HttpObjectEncoder buffer size estimation

Motivation:
HttpObjectEncoder allocates a new buffer when encoding the initial line and headers, and also allocates a buffer when encoding the trailers. The allocation always uses the default size of 256. This may lead to consistent under allocation and require a few resize/copy operations which can cause GC/memory pressure.

Modifications:
- Introduce a weighted average which tracks the historical size of encoded data and uses this as an estimate for future buffer allocations

Result:
Better approximation of buffer sizes.

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

@@ -53,6 +53,10 @@ public abstract class HttpObjectEncoder<H extends HttpMessage> extends MessageTo
     private static final ByteBuf CRLF_BUF = unreleasableBuffer(directBuffer(2).writeByte(CR).writeByte(LF));
     private static final ByteBuf ZERO_CRLF_CRLF_BUF = unreleasableBuffer(directBuffer(ZERO_CRLF_CRLF.length)
             .writeBytes(ZERO_CRLF_CRLF));
+    private static final float HEADERS_WEIGHT_NEW = 1 / 5f;
+    private static final float HEADERS_WEIGHT_HISTORICAL = 1 - HEADERS_WEIGHT_NEW;
+    private static final float TRAILERS_WEIGHT_NEW = HEADERS_WEIGHT_NEW;
+    private static final float TRAILERS_WEIGHT_HISTORICAL = HEADERS_WEIGHT_HISTORICAL;
 
     private static final int ST_INIT = 0;
     private static final int ST_CONTENT_NON_CHUNK = 1;
@@ -62,6 +66,18 @@ public abstract class HttpObjectEncoder<H extends HttpMessage> extends MessageTo
     @SuppressWarnings("RedundantFieldInitialization")
     private int state = ST_INIT;
 
+    /**
+     * Used to calculate an exponential moving average of the encoded size of the initial line and the headers for
+     * a guess for future buffer allocations.
+     */
+    private float headersEncodedSizeAccumulator = 256;
+
+    /**
+     * Used to calculate an exponential moving average of the encoded size of the trailers for
+     * a guess for future buffer allocations.
+     */
+    private float trailersEncodedSizeAccumulator = 256;
+
     @Override
     protected void encode(ChannelHandlerContext ctx, Object msg, List<Object> out) throws Exception {
         ByteBuf buf = null;
@@ -73,10 +89,12 @@ public abstract class HttpObjectEncoder<H extends HttpMessage> extends MessageTo
             @SuppressWarnings({ "unchecked", "CastConflictsWithInstanceof" })
             H m = (H) msg;
 
-            buf = ctx.alloc().buffer();
+            buf = ctx.alloc().buffer((int) headersEncodedSizeAccumulator);
             // Encode the message.
             encodeInitialLine(buf, m);
             encodeHeaders(m.headers(), buf);
+            headersEncodedSizeAccumulator = HEADERS_WEIGHT_NEW * padSizeForAccumulation(buf.readableBytes()) +
+                                            HEADERS_WEIGHT_HISTORICAL * headersEncodedSizeAccumulator;
             ByteBufUtil.writeShortBE(buf, CRLF_SHORT);
             state = isContentAlwaysEmpty(m) ? ST_CONTENT_ALWAYS_EMPTY :
                     HttpUtil.isTransferEncodingChunked(m) ? ST_CONTENT_CHUNK : ST_CONTENT_NON_CHUNK;
@@ -177,10 +195,12 @@ public abstract class HttpObjectEncoder<H extends HttpMessage> extends MessageTo
             if (headers.isEmpty()) {
                 out.add(ZERO_CRLF_CRLF_BUF.duplicate());
             } else {
-                ByteBuf buf = ctx.alloc().buffer();
+                ByteBuf buf = ctx.alloc().buffer((int) trailersEncodedSizeAccumulator);
                 ByteBufUtil.writeMediumBE(buf, ZERO_CRLF_MEDIUM);
                 encodeHeaders(headers, buf);
                 ByteBufUtil.writeShortBE(buf, CRLF_SHORT);
+                trailersEncodedSizeAccumulator = TRAILERS_WEIGHT_NEW * padSizeForAccumulation(buf.readableBytes()) +
+                                                 TRAILERS_WEIGHT_HISTORICAL * trailersEncodedSizeAccumulator;
                 out.add(buf);
             }
         } else if (contentLength == 0) {
@@ -232,6 +252,16 @@ public abstract class HttpObjectEncoder<H extends HttpMessage> extends MessageTo
         throw new IllegalStateException("unexpected message type: " + StringUtil.simpleClassName(msg));
     }
 
+    /**
+     * Add some additional overhead to the buffer. The rational is that it is better to slightly over allocate and waste
+     * some memory, rather than under allocate and require a resize/copy.
+     * @param readableBytes The readable bytes in the buffer.
+     * @return The {@code readableBytes} with some additional padding.
+     */
+    private static int padSizeForAccumulation(int readableBytes) {
+        return (readableBytes << 2) / 3;
+    }
+
     @Deprecated
     protected static void encodeAscii(String s, ByteBuf buf) {
         buf.writeCharSequence(s, CharsetUtil.US_ASCII);