feat: add fallback service target type for exit spans (#1294)

* feat: add fallback service target type for exit spans

This is the final step for complete service target fields
support. Exit spans will fallback to type and subtype similar
to what the deprecated destination resource was doing.

Assertions have been added to tests to verify the service target
type is not overwritten if previously set and that the fallback
mechanism is working properly.

* feat: allow users to disable inference of service target fields

    if the user does not call SetServiceTarget at all, we'll use inferred values for both (where possible)
    if the user calls SetServiceTarget with an empty name, this will disable inference
    if the user calls SetServiceTarget with a non-empty name, but empty type, we'll use the specified name and infer the type

* docs: update test comment to reference the correct field

Co-authored-by: Andrew Wilkins <[email protected]>

* test: add service target infer testcase with explicit name

Co-authored-by: Andrew Wilkins <[email protected]>

span.go

@@ -350,6 +350,11 @@ func (s *Span) End() {
 		// manually set the destination.service.resource
 		s.setExitSpanDestinationService()
 	}
+	if s.exit {
+		// The span was created as an exit span, but the user did not
+		// manually set the service.target fields.
+		s.setExitSpanServiceTarget()
+	}
 	if s.Duration < 0 {
 		s.Duration = time.Since(s.timestamp)
 	}
@@ -490,6 +495,40 @@ func (s *Span) setExitSpanDestinationService() {
 	})
 }
 
+func (s *Span) setExitSpanServiceTarget() {
+	fallbackType := s.Subtype
+	if fallbackType == "" {
+		fallbackType = s.Type
+	}
+
+	// Service target fields explicitly provided.
+	if s.Context.setServiceTargetCalled {
+		// if the user calls SetServiceTarget with a non-empty name, but empty type,
+		// we'll use the specified name and infer the type
+		if s.Context.serviceTarget.Type == "" && s.Context.serviceTarget.Name != "" {
+			s.Context.SetServiceTarget(ServiceTargetSpanContext{
+				Type: fallbackType,
+				Name: s.Context.serviceTarget.Name,
+			})
+		}
+		return
+	}
+
+	var fallbackName string
+	if s.Context.database.Type != "" { // database spans
+		fallbackName = s.Context.database.Instance
+	} else if s.Context.message.Queue != nil { // messaging spans
+		fallbackName = s.Context.message.Queue.Name
+	} else if s.Context.http.URL != nil { // http spans
+		fallbackName = s.Context.http.URL.Host
+	}
+
+	s.Context.SetServiceTarget(ServiceTargetSpanContext{
+		Type: fallbackType,
+		Name: fallbackName,
+	})
+}
+
 // IsExitSpan returns true if the span is an exit span.
 func (s *Span) IsExitSpan() bool {
 	if s == nil {

span_test.go

@@ -227,6 +227,10 @@ func TestStartExitSpan(t *testing.T) {
 	// the exit span will assign the value.
 	assert.Equal(t, spans[0].Context.Destination.Service.Resource, "type")
 
+	// When the context's ServiceTarget is not explicitly set, ending
+	// the exit span will assign the value.
+	assert.Equal(t, spans[0].Context.Service.Target.Type, "type")
+
 	tracer := apmtest.NewRecordingTracer()
 	defer tracer.Close()
 
@@ -1122,6 +1126,74 @@ func TestExitSpanDoesNotOverwriteDestinationServiceResource(t *testing.T) {
 	assert.Equal(t, spans[0].Context.Destination.Service.Resource, "my-custom-resource")
 }
 
