fix(sampling): validate_sampling_decision giving log warns on supported mechanisms (#13554)

#13516

https://gitlab.ddbuild.io/DataDog/apm-reliability/dd-trace-py/-/jobs/961877338#L460
=========================== short test summary info
============================
FAILED
tests/integration/test_sampling.py::test_supported_sampling_mechanism -
AssertionError: DATA_JOBS_MONITORING returned {'_dd.propagation_error':
'decoding_error'}
assert {'_dd.propagation_error': 'decoding_error'} !=
{'_dd.propagation_error': 'decoding_error'}
====== 1 failed, 104 passed, 28 skipped, 6 xpassed, 14 warnings in
26.31s ======
Test failed with exit code 1

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- The change includes or references documentation updates if necessary
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[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

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changes
- Testing strategy adequately addresses listed risks
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---------

Co-authored-by: Emmett Butler <[email protected]>
(cherry picked from commit 733711e)

Author: 2 people

ddtrace/internal/sampling.py

@@ -1,5 +1,4 @@
 import json
-import re
 from typing import TYPE_CHECKING  # noqa:F401
 from typing import Any
 from typing import Dict
@@ -71,9 +70,9 @@ class PriorityCategory(object):
     RULE_DYNAMIC = "rule_dynamic"
 
 
-# Use regex to validate trace tag value
-TRACE_TAG_RE = re.compile(r"^-([0-9])$")
-
+SAMPLING_MECHANISM_CONSTANTS = {
+    "-{}".format(value) for name, value in vars(SamplingMechanism).items() if name.isupper()
+}
 
 SpanSamplingRules = TypedDict(
     "SpanSamplingRules",
@@ -93,7 +92,7 @@ def validate_sampling_decision(
     value = meta.get(SAMPLING_DECISION_TRACE_TAG_KEY)
     if value:
         # Skip propagating invalid sampling mechanism trace tag
-        if TRACE_TAG_RE.match(value) is None:
+        if value not in SAMPLING_MECHANISM_CONSTANTS:
             del meta[SAMPLING_DECISION_TRACE_TAG_KEY]
             meta["_dd.propagation_error"] = "decoding_error"
             log.warning("failed to decode _dd.p.dm: %r", value)

releasenotes/notes/fix-validate-sampling-decision-logging-561e2ef1c0156b5e.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - | 
+    tracing: This fix resolves an issue where the library fails to decode a supported sampling mechanism, resulting in the log line: "failed to decode _dd.p.dm: ..."

tests/integration/test_sampling.py

@@ -92,6 +92,52 @@ def test_sampling_with_default_sample_rate_1_and_manual_drop(writer, tracer):
             span.set_tag(MANUAL_DROP_KEY)
 
 
+def test_supported_sampling_mechanism():
+    """
+    validate_sampling_decision should not give errors for supported sampling mechanisms
+    """
+    from ddtrace.internal.constants import SAMPLING_DECISION_TRACE_TAG_KEY
+    from ddtrace.internal.constants import SamplingMechanism
+    from ddtrace.internal.sampling import validate_sampling_decision
+
+    # This list can grow over time so we should test all of them
+    supported_mechanisms = {
+        name: getattr(SamplingMechanism, name) for name in dir(SamplingMechanism) if not name.startswith("_")
+    }
+
+    # The mechanisms we support should NOT return a decoding error
+    for mechanism, value in supported_mechanisms.items():
+        sampling_decision_validation = None
+        meta = {}
+        sampling_dm_value = "-" + str(value)
+        meta = {SAMPLING_DECISION_TRACE_TAG_KEY: sampling_dm_value}
+        sampling_decision_validation = validate_sampling_decision(meta)
+        decoding_error_result = {"_dd.propagation_error": "decoding_error"}
+        assert sampling_decision_validation != decoding_error_result, f"{mechanism} returned {decoding_error_result}"
+
+
+def test_unsupported_sampling_mechanism():
+    """
+    Unsupported sampling mechanisms actually return a decoding error in validate_sampling_decision
+    """
+    from ddtrace.internal.constants import SAMPLING_DECISION_TRACE_TAG_KEY
+    from ddtrace.internal.sampling import validate_sampling_decision
+
+    meta = {SAMPLING_DECISION_TRACE_TAG_KEY: "-999999999999"}
+    sampling_decision_validation = validate_sampling_decision(meta)
+    decoding_error_result = {"_dd.propagation_error": "decoding_error"}
+    assert (
+        sampling_decision_validation == decoding_error_result
+    ), f"Instead of getting {decoding_error_result}, received {sampling_decision_validation}"
+
+
+@pytest.mark.snapshot()
+@pytest.mark.subprocess(env={"DD_TRACE_SAMPLING_RULES": json.dumps([{"sample_rate": 0, "resource": RESOURCE}])})
+def test_extended_sampling_resource():
+    from ddtrace.trace import tracer
+    from tests.integration.test_sampling import RESOURCE
+
+
 @snapshot_parametrized_with_writers
 def test_sampling_with_default_sample_rate_1_and_manual_keep(writer, tracer):
     sampler = DatadogSampler(default_sample_rate=1)

tests/tracer/test_propagation.py

@@ -926,7 +926,7 @@ def test_extract_unicode(tracer):  # noqa: F811
         ("_dd.p.dm=-", {"_dd.propagation_error": "decoding_error"}),
         ("_dd.p.dm=--1", {"_dd.propagation_error": "decoding_error"}),
         ("_dd.p.dm=-1.0", {"_dd.propagation_error": "decoding_error"}),
-        ("_dd.p.dm=-10", {"_dd.propagation_error": "decoding_error"}),
+        ("_dd.p.dm=-13", {"_dd.propagation_error": "decoding_error"}),
     ],
 )
 def test_extract_dm(x_datadog_tags, expected_trace_tags):