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

Start with the referenced OrderedEmbeddableCollectionTest.java test and compare running it alone with running the full Hibernate Reactive suite against DB2. Trace the failure through DRDAResponse.ensureALayerDataInBuffer(), readDssHeader(), and PrepareStatementCodec.decodePayload() in the stack trace. Done means determining why the intermittent buffer error occurs and preventing it from failing the DB2 test run.

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