tracing: fix buildx tracing delegation

before this change, the threadsafe wrapper would hide
the delegate interface, which breaks the ability
to send traces.

after this change, tracing works again!

fixes docker/buildx#1847

Signed-off-by: Nick Santos <nick.santos@docker.com>

Author: nicks

util/tracing/detect/delegated/delegated.go

@@ -1,4 +1,4 @@
-package jaeger
+package delegated
 
 import (
 	"context"

util/tracing/detect/delegated/delegated_test.go

@@ -0,0 +1,18 @@
+package delegated_test
+
+import (
+	"testing"
+
+	"github.com/moby/buildkit/client"
+	"github.com/moby/buildkit/util/tracing/detect"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestDetectPreservesDelegateInterface(t *testing.T) {
+	exp, err := detect.Exporter()
+	require.NoError(t, err)
+
+	_, ok := exp.(client.TracerDelegate)
+	assert.True(t, ok, "delegated tracer expected to fulfill client.TracerDelegate interface")
+}

util/tracing/detect/detect.go

@@ -80,9 +80,7 @@ func getExporter() (sdktrace.SpanExporter, error) {
 	}
 
 	if exp != nil {
-		exp = &threadSafeExporterWrapper{
-			exporter: exp,
-		}
+		exp = makeThreadSafe(exp)
 	}
 
 	if Recorder != nil {

util/tracing/detect/threadsafe.go

@@ -7,6 +7,27 @@ import (
 	sdktrace "go.opentelemetry.io/otel/sdk/trace"
 )
 
+// Duplicated from buildkit/client.
+// Part of the external interface of our tracer.
+// Needed to ensure that we maintain this interface.
+type tracerDelegate interface {
+	sdktrace.SpanExporter
+
+	SetSpanExporter(ctx context.Context, exp sdktrace.SpanExporter) error
+}
+
+func makeThreadSafe(exp sdktrace.SpanExporter) sdktrace.SpanExporter {
+	d, isDelegate := exp.(tracerDelegate)
+	if isDelegate {
+		return &threadSafeDelegateWrapper{
+			exporter: d,
+		}
+	}
+	return &threadSafeExporterWrapper{
+		exporter: exp,
+	}
+}
+
 // threadSafeExporterWrapper wraps an OpenTelemetry SpanExporter and makes it thread-safe.
 type threadSafeExporterWrapper struct {
 	mu       sync.Mutex
@@ -24,3 +45,26 @@ func (tse *threadSafeExporterWrapper) Shutdown(ctx context.Context) error {
 	defer tse.mu.Unlock()
 	return tse.exporter.Shutdown(ctx)
 }
+
+type threadSafeDelegateWrapper struct {
+	mu       sync.Mutex
+	exporter tracerDelegate
+}
+
+func (tse *threadSafeDelegateWrapper) ExportSpans(ctx context.Context, spans []sdktrace.ReadOnlySpan) error {
+	tse.mu.Lock()
+	defer tse.mu.Unlock()
+	return tse.exporter.ExportSpans(ctx, spans)
+}
+
+func (tse *threadSafeDelegateWrapper) Shutdown(ctx context.Context) error {
+	tse.mu.Lock()
+	defer tse.mu.Unlock()
+	return tse.exporter.Shutdown(ctx)
+}
+
+func (tse *threadSafeDelegateWrapper) SetSpanExporter(ctx context.Context, exp sdktrace.SpanExporter) error {
+	tse.mu.Lock()
+	defer tse.mu.Unlock()
+	return tse.exporter.SetSpanExporter(ctx, exp)
+}