"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
```

Beginne mit dem referenzierten Test OrderedEmbeddableCollectionTest.java und vergleiche seine alleinige Ausführung mit der Ausführung der vollständigen Hibernate Reactive-Suite gegen DB2. Verfolge den Fehler anhand des Stack Traces durch DRDAResponse.ensureALayerDataInBuffer(), readDssHeader() und PrepareStatementCodec.decodePayload(). Als abgeschlossen gilt die Aufgabe, wenn ermittelt wurde, warum der intermittierende Pufferfehler auftritt, und verhindert wird, dass er den DB2-Testlauf fehlschlagen lässt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.