"Needed to have 6 in buffer but only had 0"

In the HR test suite I had to disable a test, as seen here:

https://github.com/hibernate/hibernate-reactive/blob/2ff95a963d18c863853f6258490d4ace341eb00c/hibernate-reactive-core/src/test/java/org/hibernate/reactive/OrderedEmbeddableCollectionTest.java#L32

because of an intermittent error that only occurred on DB2 when running the whole test suite (and not the single test).

From the error message it looks like this is something y'all know about already.

```
ERROR errors [vert.x-eventloop-thread-0] failed to execute statement [select orderedemb0_.id as id1_0_0_, orderedemb0_.title as title2_0_0_ from EAuthor orderedemb0_ where orderedemb0_.id=?]
ERROR errors [vert.x-eventloop-thread-0] could not load an entity: [org.hibernate.reactive.OrderedEmbeddableCollectionTest$Author#1]
java.util.concurrent.CompletionException: java.lang.IllegalStateException: Needed to have 6 in buffer but only had 0. In JDBC we would normally block here but need to find a non-blocking solution
at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:331) ~[?:?]
at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:346) ~[?:?]
at java.util.concurrent.CompletableFuture$UniApply.tryFire(CompletableFuture.java:632) ~[?:?]
at java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:506) ~[?:?]
at java.util.concurrent.CompletableFuture.completeExceptionally(CompletableFuture.java:2088) ~[?:?]
at org.hibernate.reactive.pool.impl.Handlers.lambda$toCompletionStage$0(Handlers.java:29) ~[main/:?]
at io.vertx.sqlclient.impl.SqlResultHandler.fail(SqlResultHandler.java:103) ~[vertx-sql-client-3.9.5.jar:3.9.5]
at io.vertx.sqlclient.impl.SqlResultHandler.handle(SqlResultHandler.java:89) ~[vertx-sql-client-3.9.5.jar:3.9.5]
at io.vertx.sqlclient.impl.SqlResultHandler.handle(SqlResultHandler.java:33) ~[vertx-sql-client-3.9.5.jar:3.9.5]
at io.vertx.sqlclient.impl.TransactionImpl.lambda$null$3(TransactionImpl.java:181) ~[vertx-sql-client-3.9.5.jar:3.9.5]
at io.vertx.sqlclient.impl.TransactionImpl.lambda$rollback$2(TransactionImpl.java:140) ~[vertx-sql-client-3.9.5.jar:3.9.5]
at io.vertx.sqlclient.impl.SocketConnectionBase.handleMessage(SocketConnectionBase.java:241) ~[vertx-sql-client-3.9.5.jar:3.9.5]
at io.vertx.sqlclient.impl.SocketConnectionBase.lambda$init$0(SocketConnectionBase.java:88) ~[vertx-sql-client-3.9.5.jar:3.9.5]
at io.vertx.core.net.impl.NetSocketImpl.lambda$new$2(NetSocketImpl.java:101) ~[vertx-core-3.9.5.jar:3.9.5]
at io.vertx.core.streams.impl.InboundBuffer.handleEvent(InboundBuffer.java:237) ~[vertx-core-3.9.5.jar:3.9.5]
at io.vertx.core.streams.impl.InboundBuffer.write(InboundBuffer.java:127) ~[vertx-core-3.9.5.jar:3.9.5]
at io.vertx.core.net.impl.NetSocketImpl.handleMessage(NetSocketImpl.java:357) ~[vertx-core-3.9.5.jar:3.9.5]
at io.vertx.core.impl.ContextImpl.executeTask(ContextImpl.java:366) ~[vertx-core-3.9.5.jar:3.9.5]
at io.vertx.core.impl.EventLoopContext.execute(EventLoopContext.java:43) ~[vertx-core-3.9.5.jar:3.9.5]
at io.vertx.core.impl.ContextImpl.executeFromIO(ContextImpl.java:229) ~[vertx-core-3.9.5.jar:3.9.5]
at io.vertx.core.net.impl.VertxHandler.channelRead(VertxHandler.java:163) ~[vertx-core-3.9.5.jar:3.9.5]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) ~[netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) ~[netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) ~[netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.CombinedChannelDuplexHandler$DelegatingChannelHandlerContext.fireChannelRead(CombinedChannelDuplexHandler.java:436) ~[netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.vertx.db2client.impl.codec.DB2Encoder.lambda$write$0(DB2Encoder.java:72) ~[vertx-db2-client-3.9.5.jar:3.9.5]
at io.vertx.db2client.impl.codec.DB2Decoder.decodePayload(DB2Decoder.java:97) [vertx-db2-client-3.9.5.jar:3.9.5]
