AbstractHttp2ConnectionHandlerBuilder support for HPACK huffman decoder initial size

Motivation:
Depending on the use case it may make sense to increase or decrease the initial size of the buffer used during the HPACK huffman decode process. This is currently not exposed through the AbstractHttp2ConnectionHandlerBuilder.

Modifications:
- Add a method to AbstractHttp2ConnectionHandlerBuilder which allows the initial size of the buffer used during the HPACK huffman decode prcoess to be configured.

Result:
AbstractHttp2ConnectionHandlerBuilder provides more control of codec-http2 knobs.

codec-http2/src/main/java/io/netty/handler/codec/http2/AbstractHttp2ConnectionHandlerBuilder.java

@@ -19,8 +19,11 @@ package io.netty.handler.codec.http2;
 import io.netty.handler.codec.http2.Http2HeadersEncoder.SensitivityDetector;
 import io.netty.util.internal.UnstableApi;
 
+import static io.netty.handler.codec.http2.Http2CodecUtil.DEFAULT_HEADER_LIST_SIZE;
+import static io.netty.handler.codec.http2.Http2CodecUtil.DEFAULT_INITIAL_HUFFMAN_DECODE_CAPACITY;
 import static io.netty.handler.codec.http2.Http2CodecUtil.DEFAULT_MAX_RESERVED_STREAMS;
 import static io.netty.util.internal.ObjectUtil.checkNotNull;
+import static io.netty.util.internal.ObjectUtil.checkPositive;
 import static io.netty.util.internal.ObjectUtil.checkPositiveOrZero;
 import static java.util.concurrent.TimeUnit.MILLISECONDS;
 import static java.util.concurrent.TimeUnit.SECONDS;
@@ -61,6 +64,7 @@ import static java.util.concurrent.TimeUnit.SECONDS;
  *   <li>{@link #headerSensitivityDetector(SensitivityDetector)}</li>
  *   <li>{@link #encoderEnforceMaxConcurrentStreams(boolean)}</li>
  *   <li>{@link #encoderIgnoreMaxHeaderListSize(boolean)}</li>
+ *   <li>{@link #initialHuffmanDecodeCapacity(int)}</li>
  * </ul>
  *
  * <h3>Exposing necessary methods in a subclass</h3>
@@ -102,6 +106,7 @@ public abstract class AbstractHttp2ConnectionHandlerBuilder<T extends Http2Conne
     private SensitivityDetector headerSensitivityDetector;
     private Boolean encoderEnforceMaxConcurrentStreams;
     private Boolean encoderIgnoreMaxHeaderListSize;
+    private int initialHuffmanDecodeCapacity = DEFAULT_INITIAL_HUFFMAN_DECODE_CAPACITY;
 
     /**
      * Sets the {@link Http2Settings} to use for the initial connection settings exchange.
@@ -344,6 +349,17 @@ public abstract class AbstractHttp2ConnectionHandlerBuilder<T extends Http2Conne
         return self();
     }
 
+    /**
+     * Sets the initial size of an intermediate buffer used during HPACK huffman decoding.
+     * @param initialHuffmanDecodeCapacity initial size of an intermediate buffer used during HPACK huffman decoding.
+     * @return this.
+     */
+    protected B initialHuffmanDecodeCapacity(int initialHuffmanDecodeCapacity) {
+        enforceNonCodecConstraints("initialHuffmanDecodeCapacity");
+        this.initialHuffmanDecodeCapacity = checkPositive(initialHuffmanDecodeCapacity, "initialHuffmanDecodeCapacity");
+        return self();
+    }
+
     /**
      * Create a new {@link Http2ConnectionHandler}.
      */
@@ -363,9 +379,9 @@ public abstract class AbstractHttp2ConnectionHandlerBuilder<T extends Http2Conne
 
