From d6301b7c2b21d8a0faf7ff340e50e83850b24fa9 Mon Sep 17 00:00:00 2001
From: Marc Alff <marc.alff@oracle.com>
Date: Mon, 17 Nov 2025 19:18:12 +0100
Subject: [PATCH] [SDK] Reset TraceFlags::IsSampled bit on sampler
 Decision::DROP (#3745)

---
 CHANGELOG.md                                  |  3 +
 api/include/opentelemetry/trace/trace_flags.h |  2 +-
 sdk/src/trace/tracer.cc                       |  4 ++
 sdk/test/trace/tracer_test.cc                 | 71 +++++++++++++++++++
 4 files changed, 79 insertions(+), 1 deletion(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9e02d24d24..a10ab8c4d0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -45,6 +45,9 @@ Increment the:
 * [CI] Free disk space
   [#3749](https://github.com/open-telemetry/opentelemetry-cpp/pull/3749)
 
+* [SDK] Reset TraceFlags::IsSampled bit on sampler Decision::DROP
+  [#3745](https://github.com/open-telemetry/opentelemetry-cpp/pull/3745)
+
 New Features:
 
 * [CONFIGURATION] Implement declarative configuration (config.yaml)
diff --git a/api/include/opentelemetry/trace/trace_flags.h b/api/include/opentelemetry/trace/trace_flags.h
index 0ad9dc136b..f4c7bc6a44 100644
--- a/api/include/opentelemetry/trace/trace_flags.h
+++ b/api/include/opentelemetry/trace/trace_flags.h
@@ -29,7 +29,7 @@ class TraceFlags final
 
   /**
    * Valid flags in W3C Trace Context version 2.
-   * See https://www.w3.org/TR/trace-context-1/#trace-flags
+   * See https://www.w3.org/TR/trace-context-2/#trace-flags
    */
   static constexpr uint8_t kAllW3CTraceContext2Flags = kIsSampled | kIsRandom;
 
diff --git a/sdk/src/trace/tracer.cc b/sdk/src/trace/tracer.cc
index 263638955b..4a689bbcd7 100644
--- a/sdk/src/trace/tracer.cc
+++ b/sdk/src/trace/tracer.cc
@@ -116,6 +116,10 @@ nostd::shared_ptr<opentelemetry::trace::Span> Tracer::StartSpan(
   {
     flags |= opentelemetry::trace::TraceFlags::kIsSampled;
   }
+  else
+  {
+    flags &= ~opentelemetry::trace::TraceFlags::kIsSampled;
+  }
 
 #if 1
   /* https://github.com/open-telemetry/opentelemetry-specification as of v1.29.0 */
diff --git a/sdk/test/trace/tracer_test.cc b/sdk/test/trace/tracer_test.cc
index 6e70c0130e..cec1bf496d 100644
--- a/sdk/test/trace/tracer_test.cc
+++ b/sdk/test/trace/tracer_test.cc
@@ -107,6 +107,48 @@ class MockSampler final : public Sampler
   nostd::string_view GetDescription() const noexcept override { return "MockSampler"; }
 };
 
+/**
+ * A mock sampler with ShouldSample returning
+ * a decision based on the span name.
+ */
+class MockDecisionSampler final : public Sampler
+{
+public:
+  SamplingResult ShouldSample(
+      const SpanContext & /*parent_context*/,
+      trace_api::TraceId /*trace_id*/,
+      nostd::string_view name,
+      trace_api::SpanKind /*span_kind*/,
+      const opentelemetry::common::KeyValueIterable & /*attributes*/,
+      const opentelemetry::trace::SpanContextKeyValueIterable & /*links*/) noexcept override
+  {
+    std::string span_name{name};
+
+    if (span_name.find("DROP-") != std::string::npos)
+    {
+      return {Decision::DROP,
+              nostd::unique_ptr<const std::map<std::string, opentelemetry::common::AttributeValue>>(
+                  nullptr),
+              nostd::shared_ptr<opentelemetry::trace::TraceState>(nullptr)};
+    }
+
+    if (span_name.find("RECORD_ONLY-") != std::string::npos)
+    {
+      return {Decision::RECORD_ONLY,
+              nostd::unique_ptr<const std::map<std::string, opentelemetry::common::AttributeValue>>(
+                  nullptr),
+              nostd::shared_ptr<opentelemetry::trace::TraceState>(nullptr)};
+    }
+
+    return {Decision::RECORD_AND_SAMPLE,
+            nostd::unique_ptr<const std::map<std::string, opentelemetry::common::AttributeValue>>(
+                nullptr),
+            nostd::shared_ptr<opentelemetry::trace::TraceState>(nullptr)};
+  }
+
+  nostd::string_view GetDescription() const noexcept override { return "MockDecisionSampler"; }
+};
+
 /**
  * A Mock Custom ID Generator
  */
@@ -1331,3 +1373,32 @@ TEST(Tracer, SpanCleanupWithScope)
   }
   EXPECT_EQ(4, span_data->GetSpans().size());
 }
+
+TEST(Tracer, SpanSamplerDecision)
+{
+  InMemorySpanExporter *exporter              = new InMemorySpanExporter();
+  std::shared_ptr<InMemorySpanData> span_data = exporter->GetData();
+  auto tracer = initTracer(std::unique_ptr<SpanExporter>{exporter}, new MockDecisionSampler());
+  {
+    auto span0    = tracer->StartSpan("Span0");
+    auto span1    = tracer->StartSpan("span1");
+    auto context1 = span1->GetContext();
+    EXPECT_TRUE(context1.IsValid());
+    EXPECT_TRUE(context1.IsSampled());
+    {
+      trace_api::Scope scope1(span1);
+      auto span2    = tracer->StartSpan("RECORD_ONLY-span2");
+      auto context2 = span2->GetContext();
+      EXPECT_TRUE(context2.IsValid());
+      EXPECT_FALSE(context2.IsSampled());
+      {
+        trace_api::Scope scope2(span2);
+        auto span3    = tracer->StartSpan("DROP-span3");
+        auto context3 = span3->GetContext();
+        EXPECT_TRUE(context3.IsValid());
+        EXPECT_FALSE(context3.IsSampled());
+      }
+    }
+  }
+  EXPECT_EQ(3, span_data->GetSpans().size());
+}