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Fix _dd.p.ksr formatting to use 6 significant digits without trailing zeros #288

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Fix _dd.p.ksr formatting to use 6 significant digits without trailing zeros #288

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092e49e
Fix _dd.p.ksr formatting to use 6 significant digits without trailing…
bm1549 Mar 11, 2026
28fee0b
Fix clang-format violation in trace_segment.cpp
bm1549 Mar 11, 2026
ef4d91b
Do not set _dd.p.ksr when using the DEFAULT sampling mechanism
bm1549 Mar 11, 2026
913ad91
Replace snprintf with std::to_chars for _dd.p.ksr formatting
bm1549 Mar 11, 2026
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16 changes: 13 additions & 3 deletions src/datadog/trace_segment.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,7 @@
#include <datadog/telemetry/telemetry.h>
#include <datadog/trace_segment.h>

#include <array>
#include <cassert>
#include <charconv>
#include <string>
Expand Down Expand Up @@ -321,9 +322,18 @@ void TraceSegment::make_sampling_decision_if_null() {

update_decision_maker_trace_tag();

trace_tags_.emplace_back(
tags::internal::ksr,
std::to_string(*sampling_decision_->configured_rate));
// Only set ksr when the sampling mechanism is explicit (agent rate, rule, or
// remote rule). The DEFAULT mechanism means we haven't received any
// configuration from the agent yet, so ksr would be meaningless.
if (sampling_decision_->mechanism &&
*sampling_decision_->mechanism != int(SamplingMechanism::DEFAULT)) {
std::array<char, 8> buf;
const auto [ptr, ec] = std::to_chars(buf.data(), buf.data() + buf.size(),
*sampling_decision_->configured_rate,
std::chars_format::general, 6);
assert(ec == std::errc());
trace_tags_.emplace_back(tags::internal::ksr, std::string(buf.data(), ptr));
}
}

void TraceSegment::update_decision_maker_trace_tag() {
Expand Down
16 changes: 8 additions & 8 deletions test/test_span.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -784,8 +784,8 @@ TEST_SPAN("injecting W3C tracestate header") {
{"x-datadog-parent-id", "1"},
{"x-datadog-origin", "France"},
},
// The "s:-1" and "t.ksr:0.000000" comes from the 0% sample rate.
"dd=s:-1;p:$parent_id;o:France;t.ksr:0.000000"},
// The "s:-1" and "t.ksr:0" comes from the 0% sample rate.
"dd=s:-1;p:$parent_id;o:France;t.ksr:0"},

{__LINE__,
"trace tags",
Expand All @@ -794,8 +794,8 @@ TEST_SPAN("injecting W3C tracestate header") {
{"x-datadog-parent-id", "1"},
{"x-datadog-tags", "_dd.p.foo=x,_dd.p.bar=y,ignored=wrong_prefix"},
},
// The "s:-1" and "t.ksr:0.000000" comes from the 0% sample rate.
"dd=s:-1;p:$parent_id;t.foo:x;t.bar:y;t.ksr:0.000000"},
// The "s:-1" and "t.ksr:0" comes from the 0% sample rate.
"dd=s:-1;p:$parent_id;t.foo:x;t.bar:y;t.ksr:0"},

{__LINE__,
"extra fields",
Expand Down Expand Up @@ -825,7 +825,7 @@ TEST_SPAN("injecting W3C tracestate header") {
},
// The "s:-1" comes from the 0% sample rate.
"dd=s:-1;p:$parent_id;o:France_ is a country~nation_ so is "
"______.;t.ksr:0.000000",
"______.;t.ksr:0",
},

{__LINE__,
Expand All @@ -836,7 +836,7 @@ TEST_SPAN("injecting W3C tracestate header") {
{"x-datadog-tags", "_dd.p.a;d台北x =foo,_dd.p.ok=bar"},
},
// The "s:-1" comes from the 0% sample rate.
"dd=s:-1;p:$parent_id;t.a_d______x_:foo;t.ok:bar;t.ksr:0.000000"},
"dd=s:-1;p:$parent_id;t.a_d______x_:foo;t.ok:bar;t.ksr:0"},