     private T buildFromConnection(Http2Connection connection) {
         Long maxHeaderListSize = initialSettings.maxHeaderListSize();
-        Http2FrameReader reader = new DefaultHttp2FrameReader(maxHeaderListSize == null ?
+        Http2FrameReader reader = new DefaultHttp2FrameReader(new DefaultHttp2HeadersDecoder(isValidateHeaders(),
-                new DefaultHttp2HeadersDecoder(isValidateHeaders()) :
+                maxHeaderListSize == null ? DEFAULT_HEADER_LIST_SIZE : maxHeaderListSize,
-                new DefaultHttp2HeadersDecoder(isValidateHeaders(), maxHeaderListSize));
+                initialHuffmanDecodeCapacity));
         Http2FrameWriter writer = encoderIgnoreMaxHeaderListSize == null ?
                 new DefaultHttp2FrameWriter(headerSensitivityDetector()) :
                 new DefaultHttp2FrameWriter(headerSensitivityDetector(), encoderIgnoreMaxHeaderListSize);

codec-http2/src/main/java/io/netty/handler/codec/http2/DefaultHttp2HeadersDecoder.java

@@ -20,6 +20,7 @@ import io.netty.util.internal.ObjectUtil;
 import io.netty.util.internal.UnstableApi;
 
 import static io.netty.handler.codec.http2.Http2CodecUtil.DEFAULT_HEADER_LIST_SIZE;
+import static io.netty.handler.codec.http2.Http2CodecUtil.DEFAULT_INITIAL_HUFFMAN_DECODE_CAPACITY;
 import static io.netty.handler.codec.http2.Http2Error.COMPRESSION_ERROR;
 import static io.netty.handler.codec.http2.Http2Exception.connectionError;
 
@@ -30,6 +31,7 @@ public class DefaultHttp2HeadersDecoder implements Http2HeadersDecoder, Http2Hea
 
     private final HpackDecoder hpackDecoder;
     private final boolean validateHeaders;
+
     /**
      * Used to calculate an exponential moving average of header sizes to get an estimate of how large the data
      * structure for storing headers should be.
@@ -53,7 +55,7 @@ public class DefaultHttp2HeadersDecoder implements Http2HeadersDecoder, Http2Hea
      *  (which is dangerous).
      */
     public DefaultHttp2HeadersDecoder(boolean validateHeaders, long maxHeaderListSize) {
-        this(validateHeaders, maxHeaderListSize, 32);
+        this(validateHeaders, maxHeaderListSize, DEFAULT_INITIAL_HUFFMAN_DECODE_CAPACITY);
     }
 
     /**

codec-http2/src/main/java/io/netty/handler/codec/http2/Http2CodecUtil.java

@@ -118,6 +118,7 @@ public final class Http2CodecUtil {
     public static final int SMALLEST_MAX_CONCURRENT_STREAMS = 100;
     static final int DEFAULT_MAX_RESERVED_STREAMS = SMALLEST_MAX_CONCURRENT_STREAMS;
     static final int DEFAULT_MIN_ALLOCATION_CHUNK = 1024;
+    static final int DEFAULT_INITIAL_HUFFMAN_DECODE_CAPACITY = 32;
 
     /**
      * Calculate the threshold in bytes which should trigger a {@code GO_AWAY} if a set of headers exceeds this amount.

codec-http2/src/main/java/io/netty/handler/codec/http2/Http2ConnectionHandlerBuilder.java

@@ -87,6 +87,11 @@ public final class Http2ConnectionHandlerBuilder
         return super.headerSensitivityDetector(headerSensitivityDetector);
     }
 
+    @Override
+    public Http2ConnectionHandlerBuilder initialHuffmanDecodeCapacity(int initialHuffmanDecodeCapacity) {
+        return super.initialHuffmanDecodeCapacity(initialHuffmanDecodeCapacity);
+    }
+
     @Override
     public Http2ConnectionHandler build() {
         return super.build();

codec-http2/src/main/java/io/netty/handler/codec/http2/HttpToHttp2ConnectionHandlerBuilder.java

@@ -79,6 +79,11 @@ public final class HttpToHttp2ConnectionHandlerBuilder extends
         return super.headerSensitivityDetector(headerSensitivityDetector);
     }
 
+    @Override
+    public HttpToHttp2ConnectionHandlerBuilder initialHuffmanDecodeCapacity(int initialHuffmanDecodeCapacity) {
+        return super.initialHuffmanDecodeCapacity(initialHuffmanDecodeCapacity);
+    }
+
     @Override
     public HttpToHttp2ConnectionHandler build() {
         return super.build();