HttpObjectDecoder configurable initial buffer size

Motivation:
The initial buffer size used to decode HTTP objects is currently fixed at 128. This may be too small for some use cases and create a high amount of overhead associated with resizing/copying. The user should be able to configure the initial size as they please.

Modifications:
- Make HttpObjectDecoder's AppendableCharSequence initial size configurable

Result:
Users can more finely tune initial buffer size for increased performance or to save memory.
Fixes https://github.com/netty/netty/issues/4807

codec-http/src/main/java/io/netty/handler/codec/http/HttpClientCodec.java

@@ -88,6 +88,17 @@ public final class HttpClientCodec extends CombinedChannelDuplexHandler<HttpResp
         this.failOnMissingResponse = failOnMissingResponse;
     }
 
+    /**
+     * Creates a new instance with the specified decoder options.
+     */
+    public HttpClientCodec(
+            int maxInitialLineLength, int maxHeaderSize, int maxChunkSize, boolean failOnMissingResponse,
+            boolean validateHeaders, int initialBufferSize) {
+        init(new Decoder(maxInitialLineLength, maxHeaderSize, maxChunkSize, validateHeaders, initialBufferSize),
+             new Encoder());
+        this.failOnMissingResponse = failOnMissingResponse;
+    }
+
     /**
      * Prepares to upgrade to another protocol from HTTP. Disables the {@link Encoder}.
      */
@@ -148,6 +159,11 @@ public final class HttpClientCodec extends CombinedChannelDuplexHandler<HttpResp
             super(maxInitialLineLength, maxHeaderSize, maxChunkSize, validateHeaders);
         }
 
+        Decoder(int maxInitialLineLength, int maxHeaderSize, int maxChunkSize, boolean validateHeaders,
+                int initialBufferSize) {
+            super(maxInitialLineLength, maxHeaderSize, maxChunkSize, validateHeaders, initialBufferSize);
+        }
+
         @Override
         protected void decode(
                 ChannelHandlerContext ctx, ByteBuf buffer, List<Object> out) throws Exception {

codec-http/src/main/java/io/netty/handler/codec/http/HttpObjectDecoder.java

@@ -162,7 +162,12 @@ public abstract class HttpObjectDecoder extends ByteToMessageDecoder {
     protected HttpObjectDecoder(
             int maxInitialLineLength, int maxHeaderSize, int maxChunkSize,
             boolean chunkedSupported, boolean validateHeaders) {
+        this(maxInitialLineLength, maxHeaderSize, maxChunkSize, chunkedSupported, validateHeaders, 128);
+    }
 
+    protected HttpObjectDecoder(
+            int maxInitialLineLength, int maxHeaderSize, int maxChunkSize,
+            boolean chunkedSupported, boolean validateHeaders, int initialBufferSize) {
         if (maxInitialLineLength <= 0) {
             throw new IllegalArgumentException(
                     "maxInitialLineLength must be a positive integer: " +
@@ -178,12 +183,12 @@ public abstract class HttpObjectDecoder extends ByteToMessageDecoder {
                     "maxChunkSize must be a positive integer: " +
                     maxChunkSize);
         }
+        AppendableCharSequence seq = new AppendableCharSequence(initialBufferSize);
+        lineParser = new LineParser(seq, maxInitialLineLength);
+        headerParser = new HeaderParser(seq, maxHeaderSize);
         this.maxChunkSize = maxChunkSize;
         this.chunkedSupported = chunkedSupported;
         this.validateHeaders = validateHeaders;
-        AppendableCharSequence seq = new AppendableCharSequence(128);
-        lineParser = new LineParser(seq, maxInitialLineLength);
-        headerParser = new HeaderParser(seq, maxHeaderSize);
     }
 
     @Override

codec-http/src/main/java/io/netty/handler/codec/http/HttpRequestDecoder.java

@@ -75,6 +75,12 @@ public class HttpRequestDecoder extends HttpObjectDecoder {
         super(maxInitialLineLength, maxHeaderSize, maxChunkSize, true, validateHeaders);
     }
 
+    public HttpRequestDecoder(
+            int maxInitialLineLength, int maxHeaderSize, int maxChunkSize, boolean validateHeaders,
+            int initialBufferSize) {
+        super(maxInitialLineLength, maxHeaderSize, maxChunkSize, true, validateHeaders, initialBufferSize);
+    }
+
     @Override
     protected HttpMessage createMessage(String[] initialLine) throws Exception {
         return new DefaultHttpRequest(

codec-http/src/main/java/io/netty/handler/codec/http/HttpResponseDecoder.java

@@ -106,6 +106,12 @@ public class HttpResponseDecoder extends HttpObjectDecoder {
         super(maxInitialLineLength, maxHeaderSize, maxChunkSize, true, validateHeaders);
     }
 
+    public HttpResponseDecoder(
+            int maxInitialLineLength, int maxHeaderSize, int maxChunkSize, boolean validateHeaders,
+            int initialBufferSize) {
+        super(maxInitialLineLength, maxHeaderSize, maxChunkSize, true, validateHeaders, initialBufferSize);
+    }
+
     @Override
     protected HttpMessage createMessage(String[] initialLine) {
         return new DefaultHttpResponse(

codec-http/src/main/java/io/netty/handler/codec/http/HttpServerCodec.java

@@ -51,6 +51,16 @@ public final class HttpServerCodec extends CombinedChannelDuplexHandler<HttpRequ
                 new HttpResponseEncoder());
     }
 
+    /**
+     * Creates a new instance with the specified decoder options.
+     */
+    public HttpServerCodec(int maxInitialLineLength, int maxHeaderSize, int maxChunkSize, boolean validateHeaders,
+                           int initialBufferSize) {
+        super(
+          new HttpRequestDecoder(maxInitialLineLength, maxHeaderSize, maxChunkSize, validateHeaders, initialBufferSize),
+          new HttpResponseEncoder());
+    }
+
     /**
      * Upgrades to another protocol from HTTP. Removes the {@link HttpRequestDecoder} and
      * {@link HttpResponseEncoder} from the pipeline.