fix(sampling): format _dd.p.ksr to 6 decimal places, not 6 significant digits#2086
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The cross-tracer spec for _dd.p.ksr is "up to 6 decimal digits of precision,
with trailing zeros stripped" — i.e. Go's strconv.FormatFloat(rate, 'f', 6, 64).
The previous implementation rounded to 6 *significant* digits ('g', 6, 64),
which gave the right answer for every value in the original unit test but
diverged for very small rates: 0.0000001 produced "0.0000001" instead of "0",
and 0.00000051 produced "0.00000051" instead of "0.000001".
System-tests PR DataDog/system-tests#6466 added explicit cases for those two
inputs (below_precision_rounds_to_zero, rounds_up_to_one_millionth), which
started failing for dd-trace-rs once it bumped to a libdatadog containing
this code (PR DataDog/dd-trace-rs#180, now living here after the sampling
move-out refactor).
Replace the bespoke significant-digit rounding with a plain `{rate:.6}` format
followed by trailing-zero strip; this gives the right rounding for free
(including the "0" case, since "0.000000" trims down to "0").
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
## main #2086 +/- ##
==========================================
- Coverage 73.47% 73.47% -0.01%
==========================================
Files 475 475
Lines 78999 78992 -7
==========================================
- Hits 58048 58042 -6
+ Misses 20951 20950 -1
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Jun 5, 2026
# Release proposal for libdd-sampling and its dependencies This PR contains version bumps based on public API changes and commits since last release. ## libdd-trace-utils **Next version:** `7.0.0` **Semver bump:** `major` **Tag:** `libdd-trace-utils-v7.0.0` ### Commits - feat(trace-utils)!: add dedup convenience to VecMap (#2049) - fix(trace-utils): match the Go trace agent when parsing `datadog-client-computed-*` bool headers (#2071) - revert!: add from_string to span text (#2011) (#2073) - fix(send_with_retry): follow max retries of the strategy (#2047) ## libdd-sampling (manually bumped due to breaking in #2073) **Next version:** ~~`2.1.1`~~ `3.0.0` **Semver bump:** ~~`patch`~~ `major` **Tag:** ~~`libdd-sampling-v2.1.1`~~ `libdd-sampling-v3.0.0` ### Commits - fix(sampling): format _dd.p.ksr to 6 decimal places, not 6 significant digits (#2086) - revert!: add from_string to span text (#2011) (#2073) --------- Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com> Co-authored-by: iunanua <18325288+iunanua@users.noreply.github.com> Co-authored-by: iunanua <igor.unanua@datadoghq.com>
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What does this PR do?
The cross-tracer spec for _dd.p.ksr is "up to 6 decimal digits of precision, with trailing zeros stripped" — i.e. Go's
strconv.FormatFloat(rate, 'f', 6, 64). The previous implementation rounded to 6 significant digits ('g', 6, 64), which gave the right answer for every value in the original unit test but diverged for very small rates: 0.0000001 produced "0.0000001" instead of "0", and 0.00000051 produced "0.00000051" instead of "0.000001".System-tests PR DataDog/system-tests#6466 added explicit cases for those two inputs (below_precision_rounds_to_zero, rounds_up_to_one_millionth), which started failing for dd-trace-rs once it bumped to a libdatadog containing this code (PR DataDog/dd-trace-rs#180, now living here after the sampling move-out refactor).
Replace the bespoke significant-digit rounding with a plain
{rate:.6}format followed by trailing-zero strip; this gives the right rounding for free (including the "0" case, since "0.000000" trims down to "0").