at io.vertx.db2client.impl.codec.DB2Decoder.decode(DB2Decoder.java:53) [vertx-db2-client-3.9.5.jar:3.9.5]
at io.netty.handler.codec.ByteToMessageDecoder.decodeRemovalReentryProtection(ByteToMessageDecoder.java:501) [netty-codec-4.1.49.Final.jar:4.1.49.Final]
at io.netty.handler.codec.ByteToMessageDecoder.callDecode(ByteToMessageDecoder.java:440) [netty-codec-4.1.49.Final.jar:4.1.49.Final]
at io.netty.handler.codec.ByteToMessageDecoder.channelRead(ByteToMessageDecoder.java:276) [netty-codec-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.CombinedChannelDuplexHandler.channelRead(CombinedChannelDuplexHandler.java:251) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.AbstractChannelHandlerContext.fireChannelRead(AbstractChannelHandlerContext.java:357) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.DefaultChannelPipeline$HeadContext.channelRead(DefaultChannelPipeline.java:1410) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:379) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.AbstractChannelHandlerContext.invokeChannelRead(AbstractChannelHandlerContext.java:365) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.DefaultChannelPipeline.fireChannelRead(DefaultChannelPipeline.java:919) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.nio.AbstractNioByteChannel$NioByteUnsafe.read(AbstractNioByteChannel.java:163) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.nio.NioEventLoop.processSelectedKey(NioEventLoop.java:714) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.nio.NioEventLoop.processSelectedKeysOptimized(NioEventLoop.java:650) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.nio.NioEventLoop.processSelectedKeys(NioEventLoop.java:576) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:493) [netty-transport-4.1.49.Final.jar:4.1.49.Final]
at io.netty.util.concurrent.SingleThreadEventExecutor$4.run(SingleThreadEventExecutor.java:989) [netty-common-4.1.49.Final.jar:4.1.49.Final]
at io.netty.util.internal.ThreadExecutorMap$2.run(ThreadExecutorMap.java:74) [netty-common-4.1.49.Final.jar:4.1.49.Final]
at io.netty.util.concurrent.FastThreadLocalRunnable.run(FastThreadLocalRunnable.java:30) [netty-common-4.1.49.Final.jar:4.1.49.Final]
at java.lang.Thread.run(Thread.java:834) [?:?]
Caused by: java.lang.IllegalStateException: Needed to have 6 in buffer but only had 0. In JDBC we would normally block here but need to find a non-blocking solution
at io.vertx.db2client.impl.drda.DRDAResponse.ensureALayerDataInBuffer(DRDAResponse.java:1462) ~[vertx-db2-client-3.9.5.jar:3.9.5]
at io.vertx.db2client.impl.drda.DRDAResponse.readDssHeader(DRDAResponse.java:878) ~[vertx-db2-client-3.9.5.jar:3.9.5]
at io.vertx.db2client.impl.drda.DRDAResponse.startSameIdChainParse(DRDAResponse.java:52) ~[vertx-db2-client-3.9.5.jar:3.9.5]
at io.vertx.db2client.impl.drda.DRDAQueryResponse.readDescribeInput(DRDAQueryResponse.java:81) ~[vertx-db2-client-3.9.5.jar:3.9.5]
at io.vertx.db2client.impl.drda.DRDAQueryResponse.readPrepareDescribeInputOutput(DRDAQueryResponse.java:53) ~[vertx-db2-client-3.9.5.jar:3.9.5]
at io.vertx.db2client.impl.codec.PrepareStatementCodec.decodePayload(PrepareStatementCodec.java:76) ~[vertx-db2-client-3.9.5.jar:3.9.5]
at io.vertx.db2client.impl.codec.DB2Decoder.decodePayload(DB2Decoder.java:80) ~[vertx-db2-client-3.9.5.jar:3.9.5]
... 21 more
```

Comienza con la prueba referenciada OrderedEmbeddableCollectionTest.java y compara su ejecución aislada con la ejecución de la suite completa de Hibernate Reactive contra DB2. Sigue el fallo a través de DRDAResponse.ensureALayerDataInBuffer(), readDssHeader() y PrepareStatementCodec.decodePayload() en el stack trace. Se considera terminado cuando se determine por qué ocurre el error intermitente del búfer y se evite que haga fallar la ejecución de pruebas de DB2.

Escrito por el modelo de indexación a partir del texto del issue.