go.opentelemetry.io/otel/sdk v1.35.0: Raw SDK samplers passed to WithSampler strip the ParentBased decorator, dropping incoming remote sampled spans or sampling unsampled fragments unless explicitly wrapped in ParentBased

`search_known_solution` returned sample `sha256:3f14ed288a161c7b18e8a641230a1acbbe95a6630bf2bfe7bf77cc02b2958d34` covering unwired global tracer no-op spans and batch exporter flush lifecycles.
A model would configure `trace.WithSampler(trace.NeverSample())` or `trace.WithSampler(trace.TraceIDRatioBased(ratio))` directly without `trace.ParentBased(...)`, assuming child spans automatically inherit the sampling decision of upstream parent traces.
The wrong code fails silently with a green build: all spans and context objects initialize without error, but incoming distributed traces from upstream services are silently discarded at process boundaries, permanently fragmenting the distributed trace.

{"case":{"caseId":"case:sha256:dda450a2354a3069f46fb1c37ea2bef23b1f6048a81d5c58a7bfcfd8591ff553","contract":["assert raw NeverSample drops child spans even when remote parent context is sampled","assert raw AlwaysSample forces child span recording even when remote parent context is unsampled","assert ParentBased(NeverSample) drops unparented root spans","assert ParentBased(NeverSample) records and exports child spans when remote parent is sampled","assert ParentBased(NeverSample) drops child spans when remote parent is unsampled"],"goal":"Raw SDK samplers passed to WithSampler strip the ParentBased decorator, dropping incoming remote sampled spans or sampling unsampled fragments unless explicitly wrapped in ParentBased","kind":"HOW","packages":["pkg:golang/go.opentelemetry.io/otel/sdk@v1.35.0","pkg:golang/go.opentelemetry.io/otel/trace@v1.35.0"],"schemaVersion":1,"symbols":["go.opentelemetry.io/otel/sdk/trace.NewTracerProvider","go.opentelemetry.io/otel/sdk/trace.WithSampler","go.opentelemetry.io/otel/sdk/trace.ParentBased","go.opentelemetry.io/otel/sdk/trace.NeverSample","go.opentelemetry.io/otel/sdk/trace.AlwaysSample","go.opentelemetry.io/otel/sdk/trace/tracetest.NewInMemoryExporter"]},"contractCommand":["go","test","./..."],"environment":{"arch":"x64","ecosystem":"golang","executionContext":"go","language":"go","os":"linux","packageManager":"gomod","runtime":"go","schemaVersion":1},"license":"MIT-0","packages":["pkg:golang/go.opentelemetry.io/otel/sdk@v1.35.0","pkg:golang/go.opentelemetry.io/otel/trace@v1.35.0"],"schemaVersion":1,"symbols":["go.opentelemetry.io/otel/sdk/trace.NewTracerProvider","go.opentelemetry.io/otel/sdk/trace.WithSampler","go.opentelemetry.io/otel/sdk/trace.ParentBased","go.opentelemetry.io/otel/sdk/trace.NeverSample","go.opentelemetry.io/otel/sdk/trace.AlwaysSample","go.opentelemetry.io/otel/sdk/trace/tracetest.NewInMemoryExporter"],"verifierAdapter":"golang@1"}

module sample

go 1.26.5

require (
	go.opentelemetry.io/otel/sdk v1.35.0
	go.opentelemetry.io/otel/trace v1.35.0
)

require (
	github.com/go-logr/logr v1.4.2 // indirect
	github.com/go-logr/stdr v1.2.2 // indirect
	github.com/google/uuid v1.6.0 // indirect
	go.opentelemetry.io/auto/sdk v1.1.0 // indirect
	go.opentelemetry.io/otel v1.35.0 // indirect
	go.opentelemetry.io/otel/metric v1.35.0 // indirect
	golang.org/x/sys v0.30.0 // indirect
)

