Allocating an unpooled CompositeBuffer of size 1 throws an IllegalArgumentException

Expected behavior

Allocating a new composite buffer with max components equal to 1 via Unpooled.compositeBuffer allocates a new CompositeBuffer instance.

Actual behavior

As of 4.1.32.Final, this method will now throw an IllegalStateException, indicating that the argument must be greater than or equal to two:

java.lang.IllegalArgumentException: maxNumComponents: 1 (expected: >= 2)
        at io.netty.buffer.CompositeByteBuf.<init>(CompositeByteBuf.java:62)
        at io.netty.buffer.CompositeByteBuf.<init>(CompositeByteBuf.java:72)
        at io.netty.buffer.Unpooled.compositeBuffer(Unpooled.java:351)

This is firing in one of our tests. We should fix our code to not allocate composite buffers of this size, but this was a breaking change that we weren’t expecting. This is more of a heads up about the breaking change.

This change was introduced in #8420.

Steps to reproduce

Apply the following patch:

diff --git a/buffer/src/test/java/io/netty/buffer/UnpooledTest.java b/buffer/src/test/java/io/netty/buffer/UnpooledTest.java
index f7fd30ba9..c33570f0f 100644
--- a/buffer/src/test/java/io/netty/buffer/UnpooledTest.java
+++ b/buffer/src/test/java/io/netty/buffer/UnpooledTest.java
@@ -39,6 +39,15 @@ public class UnpooledTest {
     private static final ByteBuf[] EMPTY_BYTE_BUFS = new ByteBuf[0];
     private static final byte[][] EMPTY_BYTES_2D = new byte[0][];

+    @Test
+    public void testCompositeBufferSizeOne() {
+        CompositeByteBuf byteBuf = compositeBuffer(1);
+
+        assertEquals(1, byteBuf.maxNumComponents());
+
+        byteBuf.release();
+    }
+
     @Test
     public void testCompositeWrappedBuffer() {
         ByteBuf header = buffer(12);

and run

$ mvn -pl buffer -Dtest=UnpooledTest#testCompositeBufferSizeOne test

Netty version

4.1.32.Final

JVM version (e.g. java -version)

$ java -version
java version "1.8.0_162"
Java(TM) SE Runtime Environment (build 1.8.0_162-b12)
Java HotSpot(TM) 64-Bit Server VM (build 25.162-b12, mixed mode)

OS version (e.g. uname -a)

$ uname -a
Darwin nickt.local 18.2.0 Darwin Kernel Version 18.2.0: Fri Oct  5 19:41:49 PDT 2018; root:xnu-4903.221.2~2/RELEASE_X86_64 x86_64

[nicktrav]

Sounds good. I’ll try and work up a patch today.

[normanmaurer]

@njhill @nicktrav Yeah I think failing for <= 0 makes sense and we should still allow 1 just to not break people.