tracing: add delegated exporter

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

util/tracing/detect/delegated/delegated.go

@@ -0,0 +1,66 @@
+package jaeger
+
+import (
+	"context"
+	"sync"
+
+	"github.com/moby/buildkit/util/tracing/detect"
+	sdktrace "go.opentelemetry.io/otel/sdk/trace"
+)
+
+const maxBuffer = 128
+
+var exp = &Exporter{}
+
+func init() {
+	detect.Register("delegated", func() (sdktrace.SpanExporter, error) {
+		return exp, nil
+	})
+}
+
+type Exporter struct {
+	mu       sync.Mutex
+	delegate sdktrace.SpanExporter
+	buffer   []sdktrace.ReadOnlySpan
+}
+
+var _ sdktrace.SpanExporter = &Exporter{}
+
+func (e *Exporter) ExportSpans(ctx context.Context, ss []sdktrace.ReadOnlySpan) error {
+	e.mu.Lock()
+	defer e.mu.Unlock()
+
+	if e.delegate != nil {
+		return e.delegate.ExportSpans(ctx, ss)
+	}
+
+	if len(e.buffer) > maxBuffer {
+		return nil
+	}
+
+	e.buffer = append(e.buffer, ss...)
+	return nil
+}
+
+func (e *Exporter) Shutdown(ctx context.Context) error {
+	e.mu.Lock()
+	defer e.mu.Unlock()
+
+	if e.delegate != nil {
+		return e.delegate.Shutdown(ctx)
+	}
+
+	return nil
+}
+
+func (e *Exporter) SetDelegate(ctx context.Context, del sdktrace.SpanExporter) error {
+	e.mu.Lock()
+	defer e.mu.Unlock()
+
+	e.delegate = del
+
+	if len(e.buffer) > 0 {
+		return e.delegate.ExportSpans(ctx, e.buffer)
+	}
+	return nil
+}

util/tracing/detect/detect.go

@@ -78,7 +78,11 @@ func detect() error {
 
 	sdktp := sdktrace.NewTracerProvider(sdktrace.WithSpanProcessor(sp), sdktrace.WithResource(res))
 	closers = append(closers, sdktp.Shutdown)
-	tp = sdktp
+
+	tp = &tracerProviderWithExporter{
+		TracerProvider: sdktp,
+		exp:            exp,
+	}
 
 	return nil
 }
@@ -122,3 +126,12 @@ func (serviceNameDetector) Detect(ctx context.Context) (*resource.Resource, erro
 		},
 	).Detect(ctx)
 }
+
+type tracerProviderWithExporter struct {
+	trace.TracerProvider
+	exp sdktrace.SpanExporter
+}
+
+func (t *tracerProviderWithExporter) SpanExporter() sdktrace.SpanExporter {
+	return t.exp
+}