NPE under load

[cquiroz]

I did some stress testing using web sockets and I get a NullPointerException sometimes.

edu_gemini_seqexec_web_server[ERROR] java.lang.NullPointerException
edu_gemini_seqexec_web_server[ERROR] 	at org.http4s.blaze.util.BufferTools$.go$3(BufferTools.scala:194)
edu_gemini_seqexec_web_server[ERROR] 	at org.http4s.blaze.util.BufferTools$.areDirectOrEmpty(BufferTools.scala:202)
edu_gemini_seqexec_web_server[ERROR] 	at org.http4s.blaze.channel.nio1.NIO1SocketServerGroup$SocketChannelHead.performWrite(NIO1SocketServerGroup.scala:221)
edu_gemini_seqexec_web_server[ERROR] 	at org.http4s.blaze.channel.nio1.NIO1HeadStage.writeReady(NIO1HeadStage.scala:102)
edu_gemini_seqexec_web_server[ERROR] 	at org.http4s.blaze.channel.nio1.SelectorLoop.run(SelectorLoop.scala:131)

I'm testing this using http4s version 0.16.x and running several simultaneous clients using artillery

[ChristopherDavenport]

This seems likely considering error rates seen in the TechEmpower benchmarks for both Http4s and Blaze. Just had never been able to reproduce locally.

[cquiroz]

Maybe I can reproduce my test with a minimal example, let me try to get that done soon

[bryce-anderson]

I wonder if at high load write interests are leaking through even after a socket has been unregistered interest in the write operation. If so, https://github.com/http4s/blaze/blob/v0.12.6/core/src/main/scala/org/http4s/blaze/channel/nio1/NIO1HeadStage.scala#L101 would end up with a null.

[cquiroz]

I can reproduce it agains http4s' BlazeWebSocketExample running from sbt using artillery and the configuration file below with the command

artillery run wsstress.yml

Note that I see the NPE on the release-0.16.x branch but not on master

wsstress.yml:

config:
  target: "ws://localhost:8080/http4s/ws"
  phases:
    -
      duration: 120
      arrivalRate: 50
      rampTo: 100
  ws:
    # Ignore SSL certificate errors
    # - useful in *development* with self-signed certs
    rejectUnauthorized: false
scenarios:
  -
    engine: "ws"
    flow:
      -
        send: "hello"
      -
        think: 1
      -
        send: "world"