Add ByteBuf.Unsafe.discardSomeReadBytes() to reduce discardReadBytes()

buffer/src/main/java/io/netty/buffer/ByteBuf.java

@@ -1808,6 +1808,14 @@ public interface ByteBuf extends ChannelBuf, Comparable<ByteBuf> {
          */
         ByteBuf newBuffer(int initialCapacity);
 
+        /**
+         * Similar to {@link ByteBuf#discardReadBytes()} except that this method might discard
+         * some, all, or none of read bytes depending on its internal implementation to reduce
+         * overall memory bandwidth consumption at the cost of potentially additional memory
+         * consumption.
+         */
+        void discardSomeReadBytes();
+
         /**
          * Increases the reference count of the buffer.
          */

buffer/src/main/java/io/netty/buffer/DefaultCompositeByteBuf.java

@@ -1201,6 +1201,11 @@ public class DefaultCompositeByteBuf extends AbstractByteBuf implements Composit
             return buf;
         }
 
+        @Override
+        public void discardSomeReadBytes() {
+            discardReadComponents();
+        }
+
         @Override
         public void acquire() {
             if (refCnt <= 0) {

buffer/src/main/java/io/netty/buffer/DirectByteBuf.java

@@ -413,6 +413,19 @@ public class DirectByteBuf extends AbstractByteBuf {
             return new DirectByteBuf(initialCapacity, Math.max(initialCapacity, maxCapacity()));
         }
 
+        @Override
+        public void discardSomeReadBytes() {
+            final int readerIndex = readerIndex();
+            if (readerIndex == writerIndex()) {
+                discardReadBytes();
+                return;
+            }
+
+            if (readerIndex > 0 && readerIndex >= capacity >>> 1) {
+                discardReadBytes();
+            }
+        }
+
         @Override
         public void acquire() {
             if (refCnt <= 0) {

buffer/src/main/java/io/netty/buffer/DuplicatedByteBuf.java

@@ -228,6 +228,11 @@ public class DuplicatedByteBuf extends AbstractByteBuf implements WrappedByteBuf
             return buffer.unsafe().newBuffer(initialCapacity);
         }
 
+        @Override
+        public void discardSomeReadBytes() {
+            throw new UnsupportedOperationException();
+        }
+
         @Override
         public void acquire() {
             buffer.unsafe().acquire();

buffer/src/main/java/io/netty/buffer/HeapByteBuf.java

@@ -302,6 +302,19 @@ public class HeapByteBuf extends AbstractByteBuf {
             return new HeapByteBuf(initialCapacity, Math.max(initialCapacity, maxCapacity()));
         }
 
+        @Override
+        public void discardSomeReadBytes() {
+            final int readerIndex = readerIndex();
+            if (readerIndex == writerIndex()) {
+                discardReadBytes();
+                return;
+            }
+
+            if (readerIndex > 0 && readerIndex >= capacity() >>> 1) {
+                discardReadBytes();
+            }
+        }
+
         @Override
         public void acquire() {
             if (refCnt <= 0) {

buffer/src/main/java/io/netty/buffer/SlicedByteBuf.java

@@ -295,6 +295,11 @@ public class SlicedByteBuf extends AbstractByteBuf implements WrappedByteBuf {
             return buffer.unsafe().newBuffer(initialCapacity);
         }
 
+        @Override
+        public void discardSomeReadBytes() {
+            throw new UnsupportedOperationException();
+        }
+
         @Override
         public void acquire() {
             buffer.unsafe().acquire();

codec/src/main/java/io/netty/handler/codec/ByteToByteDecoder.java

@@ -46,7 +46,6 @@ public abstract class ByteToByteDecoder extends ChannelInboundByteHandlerAdapter
         }
 
         if (out.readableBytes() > oldOutSize) {
-            in.discardReadBytes();
             ctx.fireInboundBufferUpdated();
         }
 
@@ -71,8 +70,8 @@ public abstract class ByteToByteDecoder extends ChannelInboundByteHandlerAdapter
             }
         }
 