+func TestExitSpanDoesNotOverwriteServiceTarget(t *testing.T) {
+	_, spans, _ := apmtest.WithTransaction(func(ctx context.Context) {
+		span, _ := apm.StartSpanOptions(ctx, "name", "type", apm.SpanOptions{ExitSpan: true})
+		assert.True(t, span.IsExitSpan())
+		span.Context.SetServiceTarget(apm.ServiceTargetSpanContext{
+			Type: "my-custom-resource",
+			Name: "foo",
+		})
+		span.Duration = 2 * time.Millisecond
+		span.End()
+	})
+	require.Len(t, spans, 1)
+	assert.Equal(t, spans[0].Context.Service.Target.Type, "my-custom-resource")
+	assert.Equal(t, spans[0].Context.Service.Target.Name, "foo")
+}
+
+func TestExitSpanDisableInferTarget(t *testing.T) {
+	_, spans, _ := apmtest.WithTransaction(func(ctx context.Context) {
+		span, _ := apm.StartSpanOptions(ctx, "name", "type", apm.SpanOptions{ExitSpan: true})
+		assert.True(t, span.IsExitSpan())
+		span.Context.SetDatabase(apm.DatabaseSpanContext{
+			Type:     "mysql",
+			Instance: "foo",
+		})
+		span.Context.SetServiceTarget(apm.ServiceTargetSpanContext{})
+		span.Duration = 2 * time.Millisecond
+		span.End()
+	})
+	require.Len(t, spans, 1)
+	assert.Empty(t, spans[0].Context.Service.Target.Type)
+	assert.Empty(t, spans[0].Context.Service.Target.Name)
+}
+
+func TestExitSpanInferTarget(t *testing.T) {
+	_, spans, _ := apmtest.WithTransaction(func(ctx context.Context) {
+		span, _ := apm.StartSpanOptions(ctx, "name", "type", apm.SpanOptions{ExitSpan: true})
+		assert.True(t, span.IsExitSpan())
+		span.Context.SetDatabase(apm.DatabaseSpanContext{
+			Type:     "mysql",
+			Instance: "foo",
+		})
+		span.Duration = 2 * time.Millisecond
+		span.End()
+	})
+	require.Len(t, spans, 1)
+	assert.Equal(t, spans[0].Context.Service.Target.Type, "type")
+	assert.Equal(t, spans[0].Context.Service.Target.Name, "foo")
+}
+
+func TestExitSpanInferTargetWithName(t *testing.T) {
+	_, spans, _ := apmtest.WithTransaction(func(ctx context.Context) {
+		span, _ := apm.StartSpanOptions(ctx, "name", "type", apm.SpanOptions{ExitSpan: true})
+		assert.True(t, span.IsExitSpan())
+		span.Context.SetDatabase(apm.DatabaseSpanContext{
+			Type:     "mysql",
+			Instance: "foo",
+		})
+		span.Context.SetServiceTarget(apm.ServiceTargetSpanContext{
+			Name: "bar",
+		})
+		span.Duration = 2 * time.Millisecond
+		span.End()
+	})
+	require.Len(t, spans, 1)
+	assert.Equal(t, spans[0].Context.Service.Target.Type, "type")
+	assert.Equal(t, spans[0].Context.Service.Target.Name, "bar")
+}
+
 func TestTracerStartSpanIDSpecified(t *testing.T) {
 	spanID := apm.SpanID{0, 1, 2, 3, 4, 5, 6, 7}
 	_, spans, _ := apmtest.WithTransaction(func(ctx context.Context) {

spancontext.go

@@ -44,6 +44,10 @@ type SpanContext struct {
 	// If SetDestinationService has been called, we do not auto-set its
 	// resource value on span end.
 	setDestinationServiceCalled bool
+
+	// If SetServiceTarget has been called, we do not auto-set its
+	// values on span end.
+	setServiceTargetCalled bool
 }
 
 // DatabaseSpanContext holds database span context.
@@ -273,6 +277,7 @@ func (c *SpanContext) SetDestinationService(service DestinationServiceSpanContex
 
 // SetServiceTarget sets the service target info in the context.
 func (c *SpanContext) SetServiceTarget(service ServiceTargetSpanContext) {
+	c.setServiceTargetCalled = true
 	c.serviceTarget.Type = truncateString(service.Type)
 	c.serviceTarget.Name = truncateString(service.Name)
 	c.service.Target = &c.serviceTarget