go.opentelemetry.io/otel v1.34.0: Unconfigured global text map propagator silently drops distributed trace headers and severs multi-service spans without explicit SetTextMapPropagator

`search_known_solution` answered with existing sample `sha256:3f14ed288a161c7b18e8a641230a1acbbe95a6630bf2bfe7bf77cc02b2958d34` ("Unwired OpenTelemetry tracers silently emit invalid zero-context spans while batch-buffered tracers drop spans without explicit flush or shutdown").
A model would assume calling `otel.SetTracerProvider(tp)` enables distributed context propagation out of the box or that `otel.GetTextMapPropagator()` defaults to W3C TraceContext and Baggage propagation.
The wrong code fails silently with a green build: all services record and export valid spans without errors or warnings, but outbound carriers contain zero `traceparent` headers, inbound trace contexts are ignored, and downstream services create disconnected root traces with brand new TraceIDs.

{"case":{"caseId":"case:sha256:d6f23d9c5c9d522c117f9d021da46c9696317140457078676babee9e30def0ef","contract":["assert unconfigured global text map propagator silently injects zero headers into carrier","assert unconfigured global text map propagator silently ignores incoming traceparent header and returns unlinked context","assert downstream span created from unconfigured extraction context generates a disconnected new trace ID","assert explicitly configured TraceContext and Baggage propagator injects W3C headers","assert configured propagator extracts remote span context with matching TraceID and remote flag","assert downstream span links to upstream parent TraceID and parent SpanID when propagator is configured","assert ParentBased sampler preserves sampling decision for remote sampled parent context"],"goal":"Unconfigured global text map propagator silently drops distributed trace headers and severs multi-service spans without explicit SetTextMapPropagator","kind":"HOW","packages":["pkg:golang/go.opentelemetry.io/otel@v1.34.0","pkg:golang/go.opentelemetry.io/otel/sdk@v1.34.0"],"schemaVersion":1,"symbols":["go.opentelemetry.io/otel.SetTextMapPropagator","go.opentelemetry.io/otel.GetTextMapPropagator","go.opentelemetry.io/otel/propagation.TraceContext","go.opentelemetry.io/otel/propagation.Baggage","go.opentelemetry.io/otel/sdk/trace.ParentBased"]},"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@v1.34.0","pkg:golang/go.opentelemetry.io/otel/sdk@v1.34.0"],"schemaVersion":1,"symbols":["go.opentelemetry.io/otel.SetTextMapPropagator","go.opentelemetry.io/otel.GetTextMapPropagator","go.opentelemetry.io/otel/propagation.TraceContext","go.opentelemetry.io/otel/propagation.Baggage","go.opentelemetry.io/otel/sdk/trace.ParentBased"],"verifierAdapter":"golang@1"}

module sample

go 1.26.5

require (
	go.opentelemetry.io/otel v1.34.0
	go.opentelemetry.io/otel/sdk v1.34.0
	go.opentelemetry.io/otel/trace v1.34.0
)

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/metric v1.34.0 // indirect
	golang.org/x/sys v0.29.0 // 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 v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
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 v1.34.0 h1:zRLXxLCgL1WyKsPVrgbSdMN4c0FMkDAskSTQP+0hdUY=
go.opentelemetry.io/otel v1.34.0/go.mod h1:OWFPOQ+h4G8xpyjgqo4SxJYdDQ/qmRH+wivy7zzx9oI=
go.opentelemetry.io/otel/metric v1.34.0 h1:+eTR3U0MyfWjRDhmFMxe2SsW64QrZ84AOhvqS7Y+PoQ=
go.opentelemetry.io/otel/metric v1.34.0/go.mod h1:CEDrp0fy2D0MvkXE+dPV7cMi8tWZwX3dmaIhwPOaqHE=
go.opentelemetry.io/otel/sdk v1.34.0 h1:95zS4k/2GOy069d321O8jWgYsW3MzVV+KuSPKp7Wr1A=
go.opentelemetry.io/otel/sdk v1.34.0/go.mod h1:0e/pNiaMAqaykJGKbi+tSjWfNNHMTxoC9qANsCzbyxU=
go.opentelemetry.io/otel/trace v1.34.0 h1:+ouXS2V8Rd4hp4580a8q23bg0azF2nI8cqLYnC8mh/k=
go.opentelemetry.io/otel/trace v1.34.0/go.mod h1:Svm7lSjQD7kG7KJ/MUHPVXSDGz2OX4h0M2jHBhmSfRE=
golang.org/x/sys v0.29.0 h1:TPYlXGxvx1MGTn2GiZDhnjPA9wZzZeGKHHmKhHYvgaU=
golang.org/x/sys v0.29.0/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"
	"net/http"
	"testing"

