Added NPE checkers in Instrumentation Gateway

[ValentinZakharov]

What Does This Do

The temporary solution. Added Null checker to avoid an NPE (logged to debug level).

Motivation

Reported NPE:

java.lang.NullPointerException: Cannot invoke "com.datadog.appsec.event.EventDispatcher$DataSubscriberInfoImpl.isEventDispatcher(com.datadog.appsec.event.EventDispatcher)" 
because "subscribers" is null. 
	at com.datadog.appsec.event.EventDispatcher.publishDataEvent(EventDispatcher.java:185)
	at com.datadog.appsec.event.ReplaceableEventProducerService.publishDataEvent(ReplaceableEventProducerService.java:33)
	at com.datadog.appsec.gateway.GatewayBridge.lambda$init$5(GatewayBridge.java:224)
	at com.datadog.appsec.gateway.GatewayBridge$$Lambda$151 (Source)
	at datadog.trace.api.gateway.InstrumentationGateway$6.apply(InstrumentationGateway.java:255)
	at datadog.trace.api.gateway.InstrumentationGateway$6.apply(InstrumentationGateway.java:250)
	at org.glassfish.jersey.server.internal.routing.UriRoutingContext.getPathParameters(UriRoutingContext.java:319)
	at org.glassfish.jersey.server.internal.routing.UriRoutingContext.getPathParameters(UriRoutingContext.java:297)
	at com.fsmatic.launcher.JettyEnvironment.decorateRequest(JettyEnvironment.java:284)
	at com.fsmatic.launcher.JettyEnvironment$2.decorateRequest(JettyEnvironment.java:341)
	at io.opentracing.contrib.jaxrs2.server.ServerTracingFilter.filter(ServerTracingFilter.java:78)
	at org.glassfish.jersey.server.ContainerFilteringStage.apply(ContainerFilteringStage.java:132)
	at org.glassfish.jersey.server.ContainerFilteringStage.apply(ContainerFilteringStage.java:68)
	at org.glassfish.jersey.process.internal.Stages.process(Stages.java:197)
	at org.glassfish.jersey.server.ServerRuntime$1run(ServerRuntime.java:269)

Additional Notes

• This is a temporary fix, while we are investigating the root of issue. It seems, there are something is wrong with the jersey instrumentation, that cause the problem.
• Hitting this problem should have no impact to the application other than the logged exceptions.

[cataphract]

I've looked at the code, at there seems to be a race condition indeed. Code follows this pattern:

    while (true) {
      if (this.respDataSubInfo == null) {
        this.respDataSubInfo =
            producerService.getDataSubscribers(
                KnownAddresses.RESPONSE_STATUS, KnownAddresses.RESPONSE_HEADERS_NO_COOKIES);
      }

      try {
        return producerService.publishDataEvent(this.respDataSubInfo, ctx, bundle, false);
      } catch (ExpiredSubscriberInfoException e) {
        this.respDataSubInfo = null;
      }
    }

where respDataSubInfo is a volatile field. Problem is some thread may have set the variable to null after the null check on another thread. The field needs to be copied to a local variable before the null check.

[pr-commenter]

Benchmarks

Parameters

	Baseline	Candidate
commit	1.17.0-SNAPSHOT~314c79e928	1.16.0-SNAPSHOT~765e2c3b02
config	baseline	candidate

See matching parameters

	Baseline	Candidate
module	Agent	Agent
parent	None	None

Summary

Found 0 performance improvements and 3 performance regressions! Performance is the same for 19 cases.

scenario	Δ mean execution_time
scenario:Startup-iast-IAST	worse [+0.330ms; +0.598ms] or [+2.835%; +5.141%]
scenario:Startup-iast-Remote Config	worse [+34.010µs; +66.745µs] or [+5.720%; +11.226%]
scenario:Startup-waf-Remote Config	worse [+143.454µs; +230.061µs] or [+20.934%; +33.572%]

[ValentinZakharov]

I'll try to fix the race condition issue in next PR