{__LINE__,
"replace invalid characters in trace tag value",
Expand All @@ -847,7 +847,7 @@ TEST_SPAN("injecting W3C tracestate header") {
},
// The "s:-1" comes from the 0% sample rate.
"dd=s:-1;p:$parent_id;t.wacky:hello fr_d_ how are "
"_________?;t.ksr:0.000000"},
"_________?;t.ksr:0"},

{__LINE__,
"replace equal signs with tildes in trace tag value",
Expand All @@ -857,7 +857,7 @@ TEST_SPAN("injecting W3C tracestate header") {
{"x-datadog-tags", "_dd.p.base64_thingy=d2Fra2EhIHdhaw=="},
},
// The "s:-1" comes from the 0% sample rate.
"dd=s:-1;p:$parent_id;t.base64_thingy:d2Fra2EhIHdhaw~~;t.ksr:0.000000"},
"dd=s:-1;p:$parent_id;t.base64_thingy:d2Fra2EhIHdhaw~~;t.ksr:0"},

{__LINE__,
"oversized origin truncates it and subsequent fields",
Expand Down
20 changes: 15 additions & 5 deletions test/test_trace_segment.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,9 @@
#include <datadog/tracer.h>
#include <datadog/tracer_config.h>

#include <cstdio>
#include <regex>
#include <string>
#include <vector>

#include "matchers.h"
Expand Down Expand Up @@ -310,7 +312,9 @@ TEST_CASE("TraceSegment finalization of spans") {
REQUIRE_THAT(span.tags, ContainsSubset(filtered));
// "_dd.p.dm" will be added, because we made a sampling decision.
REQUIRE(span.tags.count("_dd.p.dm") == 1);
REQUIRE(span.tags.count("_dd.p.ksr") == 1);
// "_dd.p.ksr" is NOT set because this uses the DEFAULT mechanism (no
// agent configuration received yet).
REQUIRE(span.tags.count("_dd.p.ksr") == 0);
}

SECTION("rate tags") {
Expand All @@ -325,7 +329,8 @@ TEST_CASE("TraceSegment finalization of spans") {
REQUIRE(collector->span_count() == 1);
const auto& span = collector->first_span();
REQUIRE(span.numeric_tags.at(tags::internal::agent_sample_rate) == 1.0);
REQUIRE(span.tags.at(tags::internal::ksr) == "1.000000");
// ksr is NOT set for the DEFAULT mechanism.
REQUIRE(span.tags.count(tags::internal::ksr) == 0);
}

SECTION(
Expand All @@ -348,7 +353,8 @@ TEST_CASE("TraceSegment finalization of spans") {
{
REQUIRE(collector_response->span_count() == 1);
const auto& span = collector_response->first_span();
CHECK(span.tags.at(tags::internal::ksr) == "1.000000");
// ksr is NOT set for the DEFAULT mechanism.
CHECK(span.tags.count(tags::internal::ksr) == 0);
}

collector_response->chunks.clear();
Expand All @@ -361,7 +367,7 @@ TEST_CASE("TraceSegment finalization of spans") {
REQUIRE(collector_response->span_count() == 1);
const auto& span = collector_response->first_span();
CHECK(span.numeric_tags.at(tags::internal::agent_sample_rate) == 1.0);
CHECK(span.tags.at(tags::internal::ksr) == "1.000000");
CHECK(span.tags.at(tags::internal::ksr) == "1");
}
}

Expand Down Expand Up @@ -392,7 +398,11 @@ TEST_CASE("TraceSegment finalization of spans") {
const auto& span = collector->first_span();
REQUIRE(span.numeric_tags.at(tags::internal::rule_sample_rate) ==
sample_rate);
CHECK(span.tags.at(tags::internal::ksr) == std::to_string(sample_rate));
{
char buf[32];
std::snprintf(buf, sizeof(buf), "%.6g", sample_rate);
CHECK(span.tags.at(tags::internal::ksr) == std::string(buf));
}
if (sample_rate == 1.0) {
REQUIRE(span.numeric_tags.at(
tags::internal::rule_limiter_sample_rate) == 1.0);
Expand Down
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