	"go.opentelemetry.io/otel"
	"go.opentelemetry.io/otel/baggage"
	"go.opentelemetry.io/otel/propagation"
	sdktrace "go.opentelemetry.io/otel/sdk/trace"
	"go.opentelemetry.io/otel/sdk/trace/tracetest"
	"go.opentelemetry.io/otel/trace"
)

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

	// Initialize active TracerProvider with an in-memory exporter
	exporter := tracetest.NewInMemoryExporter()
	tp := sdktrace.NewTracerProvider(sdktrace.WithSyncer(exporter))
	otel.SetTracerProvider(tp)
	defer func() {
		_ = tp.Shutdown(ctx)
		otel.SetTracerProvider(trace.NewNoopTracerProvider())
	}()

	tr := otel.Tracer("upstream-service")
	ctxParent, parentSpan := tr.Start(ctx, "client-call")
	parentSC := parentSpan.SpanContext()

	if !parentSC.IsValid() {
		t.Fatalf("parent span context must be valid")
	}

	// Add baggage to upstream context
	baggageMember, err := baggage.NewMember("session_id", "sess-98765")
	if err != nil {
		t.Fatalf("failed to create baggage member: %v", err)
	}
	bag, err := baggage.New(baggageMember)
	if err != nil {
		t.Fatalf("failed to create baggage: %v", err)
	}
	ctxParentWithBaggage := baggage.ContextWithBaggage(ctxParent, bag)

	// 1. DEFAULT UNCONFIGURED PROPAGATOR:
	// A naive model assumes otel.SetTracerProvider(tp) enables distributed context propagation
	// or that otel.GetTextMapPropagator() defaults to W3C TraceContext.
	// In reality: otel.GetTextMapPropagator() defaults to a no-op composite propagator.

	// Trap 1: Fields() returns empty slice with default propagator.
	defaultFields := otel.GetTextMapPropagator().Fields()
	if len(defaultFields) != 0 {
		t.Errorf("default unconfigured propagator must return 0 fields, got %v", defaultFields)
	}

	// Trap 2: Inject with default propagator writes ZERO headers into the carrier.
	defaultCarrier := propagation.HeaderCarrier(http.Header{})
	otel.GetTextMapPropagator().Inject(ctxParentWithBaggage, defaultCarrier)

	if len(defaultCarrier) != 0 {
		t.Fatalf("default propagator must not inject any headers, got: %v", defaultCarrier)
	}
	if defaultCarrier.Get("traceparent") != "" {
		t.Errorf("traceparent header must be empty with default unconfigured propagator")
	}
	if defaultCarrier.Get("baggage") != "" {
		t.Errorf("baggage header must be empty with default unconfigured propagator")
	}

	// Trap 3: Extract with default propagator silently ignores incoming headers and returns an unlinked context.
	incomingHeader := http.Header{}
	incomingHeader.Set("traceparent", "00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01")
	incomingHeader.Set("baggage", "session_id=sess-98765")
	extractedCtx := otel.GetTextMapPropagator().Extract(context.Background(), propagation.HeaderCarrier(incomingHeader))

	extractedSC := trace.SpanContextFromContext(extractedCtx)
	if extractedSC.IsValid() {
		t.Errorf("extracted SpanContext from unconfigured propagator must be invalid/empty")
	}
	extractedBag := baggage.FromContext(extractedCtx)
	if extractedBag.Len() != 0 {
		t.Errorf("extracted baggage from unconfigured propagator must have length 0")
	}

	// Trap 4: Downstream span created from extracted context creates a NEW random TraceID, severing distributed tracing.
	downstreamTracer := otel.Tracer("downstream-service")
	_, downstreamSpan := downstreamTracer.Start(extractedCtx, "server-handler")
	downstreamSC := downstreamSpan.SpanContext()

	if downstreamSC.TraceID().String() == "4bf92f3577b34da6a3ce929d0e0e4736" {
		t.Errorf("unconfigured propagator must NOT extract trace ID from incoming headers")
	}
	downstreamSpan.End()

	// 2. EXPLICITLY CONFIGURED PROPAGATOR:
	// Distributed tracing REQUIRES calling otel.SetTextMapPropagator with W3C TraceContext and Baggage.
	otel.SetTextMapPropagator(propagation.NewCompositeTextMapPropagator(
		propagation.TraceContext{},
		propagation.Baggage{},
	))
	defer func() {
		otel.SetTextMapPropagator(propagation.NewCompositeTextMapPropagator())
	}()

	configuredFields := otel.GetTextMapPropagator().Fields()
	if len(configuredFields) < 2 {
		t.Fatalf("configured composite propagator must advertise traceparent and baggage fields, got: %v", configuredFields)
	}