github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
github.com/go-logr/logr v1.2.2/go.mod h1:jdQByPbusPIv2/zmleS9BjJVeZ6kBagPoEUsqbVz/1A=
github.com/go-logr/logr v1.4.2 h1:6pFjapn8bFcIbiKo3XT4j/BhANplGihG6tvd+8rYgrY=
github.com/go-logr/logr v1.4.2/go.mod h1:9T104GzyrTigFIr8wt5mBrctHMim0Nb2HLGrmQ40KvY=
github.com/go-logr/stdr v1.2.2 h1:hSWxHoqTgW2S2qGc0LTAI563KZ5YKYRhT3MFKZMbjag=
github.com/go-logr/stdr v1.2.2/go.mod h1:mMo/vtBO5dYbehREoey6XUKy/eSumjCCveDpRre4VKE=
github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
github.com/google/uuid v1.6.0 h1:NIvaJDMOsjHA8n1jAhLSgzrAzy1Hgr+hNrb57e+94F0=
github.com/google/uuid v1.6.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
github.com/stretchr/testify v1.10.0 h1:Xv5erBjTwe/5IxqUQTdXv5kgmIvbHo3QQyRwhJsOfJA=
github.com/stretchr/testify v1.10.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
go.opentelemetry.io/auto/sdk v1.1.0 h1:cH53jehLUN6UFLY71z+NDOiNJqDdPRaXzTel0sJySYA=
go.opentelemetry.io/auto/sdk v1.1.0/go.mod h1:3wSPjt5PWp2RhlCcmmOial7AvC4DQqZb7a7wCow3W8A=
go.opentelemetry.io/otel v1.35.0 h1:xKWKPxrxB6OtMCbmMY021CqC45J+3Onta9MqjhnusiQ=
go.opentelemetry.io/otel v1.35.0/go.mod h1:UEqy8Zp11hpkUrL73gSlELM0DupHoiq72dR+Zqel/+Y=
go.opentelemetry.io/otel/metric v1.35.0 h1:0znxYu2SNyuMSQT4Y9WDWej0VpcsxkuklLa4/siN90M=
go.opentelemetry.io/otel/metric v1.35.0/go.mod h1:nKVFgxBZ2fReX6IlyW28MgZojkoAkJGaE8CpgeAU3oE=
go.opentelemetry.io/otel/sdk v1.35.0 h1:iPctf8iprVySXSKJffSS79eOjl9pvxV9ZqOWT0QejKY=
go.opentelemetry.io/otel/sdk v1.35.0/go.mod h1:+ga1bZliga3DxJ3CQGg3updiaAJoNECOgJREo9KHGQg=
go.opentelemetry.io/otel/trace v1.35.0 h1:dPpEfJu1sDIqruz7BHFG3c7528f6ddfSWfFDVt/xgMs=
go.opentelemetry.io/otel/trace v1.35.0/go.mod h1:WUk7DtFp1Aw2MkvqGdwiXYDZZNvA/1J8o6xRXLrIkyc=
go.uber.org/goleak v1.3.0 h1:2K3zAYmnTNqV73imy9J1T3WC+gmCePx2hEGkimedGto=
go.uber.org/goleak v1.3.0/go.mod h1:CoHD4mav9JJNrW/WLlf7HGZPjdw8EucARQHekz1X6bE=
golang.org/x/sys v0.30.0 h1:QjkSwP/36a20jFYWkSue1YwXzLmsV5Gfq7Eiy72C1uc=
golang.org/x/sys v0.30.0/go.mod h1:/VUhepiaJMQUp4+oa/7Zr1D23ma6VTLIYjOOTFZPUcA=
gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

package sample_test

import (
	"context"
	"testing"

	"go.opentelemetry.io/otel/sdk/trace"
	"go.opentelemetry.io/otel/sdk/trace/tracetest"
	traceapi "go.opentelemetry.io/otel/trace"
)

func TestSamplerParentBasedTrap(t *testing.T) {
	ctx := context.Background()

	traceID, _ := traceapi.TraceIDFromHex("4bf92f3577b34da6a3ce929d0e0e4736")
	spanID, _ := traceapi.SpanIDFromHex("00f067aa0ba902b7")

	// 1. Simulated remote parent context: SAMPLED
	sampledRemoteParentSC := traceapi.NewSpanContext(traceapi.SpanContextConfig{
		TraceID:    traceID,
		SpanID:     spanID,
		TraceFlags: traceapi.FlagsSampled,
		Remote:     true,
	})
	ctxSampledRemoteParent := traceapi.ContextWithSpanContext(ctx, sampledRemoteParentSC)

	// 2. Simulated remote parent context: UNSAMPLED (TraceFlags: 0)
	unsampledRemoteParentSC := traceapi.NewSpanContext(traceapi.SpanContextConfig{
		TraceID:    traceID,
		SpanID:     spanID,
		TraceFlags: 0,
		Remote:     true,
	})
	ctxUnsampledRemoteParent := traceapi.ContextWithSpanContext(ctx, unsampledRemoteParentSC)

