable() ? bytes : 0;
-        }
-        return 0;
+        return bytes > 0 && isWritable() ? bytes : 0;
     }
 
     @Override
     public long bytesBeforeWritable() {
-        long bytes = totalPendingSize - config().getWriteBufferLowWaterMark();
+        // +1 because writability doesn't change until the threshold is crossed (not equal to).
+        long bytes = totalPendingSize - config().getWriteBufferLowWaterMark() + 1;
         // If bytes is negative we know we are writable, but if bytes is non-negative we have to check writability.
         // Note that totalPendingSize and isWritable() use different volatile variables that are not synchronized
         // together. totalPendingSize will be updated before isWritable().
-        if (bytes > 0) {
-            return isWritable() ? 0 : bytes;
-        }
-        return 0;
+        return bytes <= 0 || isWritable() ? 0 : bytes;
     }
 
     @Override

codec-http2/src/test/java/io/netty/handler/codec/http2/Http2MultiplexTest.java

@@ -925,7 +925,7 @@ public void writabilityOfParentIsRespected() {
         assertFalse(parentChannel.isWritable());
 
         assertTrue(childChannel.isWritable());
-        assertEquals(4096, childChannel.bytesBeforeUnwritable());
+        assertEquals(4097, childChannel.bytesBeforeUnwritable());
 
         // Flush everything which simulate writing everything to the socket.
         parentChannel.flush();

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

@@ -748,29 +748,25 @@ public long totalPendingWriteBytes() {
      * This quantity will always be non-negative. If {@link #isWritable()} is {@code false} then 0.
      */
     public long bytesBeforeUnwritable() {
-        long bytes = channel.config().getWriteBufferHighWaterMark() - totalPendingSize;
+        // +1 because writability doesn't change until the threshold is crossed (not equal to).
+        long bytes = channel.config().getWriteBufferHighWaterMark() - totalPendingSize + 1;
         // If bytes is negative we know we are not writable, but if bytes is non-negative we have to check writability.
         // Note that totalPendingSize and isWritable() use different volatile variables that are not synchronized
         // together. totalPendingSize will be updated before isWritable().
-        if (bytes > 0) {
-            return isWritable() ? bytes : 0;
-        }
-        return 0;
+        return bytes > 0 && isWritable() ? bytes : 0;
     }
 
     /**
      * Get how many bytes must be drained from the underlying buffer until {@link #isWritable()} returns {@code true}.
      * This quantity will always be non-negative. If {@link #isWritable()} is {@code true} then 0.
      */
     public long bytesBeforeWritable() {
-        long bytes = totalPendingSize - channel.config().getWriteBufferLowWaterMark();
+        // +1 because writability doesn't change until the threshold is crossed (not equal to).
+        long bytes = totalPendingSize - channel.config().getWriteBufferLowWaterMark() + 1;
         // If bytes is negative we know we are writable, but if bytes is non-negative we have to check writability.
         // Note that totalPendingSize and isWritable() use different volatile variables that are not synchronized
         // together. totalPendingSize will be updated before isWritable().
-        if (bytes > 0) {
-            return isWritable() ? 0 : bytes;
-        }
-        return 0;
+        return bytes <= 0 || isWritable() ? 0 : bytes;
     }
 
     /**