Don't send context for non-sampled transactions

Fix transaction encoding to drop any accumulated
context for non-sampled transactions.

We've been relying on instrumentation to not
set context for non-sampled transactions. If
some instrumentation (e.g. custom code) sets
context, it still shouldn't be sent to the
server for non-sampled transactions.

CHANGELOG.md

@@ -5,6 +5,7 @@
  - Add Context.SetCustom (#581)
  - Add support for extracting UUID-like container IDs (#577)
  - Introduce transaction/span breakdown metrics (#564)
+ - Fixed transaction encoding to drop tags (and other context) for non-sampled transactions (#593)
 
 ## [v1.4.0](https://github.com/elastic/apm-agent-go/releases/tag/v1.4.0)
 

modelwriter.go

@@ -105,7 +105,8 @@ func (w *modelWriter) writeMetrics(m *Metrics) {
 func (w *modelWriter) buildModelTransaction(out *model.Transaction, tx *Transaction, td *TransactionData) {
 	out.ID = model.SpanID(tx.traceContext.Span)
 	out.TraceID = model.TraceID(tx.traceContext.Trace)
-	if !tx.traceContext.Options.Recorded() {
+	sampled := tx.traceContext.Options.Recorded()
+	if !sampled {
 		out.Sampled = &notSampled
 	}
 
@@ -117,8 +118,10 @@ func (w *modelWriter) buildModelTransaction(out *model.Transaction, tx *Transact
 	out.Duration = td.Duration.Seconds() * 1000
 	out.SpanCount.Started = td.spansCreated
 	out.SpanCount.Dropped = td.spansDropped
+	if sampled {
+		out.Context = td.Context.build()
+	}
 
-	out.Context = td.Context.build()
 	if len(w.cfg.sanitizedFieldNames) != 0 && out.Context != nil {
 		if out.Context.Request != nil {
 			sanitizeRequest(out.Context.Request, w.cfg.sanitizedFieldNames)

transaction_test.go

@@ -28,6 +28,7 @@ import (
 	"github.com/stretchr/testify/require"
 
 	"go.elastic.co/apm"
+	"go.elastic.co/apm/apmtest"
 	"go.elastic.co/apm/model"
 	"go.elastic.co/apm/transport"
 	"go.elastic.co/apm/transport/transporttest"
@@ -140,6 +141,21 @@ func TestTransactionEnsureParent(t *testing.T) {
 	assert.Equal(t, model.SpanID(parentSpan), payloads.Transactions[0].ParentID)
 }
 
+func TestTransactionContextNotSampled(t *testing.T) {
+	tracer := apmtest.NewRecordingTracer()
+	defer tracer.Close()
+	tracer.SetSampler(samplerFunc(func(apm.TraceContext) bool { return false }))
+
+	tx := tracer.StartTransaction("name", "type")
+	tx.Context.SetTag("foo", "bar")
+	tx.End()
+	tracer.Flush(nil)
+
+	payloads := tracer.Payloads()
+	require.Len(t, payloads.Transactions, 1)
+	assert.Nil(t, payloads.Transactions[0].Context)
+}
+
 func BenchmarkTransaction(b *testing.B) {
 	tracer, err := apm.NewTracer("service", "")
 	require.NoError(b, err)