Make GlobalTracer uses more threadsafe

I have a multithreaded app which causes some unpredictable behaviors when trying to get a globally registered tracer. A specific complication appears to have some relationship to byteman when using byteman rules. I have found that sometimes I call GlobalTracer.get() and receive a noop, while other times I get a tracer initialized from some byteman execution. I attempted to resolve this by conditionally registering the tracer myself:

        Tracer tracer;
        if (GlobalTracer.isRegistered()) {
            tracer = GlobalTracer.get();
        } else {
            tracer = TracerResolver.resolveTracer();
            GlobalTracer.register(tracer);
        }

However, this was affected by a race condition somewhere. I was able to resolve it by synchronizing on the GlobalTracer.class:

        Tracer tracer;
        synchronized (GlobalTracer.class) {
            if (GlobalTracer.isRegistered()) {
                tracer = GlobalTracer.get();
            } else {
                tracer = TracerResolver.resolveTracer();
                GlobalTracer.register(tracer);
            }
        }

but this seems like something that would be nice to incorporate into the API itself. Note that this still causes the agent to log an exception:

! java.lang.IllegalStateException: There is already a current global Tracer registered.
! at io.opentracing.util.GlobalTracer.register(GlobalTracer.java:92)
! at io.opentracing.contrib.agent.OpenTracingHelper.initTracer(OpenTracingHelper.java:80)

but it appears to work anyway.

I’ll submit a method proposal for streamlining this shortly.

[sjoerdtalsma]

Having the get unsynchronized is more important I guess.

[pavolloffay]

I will close it and open a new one for adding a registerIfAbsent(Tracer tracer)