module/apmot: fix nil-pointer deref in Inject

CHANGELOG.asciidoc

@@ -26,6 +26,7 @@ https://github.com/elastic/apm-agent-go/compare/v1.7.2...master[View commits]
 - Add "recording" config option, to dynamically disable event recording {pull}737[(#737)]
 - Enable central configuration of "stack_frames_min_duration" and "stack_trace_limit" {pull}742[(#742)]
 - Implement "CloseIdleConnections" on the Elasticsearch RoundTripper {pull}750[(#750)]
+- Fix apmot nil pointer dereference in Tracer.Inject {pull}763[(#763)]
 
 [[release-notes-1.x]]
 === Go Agent version 1.x

module/apmot/tracer.go

@@ -107,10 +107,9 @@ func (t *otTracer) Inject(sc opentracing.SpanContext, format interface{}, carrie
 		if !ok {
 			return opentracing.ErrInvalidCarrier
 		}
-		tx := spanContext.Transaction()
 		headerValue := apmhttp.FormatTraceparentHeader(spanContext.traceContext)
 		writer.Set(apmhttp.W3CTraceparentHeader, headerValue)
-		if tx.ShouldPropagateLegacyHeader() {
+		if t.tracer.ShouldPropagateLegacyHeader() {
 			writer.Set(apmhttp.ElasticTraceparentHeader, headerValue)
 		}
 		if tracestate := spanContext.traceContext.State.String(); tracestate != "" {

module/apmot/tracer_test.go

@@ -20,7 +20,10 @@ package apmot_test
 import (
 	"context"
 	"errors"
+	"net/http"
 	"net/url"
+	"os"
+	"strconv"
 	"testing"
 	"time"
 
@@ -363,6 +366,41 @@ func TestSpanFinishWithOptionsLogs(t *testing.T) {
 	assert.Equal(t, model.Time(spanFinish), errors[1].Timestamp)
 }
 
+func TestTraceInjectExtract(t *testing.T) {
+	testTraceInjectExtract(t, true)
+	testTraceInjectExtract(t, false)
+}
+
+func testTraceInjectExtract(t *testing.T, shouldPropagateLegacyHeader bool) {
+	os.Setenv("ELASTIC_APM_USE_ELASTIC_TRACEPARENT_HEADER", strconv.FormatBool(shouldPropagateLegacyHeader))
+	defer os.Unsetenv("ELASTIC_APM_USE_ELASTIC_TRACEPARENT_HEADER")
+
+	tracer, apmtracer, _ := newTestTracer()
+	defer apmtracer.Close()
+
+	span := tracer.StartSpan("span")
+	headers1 := make(http.Header)
+	carrier1 := opentracing.HTTPHeadersCarrier(headers1)
+
+	err := tracer.Inject(span.Context(), opentracing.HTTPHeaders, carrier1)
+	require.NoError(t, err)
+
+	spanContext, err := tracer.Extract(opentracing.HTTPHeaders, carrier1)
+	require.NoError(t, err)
+
+	headers2 := make(http.Header)
+	carrier2 := opentracing.HTTPHeadersCarrier(headers2)
+	err = tracer.Inject(spanContext, opentracing.HTTPHeaders, carrier2)
+	require.NoError(t, err)
+
+	assert.Equal(t, headers1, headers2)
+	if shouldPropagateLegacyHeader {
+		assert.Contains(t, headers1, "Elastic-Apm-Traceparent")
+	} else {
+		assert.NotContains(t, headers1, "Elastic-Apm-Traceparent")
+	}
+}
+
 func BenchmarkSpanSetSpanContext(b *testing.B) {
 	tags := opentracing.Tags{
 		"component":    "myComponent",

tracer.go

@@ -503,6 +503,16 @@ func (t *Tracer) Active() bool {
 	return atomic.LoadInt32(&t.active) == 1
 }
 
+// ShouldPropagateLegacyHeader reports whether instrumentation should
+// propagate the legacy "Elastic-Apm-Traceparent" header in addition to
+// the standard W3C "traceparent" header.
+//
+// This method will be removed in a future major version when we remove
+// support for propagating the legacy header.
+func (t *Tracer) ShouldPropagateLegacyHeader() bool {
+	return t.instrumentationConfig().propagateLegacyHeader
+}
+
 // SetRequestDuration sets the maximum amount of time to keep a request open
 // to the APM server for streaming data before closing the stream and starting
 // a new request.