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

## Checklist
- [x] PR author has checked that all the criteria below are met
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- The PR description articulates the motivation for the change
- The change includes tests OR the PR description describes a testing
strategy
- The PR description notes risks associated with the change, if any
- Newly-added code is easy to change
- The change follows the [library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/releasenotes.html)
- The change includes or references documentation updates if necessary
- Backport labels are set (if
[applicable](https://ddtrace.readthedocs.io/en/latest/contributing.html#backporting))

## Reviewer Checklist
- [x] Reviewer has checked that all the criteria below are met 
- Title is accurate
- All changes are related to the pull request's stated goal
- Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes
- Testing strategy adequately addresses listed risks
- Newly-added code is easy to change
- Release note makes sense to a user of the library
- If necessary, author has acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment
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[release branch maintenance
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---------

Co-authored-by: Emmett Butler <[email protected]>

Author: bqbits

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
@@ -57,9 +56,9 @@ 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",
@@ -79,7 +78,7 @@ 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

@@ -33,6 +33,45 @@ def test_sampling_with_rate_limit_3():
         tracer.trace("child").finish()
 
 
+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():

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):