caddyhttp(tracing): add traceparent to response headers - Closes #7139

modules/caddyhttp/tracing/module_test.go

@@ -3,6 +3,7 @@ package tracing
 import (
 	"context"
 	"errors"
+	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -121,20 +122,26 @@ func TestTracing_ServeHTTP_Propagation_Without_Initial_Headers(t *testing.T) {
 	if err := ot.ServeHTTP(w, req, handler); err != nil {
 		t.Errorf("ServeHTTP error: %v", err)
 	}
+
+	responseTraceParent := w.Header().Get("Traceparent")
+	if responseTraceParent == "" {
+		t.Errorf("Missing traceparent in response headers")
+	}
 }
 
 func TestTracing_ServeHTTP_Propagation_With_Initial_Headers(t *testing.T) {
 	ot := &Tracing{
 		SpanName: "mySpan",
 	}
 
+	dummyTraceParentPrefix := "00-11111111111111111111111111111111"
 	req := createRequestWithContext("GET", "https://example.com/foo")
-	req.Header.Set("traceparent", "00-11111111111111111111111111111111-1111111111111111-01")
+	req.Header.Set("traceparent", fmt.Sprintf("%s-1111111111111111-01", dummyTraceParentPrefix))
 	w := httptest.NewRecorder()
 
 	var handler caddyhttp.HandlerFunc = func(writer http.ResponseWriter, request *http.Request) error {
 		traceparent := request.Header.Get("Traceparent")
-		if !strings.HasPrefix(traceparent, "00-11111111111111111111111111111111") {
+		if !strings.HasPrefix(traceparent, dummyTraceParentPrefix) {
 			t.Errorf("Invalid traceparent: %v", traceparent)
 		}
 
@@ -152,6 +159,17 @@ func TestTracing_ServeHTTP_Propagation_With_Initial_Headers(t *testing.T) {
 	if err := ot.ServeHTTP(w, req, handler); err != nil {
 		t.Errorf("ServeHTTP error: %v", err)
 	}
+
+	responseTraceParent := w.Header().Get("Traceparent")
+	if responseTraceParent == "" {
+		t.Error("Missing traceparent in response headers")
+	} else if !strings.HasPrefix(responseTraceParent, dummyTraceParentPrefix) {
+		t.Errorf(
+			"Traceparent prefix in response headers (%s) not same as initial (%s)",
+			responseTraceParent,
+			dummyTraceParentPrefix,
+		)
+	}
 }
 
 func TestTracing_ServeHTTP_Next_Error(t *testing.T) {

modules/caddyhttp/tracing/tracer.go

@@ -93,6 +93,8 @@ func (ot *openTelemetryWrapper) serveHTTP(w http.ResponseWriter, r *http.Request
 		caddyhttp.SetVar(ctx, "trace_id", traceID)
 		// Add a span_id placeholder, accessible via `{http.vars.span_id}`.
 		caddyhttp.SetVar(ctx, "span_id", spanID)
+		// Add traceId and spanId to response headers
+		w.Header().Set("traceparent", fmt.Sprintf("00-%s-%s-01", traceID, spanID))
 		// Add the traceID and spanID to the log fields for the request.
 		if extra, ok := ctx.Value(caddyhttp.ExtraLogFieldsCtxKey).(*caddyhttp.ExtraLogFields); ok {
 			extra.Add(zap.String("traceID", traceID))