fix(propagation): never clobber extracted decisionmaker tags... [backport 2.7] (#8719)

This pull request resolves [AIT-9945] by removing the condition gating
the logic that avoids clobbering the `_dd.p.dm` tag when it's present on
propagated headers.

## Checklist

- [x] Change(s) are motivated and described in the PR description

[AIT-9945]:
https://datadoghq.atlassian.net/browse/AIT-9945?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: emmettbutler

ddtrace/propagation/http.py

@@ -322,9 +322,10 @@ def _extract(headers):
                 del meta[_HIGHER_ORDER_TRACE_ID_BITS]
                 log.warning("malformed_tid: %s. Failed to decode trace id from http headers", trace_id_hob_hex)
 
-        if sampling_priority == USER_KEEP:
-            if not meta:
-                meta = {}
+        if not meta:
+            meta = {}
+
+        if not meta.get(SAMPLING_DECISION_TRACE_TAG_KEY):
             meta[SAMPLING_DECISION_TRACE_TAG_KEY] = f"-{SamplingMechanism.TRACE_SAMPLING_RULE}"
 
         # Try to parse values into their expected types

releasenotes/notes/clobber-dm-b494dc48a3217371.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    propagation: This fix resolves an issue where the sampling decision-maker tag in tracestate propagation headers
+    was clobbered by a default value.

tests/tracer/test_propagation.py

@@ -286,6 +286,7 @@ def test_extract(tracer):  # noqa: F811
         assert span.context.dd_origin == "synthetics"
         assert span.context._meta == {
             "_dd.origin": "synthetics",
+            "_dd.p.dm": "-3",
             "_dd.p.test": "value",
         }
         with tracer.trace("child_span") as child_span:
@@ -295,6 +296,7 @@ def test_extract(tracer):  # noqa: F811
             assert child_span.context.dd_origin == "synthetics"
             assert child_span.context._meta == {
                 "_dd.origin": "synthetics",
+                "_dd.p.dm": "-3",
                 "_dd.p.test": "value",
             }
 
@@ -498,6 +500,7 @@ def test_extract_unicode(tracer):  # noqa: F811
 
         assert span.context._meta == {
             "_dd.origin": "synthetics",
+            "_dd.p.dm": "-3",
             "_dd.p.test": "value",
         }
         with tracer.trace("child_span") as child_span:
@@ -507,6 +510,7 @@ def test_extract_unicode(tracer):  # noqa: F811
             assert child_span.context.dd_origin == "synthetics"
             assert child_span.context._meta == {
                 "_dd.origin": "synthetics",
+                "_dd.p.dm": "-3",
                 "_dd.p.test": "value",
             }
 
@@ -560,6 +564,7 @@ def test_WSGI_extract(tracer):  # noqa: F811
         assert span.context._meta == {
             "_dd.origin": "synthetics",
             "_dd.p.test": "value",
+            "_dd.p.dm": "-3",
         }
 
 
@@ -583,6 +588,7 @@ def test_extract_invalid_tags(tracer):  # noqa: F811
         assert span.context.dd_origin == "synthetics"
         assert span.context._meta == {
             "_dd.origin": "synthetics",
+            "_dd.p.dm": "-3",
             "_dd.propagation_error": "decoding_error",
         }
 
@@ -607,6 +613,7 @@ def test_extract_tags_large(tracer):  # noqa: F811
         assert span.context.dd_origin == "synthetics"
         assert span.context._meta == {
             "_dd.origin": "synthetics",
+            "_dd.p.dm": "-3",
             "_dd.propagation_error": "extract_max_size",
         }
 
