fix(asgi): process response header (#4443) (#4504) (#4543)

When using ASGI middleware response headers were not processed.

Co-authored-by: Vojtech Orgon <[email protected]>
Co-authored-by: Kyle Verhoog <[email protected]>
Co-authored-by: Brett Langdon <[email protected]>
Co-authored-by: Brett Langdon <[email protected]>
(cherry picked from commit d1881e4)

Co-authored-by: Vojta Orgoň <[email protected]>

Author: mergify[bot]

ddtrace/contrib/asgi/middleware.py

@@ -177,9 +177,10 @@ async def wrapped_send(message):
             else:
                 status_code = None
 
-            if "headers" in message:
-                response_headers = message["headers"]
-            else:
+            try:
+                response_headers = _extract_headers(message)
+            except Exception:
+                log.warning("failed to extract response headers", exc_info=True)
                 response_headers = None
 
             trace_utils.set_http_meta(

releasenotes/notes/Fix-for-ASGI-middleware-not-processing-response-headers-bc117dd67210ccb8.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    ASGI: response headers are correctly processed instead of ignored

tests/contrib/asgi/test_asgi.py

@@ -542,3 +542,16 @@ async def test_host_header(scope, tracer, test_spans, host):
         assert test_spans.spans
         request_span = test_spans.spans[0]
         assert request_span.get_tag("http.url") == "http://%s/" % (host,)
+
+
+@pytest.mark.asyncio
+async def test_response_headers(scope, tracer, test_spans):
+    with override_http_config("asgi", {"trace_headers": ["content-type"]}):
+        app = TraceMiddleware(basic_app, tracer=tracer)
+        async with httpx.AsyncClient(app=app) as client:
+            response = await client.get("http://testserver/")
+            assert response.status_code == 200
+
+            assert test_spans.spans
+            request_span = test_spans.spans[0]
+            assert "http.response.headers.content-type" in request_span.get_tags().keys()