Presumably in testSynchronousPost:

PlainVariableLengthServer: got connection Start sending 16384
... 16K sent.
POST /http1/variable/bar?length=106
... 16K sent.
... 16K sent.
... 16K sent.
... 16K sent.
PlainVariableLengthServer: writing 106 bytes
... 16K sent.
... 16K sent.
... 16K sent.
... 16K sent.
Timeout refired 960 times


Apr 24, 2019 2:28:11 PM jdk.internal.net.http.SocketTube$InternalReadPublisher$InternalReadSubscription handleSubscribeEvent
INFO: CHANNEL: Start reading from java.nio.channels.SocketChannel[connected local=/127.0.0.1:35956 remote=localhost/127.0.0.1:32914]
Apr 24, 2019 2:28:11 PM jdk.internal.net.http.Http1Request logHeaders
INFO: HEADERS: REQUEST HEADERS:
POST /http1/variable/bar?length=106 HTTP/1.1
Connection: Upgrade, HTTP2-Settings
Host: localhost:32914
HTTP2-Settings: AAEAAEAAAAIAAAABAAMAAABkAAQBAAAAAAUAAEAA
Transfer-encoding: chunked
Upgrade: h2c
User-Agent: Java-http-client/13-internal

Apr 24, 2019 2:28:11 PM jdk.internal.net.http.Http1AsyncReceiver$Http1TubeSubscriber onSubscribe
INFO: CHANNEL: HTTP/1 read subscriber got subscription from http://localhost:32914/http1/variable/bar?length=106 POST
Apr 24, 2019 2:28:11 PM jdk.internal.net.http.SocketTube$InternalWriteSubscriber startSubscription
INFO: CHANNEL: Start requesting bytes for writing to channel: java.nio.channels.SocketChannel[connected local=/127.0.0.1:35956 remote=localhost/127.0.0.1:32914]
Apr 24, 2019 2:28:11 PM jdk.internal.net.http.SocketTube$InternalReadPublisher$InternalReadSubscription read
INFO: CHANNEL: Raising error with subscriber for java.nio.channels.SocketChannel[connected local=/127.0.0.1:35956 remote=localhost/127.0.0.1:32914]: java.net.SocketException: Connection reset
Apr 24, 2019 2:28:11 PM jdk.internal.net.http.Http1AsyncReceiver onReadError
INFO: ERROR: HTTP/1 read subscriber recorded error: http://localhost:32914/http1/variable/bar?length=106 POST - java.net.SocketException: Connection reset
Apr 24, 2019 2:28:11 PM jdk.internal.net.http.SocketTube$InternalReadPublisher$InternalReadSubscription read
INFO: CHANNEL: Stopped reading from channel java.nio.channels.SocketChannel[connected local=/127.0.0.1:35956 remote=localhost/127.0.0.1:32914]

then nothing happens - all HttpClient start shutting down after a while...