From 71af6bc7f435eff9d45ad86d66dcd6b90e0c695f Mon Sep 17 00:00:00 2001
From: Benjamin De Bernardi <benjamin.debernardi@datadoghq.com>
Date: Tue, 9 Dec 2025 16:43:13 +0100
Subject: [PATCH] fix(ddtrace/opentelemetry): fix inherited sampling decision
 from otel being ignored

---
 ddtrace/opentelemetry/tracer.go      | 18 ++++++++++++++
 ddtrace/opentelemetry/tracer_test.go | 21 ++++++++++++++++
 ddtrace/tracer/spancontext.go        | 36 ++++++++++++++++++++++++----
 3 files changed, 70 insertions(+), 5 deletions(-)

diff --git a/ddtrace/opentelemetry/tracer.go b/ddtrace/opentelemetry/tracer.go
index a8caaf039e..00bfeeafb3 100644
--- a/ddtrace/opentelemetry/tracer.go
+++ b/ddtrace/opentelemetry/tracer.go
@@ -166,3 +166,21 @@ func (c *otelCtxToDDCtx) SpanID() uint64 {
 }
 
 func (c *otelCtxToDDCtx) ForeachBaggageItem(_ func(k, v string) bool) {}
+
+// SamplingDecision returns the sampling decision of the span context.
+func (c *otelCtxToDDCtx) SamplingDecision() uint32 {
+	if c.oc.IsSampled() {
+		return 2 // decisionKeep
+	}
+	return 1 // decisionDrop
+}
+
+// Priority returns the sampling priority of the span context.
+func (c *otelCtxToDDCtx) Priority() *float64 {
+	if c.oc.IsSampled() {
+		p := float64(ext.PriorityAutoKeep)
+		return &p
+	}
+	p := float64(ext.PriorityAutoReject)
+	return &p
+}
diff --git a/ddtrace/opentelemetry/tracer_test.go b/ddtrace/opentelemetry/tracer_test.go
index 98de1eb5f2..aaa0a75832 100644
--- a/ddtrace/opentelemetry/tracer_test.go
+++ b/ddtrace/opentelemetry/tracer_test.go
@@ -398,3 +398,24 @@ func TestMergeOtelDDBaggage(t *testing.T) {
 		assert.Equal("otelValue", value)
 	})
 }
+
+func Test_DDOpenTelemetryTracer(t *testing.T) {
+	ddOTelTracer := NewTracerProvider(
+		tracer.WithSamplingRules([]tracer.SamplingRule{
+			{Rate: 0}, // This should be applied only when a brand new root span is started and should be ignored for a non-root span
+		}),
+	).Tracer("")
+
+	parentSpanContext := oteltrace.NewSpanContext(oteltrace.SpanContextConfig{
+		TraceID:    oteltrace.TraceID{0xAA},
+		SpanID:     oteltrace.SpanID{0x01},
+		TraceFlags: oteltrace.FlagsSampled, // the parent span is sampled, so its child spans should be sampled too
+	})
+	ctx := oteltrace.ContextWithSpanContext(context.Background(), parentSpanContext)
+	_, span := ddOTelTracer.Start(ctx, "test")
+	span.End()
+
+	childSpanContext := span.SpanContext()
+	assert.Equal(t, parentSpanContext.TraceID(), childSpanContext.TraceID())
+	assert.True(t, childSpanContext.IsSampled(), "parent span is sampled, but child span is not sampled") // this test fails
+}
diff --git a/ddtrace/tracer/spancontext.go b/ddtrace/tracer/spancontext.go
index aeee388b1f..95df341199 100644
--- a/ddtrace/tracer/spancontext.go
+++ b/ddtrace/tracer/spancontext.go
@@ -116,11 +116,16 @@ type SpanContext struct {
 	baggageOnly bool       // when true, indicates this context only propagates baggage items and should not be used for distributed tracing fields
 }
 
+// Private interface for span contexts that can propagate sampling decisions.
+type spanContextWithSamplingDecision interface {
+	SamplingDecision() uint32
+	Priority() *float64
+}
+
 // Private interface for converting v1 span contexts to v2 ones.
 type spanContextV1Adapter interface {
-	SamplingDecision() uint32
+	spanContextWithSamplingDecision
 	Origin() string
-	Priority() *float64
 	PropagatingTags() map[string]string
 	Tags() map[string]string
 }
@@ -137,14 +142,35 @@ func FromGenericCtx(c ddtrace.SpanContext) *SpanContext {
 		sc.baggage[k] = v
 		return true
 	})
+
+	ctxSpl, ok := c.(spanContextWithSamplingDecision)
+	if !ok {
+		return &sc
+	}
+
+	// If the generic context has a sampling decision, set it on the trace
+	// along with the priority if it exists.
+	// After setting the sampling decision, lock the trace so the decision is
+	// respected and avoid re-sampling.
+	if sDecision := samplingDecision(ctxSpl.SamplingDecision()); sDecision != decisionNone {
+		sc.trace = newTrace()
+		sc.trace.samplingDecision = sDecision
+
+		if p := ctxSpl.Priority(); p != nil {
+			sc.setSamplingPriority(int(*p), samplernames.Unknown)
+			sc.trace.setLocked(true)
+		}
+	}
+
 	ctx, ok := c.(spanContextV1Adapter)
 	if !ok {
 		return &sc
 	}
+
 	sc.origin = ctx.Origin()
-	sc.trace = newTrace()
-	sc.trace.priority = ctx.Priority()
-	sc.trace.samplingDecision = samplingDecision(ctx.SamplingDecision())
+	if sc.trace == nil {
+		sc.trace = newTrace()
+	}
 	sc.trace.tags = ctx.Tags()
 	sc.trace.propagatingTags = ctx.PropagatingTags()
 	return &sc