	// TRAP 1: Passing raw NeverSample() to WithSampler removes the ParentBased decorator.
	// A naive model configures NeverSample() on a worker to prevent local root traces,
	// expecting incoming distributed traces from API gateways to still be recorded.
	rawNeverExporter := tracetest.NewInMemoryExporter()
	rawNeverTP := trace.NewTracerProvider(
		trace.WithSyncer(rawNeverExporter),
		trace.WithSampler(trace.NeverSample()),
	)
	defer rawNeverTP.Shutdown(ctx)

	rawTracer := rawNeverTP.Tracer("test-worker")
	_, rawChildSpan := rawTracer.Start(ctxSampledRemoteParent, "child-of-sampled")
	rawChildSpan.End()

	// Proving the trap: raw NeverSample drops the child span despite the sampled remote parent.
	if rawChildSpan.SpanContext().IsSampled() {
		t.Errorf("raw NeverSample unexpectedly sampled child span")
	}
	if len(rawNeverExporter.GetSpans()) != 0 {
		t.Errorf("expected 0 spans in exporter for raw NeverSample, got %d", len(rawNeverExporter.GetSpans()))
	}

	// TRAP 2: Passing raw AlwaysSample() to WithSampler forces recording on unsampled incoming traces.
	// A naive model configures AlwaysSample(), expecting unsampled requests (e.g. health checks) to stay dropped.
	rawAlwaysExporter := tracetest.NewInMemoryExporter()
	rawAlwaysTP := trace.NewTracerProvider(
		trace.WithSyncer(rawAlwaysExporter),
		trace.WithSampler(trace.AlwaysSample()),
	)
	defer rawAlwaysTP.Shutdown(ctx)

	alwaysTracer := rawAlwaysTP.Tracer("test-service")
	_, alwaysChildSpan := alwaysTracer.Start(ctxUnsampledRemoteParent, "child-of-unsampled")
	alwaysChildSpan.End()

	// Proving the trap: raw AlwaysSample generates orphan spans for unsampled parents.
	if !alwaysChildSpan.SpanContext().IsSampled() {
		t.Errorf("raw AlwaysSample failed to sample child span")
	}
	if len(rawAlwaysExporter.GetSpans()) != 1 {
		t.Errorf("expected 1 orphan span exported from raw AlwaysSample, got %d", len(rawAlwaysExporter.GetSpans()))
	}

	// SOLUTION: Wrap root sampler in ParentBased(...) to preserve distributed trace fidelity.
	// - Root spans (no parent): use root sampler decision (NeverSample -> dropped).
	// - Remote parent sampled: sampled and exported.
	// - Remote parent unsampled: dropped without creating orphans.
	correctExporter := tracetest.NewInMemoryExporter()
	correctTP := trace.NewTracerProvider(
		trace.WithSyncer(correctExporter),
		trace.WithSampler(trace.ParentBased(trace.NeverSample())),
	)
	defer correctTP.Shutdown(ctx)

	correctTracer := correctTP.Tracer("test-worker")

	// Assertion 1: Root span without parent is dropped by root NeverSample
	_, rootSpan := correctTracer.Start(ctx, "root-span")
	rootSpan.End()
	if rootSpan.SpanContext().IsSampled() {
		t.Errorf("ParentBased(NeverSample) must not sample root span")
	}

	// Assertion 2: Sampled remote parent is preserved and sampled
	_, validChildSpan := correctTracer.Start(ctxSampledRemoteParent, "valid-child")
	validChildSpan.End()
	if !validChildSpan.SpanContext().IsSampled() {
		t.Errorf("ParentBased(NeverSample) must sample child when remote parent is sampled")
	}

	// Assertion 3: Unsampled remote parent is preserved and dropped
	_, droppedChildSpan := correctTracer.Start(ctxUnsampledRemoteParent, "unsampled-child")
	droppedChildSpan.End()
	if droppedChildSpan.SpanContext().IsSampled() {
		t.Errorf("ParentBased(NeverSample) must not sample child when remote parent is unsampled")
	}

	// Assertion 4: Only the valid child span was recorded and exported
	spans := correctExporter.GetSpans()
	if len(spans) != 1 {
		t.Fatalf("expected exactly 1 span in correct exporter, got %d", len(spans))
	}
	if spans[0].Name != "valid-child" {
		t.Errorf("expected span name 'valid-child', got %q", spans[0].Name)
	}
	if spans[0].SpanContext.TraceID() != traceID {
		t.Errorf("expected TraceID %s, got %s", traceID, spans[0].SpanContext.TraceID())
	}
	if spans[0].Parent.SpanID() != spanID {
		t.Errorf("expected Parent.SpanID %s, got %s", spanID, spans[0].Parent.SpanID())
	}
}