Message Title

When content-length is defined, the encoder is a LengthDelimitedEncoder, and even a 0-length write will trigger an exception when the content length has been reached.
The current code allows 0-length writes, and we should protect ourselves against that.

Note: this problem only affects 5.8.0.CR1, but was only witnessed while working on

HSEARCH-2849 In Progress . It is possible that the bug never actually happens in 5.8.0.CR1 because

HSEARCH-2849 In Progress makes it content-length definition rarer.

        at org.hibernate.search.elasticsearch.work.impl.BulkWork.lambda$execute$1(BulkWork.java:75)
        at org.hibernate.search.util.impl.Futures.lambda$handler$1(Futures.java:57)
        at java.util.concurrent.CompletableFuture.uniExceptionally(CompletableFuture.java:870)
        at java.util.concurrent.CompletableFuture$UniExceptionally.tryFire(CompletableFuture.java:852)
        at java.util.concurrent.CompletableFuture.postComplete(CompletableFuture.java:474)
        at java.util.concurrent.CompletableFuture.completeExceptionally(CompletableFuture.java:1977)
        at org.hibernate.search.elasticsearch.client.impl.DefaultElasticsearchClient$1.onFailure(DefaultElasticsearchClient.java:108)
        at org.elasticsearch.client.RestClient$FailureTrackingResponseListener.onDefinitiveFailure(RestClient.java:605)
        at org.elasticsearch.client.RestClient$1.retryIfPossible(RestClient.java:396)
        at org.elasticsearch.client.RestClient$1.failed(RestClient.java:375)
        at org.apache.http.concurrent.BasicFuture.failed(BasicFuture.java:134)
        at org.apache.http.impl.nio.client.AbstractClientExchangeHandler.failed(AbstractClientExchangeHandler.java:419)
        at org.apache.http.nio.protocol.HttpAsyncRequestExecutor.exception(HttpAsyncRequestExecutor.java:155)
        at org.apache.http.impl.nio.DefaultNHttpClientConnection.produceOutput(DefaultNHttpClientConnection.java:310)
        at org.apache.http.impl.nio.client.InternalIODispatch.onOutputReady(InternalIODispatch.java:86)
        at org.apache.http.impl.nio.client.InternalIODispatch.onOutputReady(InternalIODispatch.java:39)
        at org.apache.http.impl.nio.reactor.AbstractIODispatch.outputReady(AbstractIODispatch.java:152)
        at org.apache.http.impl.nio.reactor.BaseIOReactor.writable(BaseIOReactor.java:188)
        at org.apache.http.impl.nio.reactor.AbstractIOReactor.processEvent(AbstractIOReactor.java:341)
        at org.apache.http.impl.nio.reactor.AbstractIOReactor.processEvents(AbstractIOReactor.java:315)
        at org.apache.http.impl.nio.reactor.AbstractIOReactor.execute(AbstractIOReactor.java:276)
        at org.apache.http.impl.nio.reactor.BaseIOReactor.execute(BaseIOReactor.java:104)
        at org.apache.http.impl.nio.reactor.AbstractMultiworkerIOReactor$Worker.run(AbstractMultiworkerIOReactor.java:588)
        at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.IllegalStateException: Encoding process already completed
        at org.apache.http.util.Asserts.check(Asserts.java:34)
        at org.apache.http.impl.nio.codecs.AbstractContentEncoder.assertNotCompleted(AbstractContentEncoder.java:89)
        at org.apache.http.impl.nio.codecs.LengthDelimitedEncoder.write(LengthDelimitedEncoder.java:102)
        at org.hibernate.search.elasticsearch.util.impl.GsonHttpEntity$ProgressiveCharBufferWriter.fullyWrite(GsonHttpEntity.java:435)
        at org.hibernate.search.elasticsearch.util.impl.GsonHttpEntity$ProgressiveCharBufferWriter.flushCurrentBuffer(GsonHttpEntity.java:420)
        at org.hibernate.search.elasticsearch.util.impl.GsonHttpEntity$ProgressiveCharBufferWriter.attemptFlushPendingBuffers(GsonHttpEntity.java:361)
        at org.hibernate.search.elasticsearch.util.impl.GsonHttpEntity$ProgressiveCharBufferWriter.flushToOutput(GsonHttpEntity.java:453)
        at org.hibernate.search.elasticsearch.util.impl.GsonHttpEntity.produceContent(GsonHttpEntity.java:274)
        at org.apache.http.nio.protocol.BasicAsyncRequestProducer.produceContent(BasicAsyncRequestProducer.java:125)
        at org.apache.http.impl.nio.client.MainClientExec.produceContent(MainClientExec.java:262)
        at org.apache.http.impl.nio.client.DefaultClientExchangeHandlerImpl.produceContent(DefaultClientExchangeHandlerImpl.java:136)
        at org.apache.http.nio.protocol.HttpAsyncRequestExecutor.outputReady(HttpAsyncRequestExecutor.java:241)
        at org.apache.http.impl.nio.DefaultNHttpClientConnection.produceOutput(DefaultNHttpClientConnection.java:290)
        ... 10 more