Add active_root_span to Tracer

[delner]

Overview

To aid the traversal of active spans (for example to tag a trace with tags), this pull request introduces the active_root_span function to the Tracer, which works like the active_span function.

Usage

# e.g. adding tag to active root span
current_root_span = Datadog.tracer.active_root_span
current_root_span.set_tag('my_tag', 'my_value') unless current_root_span.nil?

[delner]

@palazzem Are we happy with this implementation? We could alternatively implement current_root_span by just using the first span on @trace, or change the criteria of if @trace.empty? to if span.parent.nil?, which might yield better results. I didn't do that in this first pass because of how context flushing might cause side effects with this logic. Let me know what you think.

[pawelchcki]

👍

[delner]

I think we need to change the criteria to include if span.parent.nil? because otherwise we might get some funny behavior with distributed traces: the second service in the distributed trace would return some non-parent span as the root, when it probably should return nil.

[palazzem]

span.parent_id is not 0 for distributed traces. In that case, people should tag the local root span (not the global one) so we should always return the root span for this context (and not for the global distributed trace).

Consider that to make it work with Trace Search & Analytics, any span can be tagged, so this "find the root span" is only a helper to retrieve the start of the request, start of the task.

[delner]

Really? You'd like the "local root span"? Even if it isn't the root span for the trace?

I thought this would have been considered a bug, since you might end up with any span on the trace (depending on how many services the distributed trace is composed of), which makes the function a bit arbitrary.

If that's what you want, I can revert these changes.

[palazzem]

Yes the helper should help finding the root span for that local trace. It's not needed for Trace Search & Analytics, but it's very helpful in most of the cases (i.e. retrieving anywhere the Rack span). It's not meant to find the "global" root span of a distributed trace, just to help people in finding root spans created by our OOTB instrumentation.

since you might end up with any span on the trace

But it will be always the first root span created in this trace / local trace, right?

[delner]

@palazzem Yes, in the sense that:

Span 1 --> Span 2 ---[Service call]---> Span 3 --> Span 4 --> #active_root_span

Will yield Span 3. So if that's what you'd like, all I have to do is revert those changes. I had considered that a bug, and made it return nil with my most recent change.

Changes made as requested.