Support composite buffer creation without array alloc and copy

Motivation:

Unpooled.unmodifiableBuffer() is currently used to efficiently write
arrays of ByteBufs via FixedCompositeByteBuf, but involves an allocation
and content-copy of the provided ByteBuf array which in many (most?)
cases shouldn't be necessary.

Modifications:

Modify the internal FixedCompositeByteBuf class to support wrapping the
provided ByteBuf array directly. Control this behaviour with a
constructor flag and expose the "unsafe" version via a new
Unpooled.wrappedUnmodifiableBuffer(ByteBuf...) method.

Result:

Less garbage on IO paths. I would guess pretty much all existing usage
of unmodifiableBuffer() could use the copy-free version but assume it's
not safe to change its default behaviour.

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

@@ -39,7 +39,7 @@ final class FixedCompositeByteBuf extends AbstractReferenceCountedByteBuf {
     private final int capacity;
     private final ByteBufAllocator allocator;
     private final ByteOrder order;
-    private final Object[] buffers;
+    private final ByteBuf[] buffers;
     private final boolean direct;
 
     FixedCompositeByteBuf(ByteBufAllocator allocator, ByteBuf... buffers) {
@@ -52,8 +52,7 @@ final class FixedCompositeByteBuf extends AbstractReferenceCountedByteBuf {
             direct = false;
         } else {
             ByteBuf b = buffers[0];
-            this.buffers = new Object[buffers.length];
-            this.buffers[0] = b;
+            this.buffers = buffers;
             boolean direct = true;
             int nioBufferCount = b.nioBufferCount();
             int capacity = b.readableBytes();
@@ -68,7 +67,6 @@ final class FixedCompositeByteBuf extends AbstractReferenceCountedByteBuf {
                 if (!b.isDirect()) {
                     direct = false;
                 }
-                this.buffers[i] = b;
             }
             this.nioBufferCount = nioBufferCount;
             this.capacity = capacity;
@@ -232,20 +230,14 @@ private Component findComponent(int index) {
         int readable = 0;
         for (int i = 0 ; i < buffers.length; i++) {
             Component comp = null;
-            ByteBuf b;
-            Object obj = buffers[i];
-            boolean isBuffer;
-            if (obj instanceof ByteBuf) {
-                b = (ByteBuf) obj;
-                isBuffer = true;
-            } else {
-                comp = (Component) obj;
+            ByteBuf b = buffers[i];
+            if (b instanceof Component) {
+                comp = (Component) b;
                 b = comp.buf;
-                isBuffer = false;
             }
             readable += b.readableBytes();
             if (index < readable) {
-                if (isBuffer) {
+                if (comp == null) {
                     // Create a new component and store it in the array so it not create a new object
                     // on the next access.
                     comp = new Component(i, readable - b.readableBytes(), b);
@@ -261,11 +253,8 @@ private Component findComponent(int index) {
      * Return the {@link ByteBuf} stored at the given index of the array.
      */
     private ByteBuf buffer(int i) {
-        Object obj = buffers[i];
-        if (obj instanceof ByteBuf) {
-            return (ByteBuf) obj;
-        }
-        return ((Component) obj).buf;
+        ByteBuf b = buffers[i];
+        return b instanceof Component ? ((Component) b).buf : b;
     }
 
     @Override
@@ -684,17 +673,16 @@ public String toString() {
         return result + ", components=" + buffers.length + ')';
     }
 
-    private static final class Component {
+    private static final class Component extends WrappedByteBuf {
         private final int index;
         private final int offset;
-        private final ByteBuf buf;
         private final int endOffset;
 
         Component(int index, int offset, ByteBuf buf) {
+            super(buf);
             this.index = index;
             this.offset = offset;
             endOffset = offset + buf.readableBytes();
-            this.buf = buf;
         }
     }
 }

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

@@ -22,6 +22,7 @@
 import java.nio.CharBuffer;
 import java.nio.charset.Charset;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.List;
 
 
@@ -883,7 +884,31 @@ public static ByteBuf unreleasableBuffer(ByteBuf buf) {
      * not try to slice the given {@link ByteBuf}s to reduce GC-Pressure.
      */
     public static ByteBuf unmodifiableBuffer(ByteBuf... buffers) {
-        return new FixedCompositeByteBuf(ALLOC, buffers);
+        return wrappedUnmodifiableBuffer(true, buffers);
+    }
+
+    /**
+     * Wrap the given {@link ByteBuf}s in an unmodifiable {@link ByteBuf}. Be aware the returned {@link ByteBuf} will
+     * not try to slice the given {@link ByteBuf}s to reduce GC-Pressure.
+     *
+     * The returned {@link ByteBuf} wraps the provided array directly, and so should not be subsequently modified.
+     */
+    public static ByteBuf wrappedUnmodifiableBuffer(ByteBuf... buffers) {
+        return wrappedUnmodifiableBuffer(false, buffers);
+    }
+
+    private static ByteBuf wrappedUnmodifiableBuffer(boolean copy, ByteBuf... buffers) {
+        switch (buffers.length) {
+        case 0:
+            return EMPTY_BUFFER;
+        case 1:
+            return buffers[0].asReadOnly();
+        default:
+            if (copy) {
+                buffers = Arrays.copyOf(buffers, buffers.length, ByteBuf[].class);
+            }
+            return new FixedCompositeByteBuf(ALLOC, buffers);
+        }
     }
 
     private Unpooled() {