W3C propagates the wrong parent spanID

[pnerg]

Looks like the W3C context propagation uses a new spanID as the parent.

Did a small test app that received a request and forwarded it again to itself just to see what headers are propagated.
So the flow is postman -> app (/forward) -> app (/hello) and response all the way back in the chain

Test 1 - Sent traceparent header to app

Sent the header traceparent: Some(00-bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb-0000000000000000-01)
The printout is like

Got request for '/forward' with w3c header [Some(00-bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb-0000000000000000-01)] and parsed/generated context traceId[bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb], spanId[b4e99c809b29b7fa], parentSpanId[0000000000000000]
Got request for '/hello' with w3c header [Some(00-BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB-A4B5DFB07D82F430-01)] and parsed/generated context traceId[], spanId[230e25c602c0b2bc], parentSpanId[]

We see from the printout that the W3C propagation has read the headers and created a new local spanID (b4e99c809b29b7fa) as expected.
Trace and parent span identifiers are set in the local context.
However when the request is sent to the next part (/hello) we can see that the header is an odd parentSpanID (A4B5DFB07D82F430). This should have been 'b4e99c809b29b7fa'

The "empty" trace/span ids in the log is due to #985

Test 2 - No headers are sent to the app

The printout is like

Got request for '/forward' with w3c header [None] and parsed/generated context traceId[1aaf4ba3617d76f2441f0fc3879a1a3f], spanId[b16fd11b8393437c], parentSpanId[]
Got request for '/hello' with w3c header [Some(00-1AAF4BA3617D76F2441F0FC3879A1A3F-45B2C2A25AE39FB3-00)] and parsed/generated context traceId[], spanId[a547524af1953855], parentSpanId[]

Same situation, we see a new context is created with trace/span identifiers. But again when the context is propagated we see the sent parentId is '45B2C2A25AE39FB3' but the span that sent the request was 'b16fd11b8393437c'

Conclusion

I'm assuming a new spanID has been created before passing the context down to the
writer operation
The code simply writes the header as:

s"$Version-${idToHex(parent.trace.id, 32)}-${idToHex(parent.id, 16)}-${samplingDecision}"

I'm guessing the parent.id is a newly created spanID and not the current one doing the actual HTTP request.
This is the difference between B3 and W3C propagation, the sending side in B3 allocates a new SpanId and sends over.
W3C only sends the parentID and lets the receiving end deal with creating the new SpanID.
Guessing this leaks into the propagation by allocating a new SpanId before providing the context to the writer operation.

[SimunKaracic]

Fixed in 2.1.16!