ASGI `TraceMiddleware` does not process response headers

Summary of problem

When using ASGI TraceMiddleware the response headers are not processed into a trace.

Which version of dd-trace-py are you using?

1.5.3 but the same problem is in previous versions

How can we reproduce your problem?

• Setup a minimal ASGI application using TraceMiddleware that is responding with known a header
• add this header to the ddtrace.config.trace_headers()
• produce a trace

Produced trace has only request headers (http.request.headers.*) but no response header (http.response.headers.*).

Probable cause

I did a little digging up and found that code is using response headers directly from ASGI http.response.start message (see code). However those headers are not strings but bytes so they are not picked up ("the-header" != b"the-header"). This part requires the same treatment as request headers (see code).

I will try to create PR for it if you don't mind.

[mabdinur]

Hey @villlem,

Thanks for bringing this issue to our attention. Feel free to open a PR, I'd be happy to review it 😄

This issue will require a "fix" release note and test. Here's some docs that could help. Feel free to give it your best effort and I can fill in the gaps:

• creating a release note: https://github.com/DataDog/dd-trace-py/blob/1.x/README.md#release-notes
• Asgi test suite: https://github.com/DataDog/dd-trace-py/blob/1.x/tests/contrib/asgi/test_asgi.py
• Running tests: https://github.com/DataDog/dd-trace-py/blob/1.x/README.md#running-tests-locally

Hi @mabdinur, I created the PR. Can someone take a look if this is what you want?

[ZStriker19]

Hi @villlem the commit has been merged and is in release 1.6.1. Could you please try it out and let us know if it fixes the issue properly for you?

Everything looks fine by me. Thanks.