	// Now Inject writes the W3C traceparent and baggage headers
	configuredCarrier := propagation.HeaderCarrier(http.Header{})
	otel.GetTextMapPropagator().Inject(ctxParentWithBaggage, configuredCarrier)

	traceparent := configuredCarrier.Get("traceparent")
	if traceparent == "" {
		t.Fatalf("configured propagator must inject traceparent header")
	}
	bagHeader := configuredCarrier.Get("baggage")
	if bagHeader == "" {
		t.Fatalf("configured propagator must inject baggage header")
	}

	// Now Extract properly extracts the remote span context and baggage
	serverCtx := otel.GetTextMapPropagator().Extract(context.Background(), configuredCarrier)
	extractedRemoteSC := trace.SpanContextFromContext(serverCtx)
	if !extractedRemoteSC.IsValid() {
		t.Fatalf("extracted SpanContext must be valid")
	}
	if !extractedRemoteSC.IsRemote() {
		t.Errorf("extracted SpanContext must be marked as Remote")
	}
	if extractedRemoteSC.TraceID() != parentSC.TraceID() {
		t.Errorf("extracted TraceID %s does not match parent TraceID %s", extractedRemoteSC.TraceID(), parentSC.TraceID())
	}
	if extractedRemoteSC.SpanID() != parentSC.SpanID() {
		t.Errorf("extracted SpanID %s does not match parent SpanID %s", extractedRemoteSC.SpanID(), parentSC.SpanID())
	}

	serverBag := baggage.FromContext(serverCtx)
	if serverBag.Member("session_id").Value() != "sess-98765" {
		t.Errorf("expected baggage member 'session_id'='sess-98765', got %q", serverBag.Member("session_id").Value())
	}

	// Downstream span inherits the upstream TraceID and links as child of upstream SpanID
	_, linkedSpan := downstreamTracer.Start(serverCtx, "linked-server-handler")
	linkedSC := linkedSpan.SpanContext()
	if linkedSC.TraceID() != parentSC.TraceID() {
		t.Errorf("linked span TraceID %s must match upstream TraceID %s", linkedSC.TraceID(), parentSC.TraceID())
	}
	if linkedSC.SpanID() == parentSC.SpanID() {
		t.Errorf("linked span must have its own unique SpanID, not equal to parent")
	}
	linkedSpan.End()
	parentSpan.End()
}

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

	// Simulate incoming remote context that was sampled upstream (Sampled flag = 1)
	upstreamTraceID, _ := trace.TraceIDFromHex("4bf92f3577b34da6a3ce929d0e0e4736")
	upstreamSpanID, _ := trace.SpanIDFromHex("00f067aa0ba902b7")
	remoteSC := trace.NewSpanContext(trace.SpanContextConfig{
		TraceID:    upstreamTraceID,
		SpanID:     upstreamSpanID,
		TraceFlags: trace.FlagsSampled,
		Remote:     true,
	})
	remoteCtx := trace.ContextWithRemoteSpanContext(ctx, remoteSC)

	// Trap 5: Standalone sdktrace.NeverSample() ignores upstream sampled decision.
	standaloneNeverTP := sdktrace.NewTracerProvider(
		sdktrace.WithSampler(sdktrace.NeverSample()),
	)
	defer func() { _ = standaloneNeverTP.Shutdown(ctx) }()

	standaloneSpan := standaloneNeverTP.Tracer("svc").Start
	_, droppedSpan := standaloneSpan(remoteCtx, "standalone-dropped")
	if droppedSpan.SpanContext().IsSampled() {
		t.Errorf("standalone NeverSample unexpectedly sampled span")
	}
	if droppedSpan.IsRecording() {
		t.Errorf("standalone NeverSample span should not be recording")
	}
	droppedSpan.End()

	// Correct pattern: ParentBased(NeverSample()) respects upstream sampling decision.
	// When remote parent is sampled, ParentBased keeps downstream span sampled!
	parentBasedTP := sdktrace.NewTracerProvider(
		sdktrace.WithSampler(sdktrace.ParentBased(sdktrace.NeverSample())),
	)
	defer func() { _ = parentBasedTP.Shutdown(ctx) }()

	_, preservedSpan := parentBasedTP.Tracer("svc").Start(remoteCtx, "parent-preserved")
	if !preservedSpan.SpanContext().IsSampled() {
		t.Errorf("ParentBased sampler must preserve upstream sampled decision for remote parent")
	}
	if !preservedSpan.IsRecording() {
		t.Errorf("ParentBased sampled span must be recording")
	}
	preservedSpan.End()
}