detect: refactor the default resource detector for detect

This refactors the resource detector to ensure the detected resource is
initialized before use. It also removes the duplicate logic for setting
the service name through the environment variable.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

util/tracing/detect/detect.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/moby/buildkit/util/bklog"
 	"github.com/pkg/errors"
+	"go.opentelemetry.io/otel"
 	"go.opentelemetry.io/otel/exporters/prometheus"
 	"go.opentelemetry.io/otel/metric"
 	sdkmetric "go.opentelemetry.io/otel/sdk/metric"
@@ -29,8 +30,6 @@ type detector struct {
 var ServiceName string
 var Recorder *TraceRecorder
 
-var Resource *resource.Resource
-
 var detectors map[string]detector
 var once sync.Once
 var tp trace.TracerProvider
@@ -114,17 +113,7 @@ func detect() error {
 		return err
 	}
 
-	if Resource == nil {
-		res, err := resource.Detect(context.Background(), serviceNameDetector{})
-		if err != nil {
-			return err
-		}
-		res, err = resource.Merge(resource.Default(), res)
-		if err != nil {
-			return err
-		}
-		Resource = res
-	}
+	res := Resource()
 
 	// enable log with traceID when valid exporter
 	if texp != nil {
@@ -138,7 +127,7 @@ func detect() error {
 
 		sdktp := sdktrace.NewTracerProvider(
 			sdktrace.WithSpanProcessor(sp),
-			sdktrace.WithResource(Resource),
+			sdktrace.WithResource(res),
 		)
 		closers = append(closers, sdktp.Shutdown)
 
@@ -164,7 +153,7 @@ func detect() error {
 
 	if len(readers) > 0 {
 		opts := make([]sdkmetric.Option, 0, len(readers)+1)
-		opts = append(opts, sdkmetric.WithResource(Resource))
+		opts = append(opts, sdkmetric.WithResource(res))
 		for _, r := range readers {
 			opts = append(opts, sdkmetric.WithReader(r))
 		}
@@ -220,16 +209,42 @@ func Shutdown(ctx context.Context) error {
 	return nil
 }
 
+var (
+	detectedResource     *resource.Resource
+	detectedResourceOnce sync.Once
+)
+
+func Resource() *resource.Resource {
+	detectedResourceOnce.Do(func() {
+		res, err := resource.New(context.Background(),
+			resource.WithDetectors(serviceNameDetector{}),
+			resource.WithFromEnv(),
+			resource.WithTelemetrySDK(),
+		)
+		if err != nil {
+			otel.Handle(err)
+		}
+		detectedResource = res
+	})
+	return detectedResource
+}
+
+// OverrideResource overrides the resource returned from Resource.
+//
+// This must be invoked before Resource is called otherwise it is a no-op.
+func OverrideResource(res *resource.Resource) {
+	detectedResourceOnce.Do(func() {
+		detectedResource = res
+	})
+}
+
 type serviceNameDetector struct{}
 
 func (serviceNameDetector) Detect(ctx context.Context) (*resource.Resource, error) {
 	return resource.StringDetector(
 		semconv.SchemaURL,
 		semconv.ServiceNameKey,
 		func() (string, error) {
-			if n := os.Getenv("OTEL_SERVICE_NAME"); n != "" {
-				return n, nil
-			}
 			if ServiceName != "" {
 				return ServiceName, nil
 			}