fix(tracing): format _dd.p.ksr with decimal notation instead of scientific notation

ddtrace/internal/sampling.py

@@ -1,4 +1,5 @@
 import json
+import math
 from typing import Any
 from typing import Optional
 from typing import TypedDict
@@ -46,6 +47,17 @@ class PriorityCategory(object):
 
 KNUTH_SAMPLE_RATE_KEY = "_dd.p.ksr"
 
+
+def _format_ksr(rate: float) -> str:
+    """Format a sampling rate for _dd.p.ksr: up to 6 decimal digits, trailing zeros stripped.
+
+    Uses explicit rounding via math.floor(x + 0.5) to avoid Python's banker's rounding
+    which would round 0.0000005 down to 0 instead of up to 0.000001.
+    """
+    rounded = math.floor(rate * 1e6 + 0.5) / 1e6
+    return f"{rounded:.6f}".rstrip("0").rstrip(".")
+
+
 SpanSamplingRules = TypedDict(
     "SpanSamplingRules",
     {
@@ -243,10 +255,10 @@ def _set_sampling_tags(span: Span, sampled: bool, sample_rate: float, mechanism:
         SamplingMechanism.REMOTE_DYNAMIC_TRACE_SAMPLING_RULE,
     ):
         span._set_attribute(_SAMPLING_RULE_DECISION, sample_rate)
-        span._set_attribute(KNUTH_SAMPLE_RATE_KEY, f"{sample_rate:.6g}")
+        span._set_attribute(KNUTH_SAMPLE_RATE_KEY, _format_ksr(sample_rate))
     elif mechanism == SamplingMechanism.AGENT_RATE_BY_SERVICE:
         span._set_attribute(_SAMPLING_AGENT_DECISION, sample_rate)
-        span._set_attribute(KNUTH_SAMPLE_RATE_KEY, f"{sample_rate:.6g}")
+        span._set_attribute(KNUTH_SAMPLE_RATE_KEY, _format_ksr(sample_rate))
     # Set the sampling priority
     priorities = SAMPLING_MECHANISM_TO_PRIORITIES[mechanism]
     priority_index = _KEEP_PRIORITY_INDEX if sampled else _REJECT_PRIORITY_INDEX

releasenotes/notes/fix-ksr-scientific-notation-e7d1adabe548eb40.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    tracing: Fix ``_dd.p.ksr`` span tag formatting for very small sampling rates.
+    Previously, rates below 0.001 could be output in scientific notation (e.g.
+    ``1e-06``). Now always uses decimal notation with up to 6 decimal digits.

tests/tracer/test_sampler.py

@@ -889,3 +889,32 @@ def span():
 def test_trace_tag(span, sampling_mechanism, expected):
     span._set_sampling_decision_maker(sampling_mechanism)
     assert span.context._meta["_dd.p.dm"] == expected
+
+
+@pytest.mark.parametrize(
+    "sample_rate,expected_ksr",
+    [
+        (1.0, "1"),
+        (0.5, "0.5"),
+        (0.000001, "0.000001"),
+        (0.0000001, "0"),
+        (0.0000005, "0.000001"),
+        (0.7654321, "0.765432"),
+    ],
+    ids=[
+        "rate_1_strips_trailing_zeros",
+        "simple_rate",
+        "six_decimal_precision_boundary",
+        "below_precision_rounds_to_zero",
+        "rounds_up_to_one_millionth",
+        "truncation_at_six_decimals",
+    ],
+)
+def test_ksr_formatting(span, sample_rate, expected_ksr):
+    """_dd.p.ksr is formatted with up to 6 decimal digits, trailing zeros stripped, no scientific notation."""
+    from ddtrace.internal.sampling import KNUTH_SAMPLE_RATE_KEY
+    from ddtrace.internal.sampling import SamplingMechanism
+    from ddtrace.internal.sampling import _set_sampling_tags
+
+    _set_sampling_tags(span, True, sample_rate, SamplingMechanism.LOCAL_USER_TRACE_SAMPLING_RULE)
+    assert span._meta.get(KNUTH_SAMPLE_RATE_KEY) == expected_ksr