+        in.unsafe().discardSomeReadBytes();
         if (out.readableBytes() > oldOutSize) {
-            in.discardReadBytes();
             ctx.fireInboundBufferUpdated();
         }
     }

codec/src/main/java/io/netty/handler/codec/ByteToByteEncoder.java

@@ -44,10 +44,7 @@ public abstract class ByteToByteEncoder extends ChannelOutboundByteHandlerAdapte
             }
         }
 
-        if (out.readableBytes() > oldOutSize) {
-            in.discardReadBytes();
-        }
-
+        in.unsafe().discardSomeReadBytes();
         ctx.flush(future);
     }
 

codec/src/main/java/io/netty/handler/codec/ByteToMessageDecoder.java

@@ -52,7 +52,6 @@ public abstract class ByteToMessageDecoder<O>
 
         try {
             if (CodecUtil.unfoldAndAdd(ctx, decodeLast(ctx, in), true)) {
-                in.discardReadBytes();
                 ctx.fireInboundBufferUpdated();
             }
         } catch (Throwable t) {
@@ -93,9 +92,10 @@ public abstract class ByteToMessageDecoder<O>
                     break;
                 }
             } catch (Throwable t) {
+                in.unsafe().discardSomeReadBytes();
+
                 if (decoded) {
                     decoded = false;
-                    in.discardReadBytes();
                     ctx.fireInboundBufferUpdated();
                 }
 
@@ -107,8 +107,9 @@ public abstract class ByteToMessageDecoder<O>
             }
         }
 
+        in.unsafe().discardSomeReadBytes();
+
         if (decoded) {
-            in.discardReadBytes();
             ctx.fireInboundBufferUpdated();
         }
     }

codec/src/main/java/io/netty/handler/codec/ReplayingDecoder.java

@@ -455,8 +455,10 @@ public abstract class ReplayingDecoder<O, S> extends ByteToMessageDecoder<O> {
     }
 
     private void fireInboundBufferUpdated(ChannelHandlerContext ctx, ByteBuf in) {
-        checkpoint -= in.readerIndex();
-        in.discardReadBytes();
+        final int oldReaderIndex = in.readerIndex();
+        in.unsafe().discardSomeReadBytes();
+        final int newReaderIndex = in.readerIndex();
+        checkpoint -= oldReaderIndex - newReaderIndex;
         ctx.fireInboundBufferUpdated();
     }
 }

codec/src/main/java/io/netty/handler/codec/serialization/CompatibleObjectEncoder.java

@@ -100,7 +100,7 @@ public class CompatibleObjectEncoder extends MessageToByteEncoder<Object> {
                     oos.reset();
 
                     // Also discard the byproduct to avoid OOM on the sending side.
-                    out.discardReadBytes();
+                    out.unsafe().discardSomeReadBytes();
                 }
             }
 

handler/src/main/java/io/netty/handler/ssl/SslHandler.java

@@ -336,7 +336,7 @@ public class SslHandler
         final ByteBuf in = ctx.outboundByteBuffer();
         final ByteBuf out = ctx.nextOutboundByteBuffer();
 
-        out.discardReadBytes();
+        out.unsafe().discardSomeReadBytes();
 
         // Do not encrypt the first write request if this handler is
         // created with startTLS flag turned on.
@@ -398,9 +398,7 @@ public class SslHandler
             setHandshakeFailure(e);
             throw e;
         } finally {
-            if (bytesProduced > 0) {
-                in.discardReadBytes();
-            }
+            in.unsafe().discardSomeReadBytes();
             ctx.flush(future);
         }
     }

transport/src/test/java/io/netty/channel/local/LocalTransportThreadModelTest.java

@@ -441,7 +441,7 @@ public class LocalTransportThreadModelTest {
                     out.add(msg);
                 }
             }
-            in.discardReadBytes();
+            in.unsafe().discardSomeReadBytes();
             if (swallow) {
                 future.setSuccess();
             } else {