Fix _dd.p.ksr scientific notation for very small sampling rates

[bm1549]

Description

Fix _dd.p.ksr span tag formatting for very small sampling rates. Rates below 0.001 were formatted using C's %.6g format which outputs scientific notation (e.g. 0.000001 → "1e-06"). Changed to explicit integer-level rounding via floor(rate * 1e6 + 0.5) and %.6f formatting to always produce decimal notation with up to 6 decimal digits, trailing zeros stripped.

Fixes APMAPI-1869

Related PRs:

• dd-trace-py: fix(tracing): format _dd.p.ksr with decimal notation instead of scientific notation dd-trace-py#17086
• dd-trace-rb: Fix _dd.p.ksr scientific notation for very small sampling rates dd-trace-rb#5497
• dd-trace-js: fix(tracing): format _dd.p.ksr with decimal notation instead of scientific notation dd-trace-js#7846
• system-tests: Activate _dd.p.ksr tests for all languages system-tests#6466

Testing

• Updated test description in tests/ext/priority_sampling/028-ksr-tag-formatting.phpt
• Added new test tests/ext/priority_sampling/029-ksr-tag-small-rate-formatting.phpt verifying:
  • Rate 0.000001 → "0.000001" (decimal notation, not "1e-06")
  • No scientific notation characters in output

Reviewer checklist

• Test coverage seems ok.
• Appropriate labels assigned.

[datadog-datadog-prod-us1-2]

⚠️ Tests

✨ Fix all issues with BitsAI or with Cursor

⚠️ Warnings

❄️ 10 New flaky tests detected

tmp/build_extension/tests/ext/priority_sampling/028-ksr-tag-formatting.phpt (_dd.p.ksr propagated tag formats rate with up to 6 decimal digits and no trailing zeros) from PHP.tmp.build_extension.tests.ext.priority_sampling     (Fix with Cursor)

001+ _dd.p.ksr = -
002+ is_string = false
001- _dd.p.ksr = 0.765432
002- is_string = true

tmp/build_extension/tests/ext/priority_sampling/029-ksr-tag-small-rate-formatting.phpt (_dd.p.ksr uses decimal notation (not scientific) for very small sampling rates) from PHP.tmp.build_extension.tests.ext.priority_sampling     (Fix with Cursor)

001+ _dd.p.ksr = -
001- _dd.p.ksr = 0.000001
     no_sci_notation = true

tmp/build_extension/tests/ext/priority_sampling/030-ksr-tag-rounding-boundary.phpt (_dd.p.ksr rounds small rates correctly at 6-decimal boundary) from PHP.tmp.build_extension.tests.ext.priority_sampling     (Fix with Cursor)

001+ _dd.p.ksr = -
001- _dd.p.ksr = 0.000001
     no_sci_notation = true

View all

🧪 1056 Tests failed

testSearchPhpBinaries from integration.DDTrace\Tests\Integration\PHPInstallerTest     (Fix with Cursor)

DDTrace\Tests\Integration\PHPInstallerTest::testSearchPhpBinaries
Test code or tested code printed unexpected output: Searching for available php binaries, this operation might take a while.

testSimplePushAndProcess from laravel-58-test.DDTrace\Tests\Integrations\Laravel\V5_8\QueueTest     (Fix with Cursor)

DDTrace\Tests\Integrations\Laravel\V5_8\QueueTest::testSimplePushAndProcess
Test code or tested code printed unexpected output: spanLinksTraceId: 69c3fd3e000000006ebd1f45a05e5f74
tid: 69c3fd3e00000000
hexProcessTraceId: 6ebd1f45a05e5f74
hexProcessSpanId: 57ff2a7a8b780a8b
processTraceId: 7979568498674130804
processSpanId: 6340833506175224459

phpvfscomposer://tests/vendor/phpunit/phpunit/phpunit:106

testSimplePushAndProcess from laravel-8x-test.DDTrace\Tests\Integrations\Laravel\V8_x\QueueTest     (Fix with Cursor)

DDTrace\Tests\Integrations\Laravel\V8_x\QueueTest::testSimplePushAndProcess
Test code or tested code printed unexpected output: spanLinksTraceId: 69c3fd7100000000dbd7a407e1981b36
tid: 69c3fd7100000000
hexProcessTraceId: dbd7a407e1981b36
hexProcessSpanId: df425d8e8b020e84
processTraceId: 15841310568055577398
processSpanId: 16087523685719740036

View all

ℹ️ Info

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 60.63% (+0.01%)

_{This comment will be updated automatically if new data arrives.
🔗 Commit SHA: 67b8312 | Docs | Datadog PR Page | Was this helpful? React with 👍/👎 or give us feedback!}

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.74%. Comparing base (55eb9e5) to head (67b8312).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3721      +/-   ##
==========================================
+ Coverage   68.73%   68.74%   +0.01%     
==========================================
  Files         166      166              
  Lines       19030    19030              
  Branches     1797     1797              
==========================================
+ Hits        13080    13082       +2     
+ Misses       5135     5132       -3     
- Partials      815      816       +1     

Flag	Coverage Δ
helper-rust-integration	78.82% <ø> (ø)
helper-rust-unit	49.36% <ø> (ø)

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see 1 file with indirect coverage changes

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[pr-commenter]

Benchmarks [ tracer ]

Benchmark execution time: 2026-03-25 16:29:33

Comparing candidate commit 67b8312 in PR branch brian.marks/fix-ksr-scientific-notation with baseline commit 55eb9e5 in branch master.

Found 0 performance improvements and 8 performance regressions! Performance is the same for 184 metrics, 2 unstable metrics.

scenario:ContextPropagationBench/benchInject64Bit

• 🟥 execution_time [+55.226ns; +146.774ns] or [+2.517%; +6.690%]

scenario:ContextPropagationBench/benchInject64Bit-opcache

• 🟥 execution_time [+84.368ns; +205.632ns] or [+3.877%; +9.450%]

scenario:MessagePackSerializationBench/benchMessagePackSerialization

• 🟥 execution_time [+4.041µs; +5.239µs] or [+4.128%; +5.350%]

scenario:MessagePackSerializationBench/benchMessagePackSerialization-opcache

• 🟥 execution_time [+5.844µs; +6.956µs] or [+5.745%; +6.839%]

scenario:SamplingRuleMatchingBench/benchGlobMatching2-opcache

• 🟥 execution_time [+58.089ns; +184.111ns] or [+2.245%; +7.116%]

scenario:SamplingRuleMatchingBench/benchGlobMatching3-opcache

• 🟥 execution_time [+70.363ns; +239.237ns] or [+2.588%; +8.798%]

scenario:SamplingRuleMatchingBench/benchRegexMatching1

• 🟥 execution_time [+37.049ns; +113.951ns] or [+2.531%; +7.784%]

scenario:SamplingRuleMatchingBench/benchRegexMatching2

• 🟥 execution_time [+34.800ns; +115.800ns] or [+2.342%; +7.794%]