@@ -1180,6 +1187,7 @@ def test_extract_tracecontext(headers, expected_context):
             "span_id": 5678,
             "sampling_priority": 1,
             "dd_origin": "synthetics",
+            "meta": {"_dd.p.dm": "-3"},
         },
     ),
     (
@@ -1191,6 +1199,7 @@ def test_extract_tracecontext(headers, expected_context):
             "span_id": 5678,
             "sampling_priority": 1,
             "dd_origin": "synthetics",
+            "meta": {"_dd.p.dm": "-3"},
         },
     ),
     (
@@ -1220,6 +1229,7 @@ def test_extract_tracecontext(headers, expected_context):
             "span_id": 5678,
             "sampling_priority": 1,
             "dd_origin": "synthetics",
+            "meta": {"_dd.p.dm": "-3"},
         },
     ),
     (
@@ -1242,6 +1252,7 @@ def test_extract_tracecontext(headers, expected_context):
             "span_id": 5678,
             "sampling_priority": 1,
             "dd_origin": "synthetics",
+            "meta": {"_dd.p.dm": "-3"},
         },
     ),
     (
@@ -1253,6 +1264,7 @@ def test_extract_tracecontext(headers, expected_context):
             "span_id": 5678,
             "sampling_priority": 1,
             "dd_origin": "synthetics",
+            "meta": {"_dd.p.dm": "-3"},
         },
     ),
     (
@@ -1485,6 +1497,7 @@ def test_extract_tracecontext(headers, expected_context):
             "span_id": 5678,
             "sampling_priority": 1,
             "dd_origin": "synthetics",
+            "meta": {"_dd.p.dm": "-3"},
             "span_links": [
                 SpanLink(
                     trace_id=TRACE_ID,
@@ -1512,6 +1525,7 @@ def test_extract_tracecontext(headers, expected_context):
             "span_id": 5678,
             "sampling_priority": 1,
             "dd_origin": "synthetics",
+            "meta": {"_dd.p.dm": "-3"},
             "span_links": [
                 SpanLink(
                     trace_id=TRACE_ID,
@@ -1551,6 +1565,7 @@ def test_extract_tracecontext(headers, expected_context):
             "span_id": 5678,
             "sampling_priority": 1,
             "dd_origin": "synthetics",
+            "meta": {"_dd.p.dm": "-3"},
             "span_links": [
                 SpanLink(
                     trace_id=TRACE_ID,
@@ -1585,6 +1600,7 @@ def test_extract_tracecontext(headers, expected_context):
             "span_id": 5678,
             "sampling_priority": 1,
             "dd_origin": "synthetics",
+            "meta": {"_dd.p.dm": "-3"},
         },
     ),
     (
@@ -1596,6 +1612,7 @@ def test_extract_tracecontext(headers, expected_context):
             "span_id": 5678,
             "sampling_priority": 1,
             "dd_origin": "synthetics",
+            "meta": {"_dd.p.dm": "-3"},
         },
     ),
     (
@@ -1664,6 +1681,7 @@ def test_extract_tracecontext(headers, expected_context):
             "span_id": 5678,
             "sampling_priority": 1,
             "dd_origin": "synthetics",
+            "meta": {"_dd.p.dm": "-3"},
         },
     ),
     # Testing that order matters
@@ -1732,7 +1750,7 @@ def test_extract_tracecontext(headers, expected_context):
             "span_id": 5678,
             "sampling_priority": 1,
             "dd_origin": "synthetics",
-            "meta": {"tracestate": TRACECONTEXT_HEADERS_VALID[_HTTP_HEADER_TRACESTATE]},
+            "meta": {"tracestate": TRACECONTEXT_HEADERS_VALID[_HTTP_HEADER_TRACESTATE], "_dd.p.dm": "-3"},
         },
     ),
     # testing that tracestate is not added when tracecontext style comes later and does not match first style's trace-id
@@ -1745,6 +1763,7 @@ def test_extract_tracecontext(headers, expected_context):
             "span_id": 5678,
             "sampling_priority": 1,
             "dd_origin": "synthetics",
+            "meta": {"_dd.p.dm": "-3"},
             "span_links": [
                 SpanLink(
                     trace_id=TRACE_ID,
@@ -2045,7 +2064,7 @@ def test_DD_TRACE_PROPAGATION_STYLE_EXTRACT_overrides_DD_TRACE_PROPAGATION_STYLE
         Context(
             trace_id=13088165645273925489,
             span_id=5678,
-            meta={"_dd.origin": "synthetics"},
+            meta={"_dd.origin": "synthetics", "_dd.p.dm": "-3"},
             metrics={"_sampling_priority_v1": 1},
             span_links=[
                 SpanLink(
@@ -2088,7 +2107,14 @@ def test_DD_TRACE_PROPAGATION_STYLE_EXTRACT_overrides_DD_TRACE_PROPAGATION_STYLE
         Context(
             trace_id=7277407061855694839,
             span_id=5678,
-            meta={"_dd.origin": "synthetics", "tracestate": "dd=s:2;o:rum;t.dm:-4;t.usr.id:baz64,congo=t61rcWkgMzE"},
+            # it's weird that both _dd.p.dm and tracestate.t.dm are set here. as far as i know, this is the expected
+            # behavior for this chaotic set of headers, specifically when STYLE_DATADOG precedes STYLE_W3C_TRACECONTEXT
+            # in the styles configuration
+            meta={
+                "_dd.p.dm": "-3",
+                "_dd.origin": "synthetics",
+                "tracestate": "dd=s:2;o:rum;t.dm:-4;t.usr.id:baz64,congo=t61rcWkgMzE",
+            },
             metrics={"_sampling_priority_v1": 1},
             span_links=[
                 SpanLink(
@@ -2105,7 +2131,7 @@ def test_DD_TRACE_PROPAGATION_STYLE_EXTRACT_overrides_DD_TRACE_PROPAGATION_STYLE
 
 
 @pytest.mark.parametrize("name,styles,headers,expected_context", FULL_CONTEXT_EXTRACT_FIXTURES)
-def test_mutliple_context_interactions(name, styles, headers, expected_context):
+def test_multiple_context_interactions(name, styles, headers, expected_context):
     with override_global_config(dict(_propagation_style_extract=styles)):
         context = HTTPPropagator.extract(headers)
         assert context == expected_context