Errors when x-datadog-sampling-priority header is present but x-datadog-tags is not

[hiddewie]

Nginx version: 1.23.1
Datadog opentracing version: 1.3.4 (issue is also present in 1.3.3, but not in 1.3.2)
Nginx opentracing version: 0.26.0

Nginx server config

server {
  listen 9000 default_server;

  opentracing on;
  opentracing_trace_locations off;
  opentracing_tag http_user_agent $http_user_agent;
  opentracing_operation_name nginx.request;
  opentracing_propagate_context;

  location / {
    # proxy ...
  }
}

When the request contains the headers

x-datadog-parent-id: 628590525536830299
x-datadog-sampling-priority: 0
x-datadog-trace-id: 8761565436669190370

the following error is emitted

2022/09/06 08:33:46 [error] 102#102: *17 no opentracing context value found for span context key x_datadog_tags for request 00007FF9B25D2450, client: 127.0.0.1, server: , request: "GET / HTTP/1.1", host: "localhost:9000"

If I add the header

x-datadog-tags: a=b

the nginx error is not emitted anymore.

I suspect the pull request #230

[sanglt]

Yes we have the same issue and need to rollback to v1.3.2

[dgoffredo]

Thanks for reporting this.

I probably need a branch like this for x-datadog-tags:

dd-opentracing-cpp/src/span_context.cpp

Lines 391 to 397 in 9b9248f

	} else if (nginx_opentracing_compatibility_hack_) {
	// See the comment in the header file on nginx_opentracing_compatibility_hack_.
	result = writer.Set(headers_impl.sampling_priority_header, "1");
	if (!result) {
	return result;
	}
	}

. nginx-opentracing needs to know in advance which HTTP headers we might add to the upstream request. It does this by pretending to serialize some trace context, and then inspects the headers written. The tracer is expected to detect this pretend serialization and write all tags that it ever might. The issue probably is that I didn't add x-datadog-tags there.

Our nginx integration test didn't catch this. I'll play with it and propose a fix.

[dgoffredo]

#242

[hiddewie]

Thanks for looking into this quickly!

[dgoffredo]

https://github.com/DataDog/dd-opentracing-cpp/releases/tag/v1.3.5