othttp: add WithSpanFormatter option (#617)

* othttp: add WithSpanFormatter option

* plugin/othttp: add span formatter test

* remove typo

* preserve operation && change option name

* nil check

* fix comment typo

Co-Authored-By: Rahul Patel <rghetia@yahoo.com>

* add default formatter test case

Co-authored-by: Rahul Patel <rghetia@yahoo.com>

internal/trace/mock_span.go

@@ -28,6 +28,7 @@ import (
 type MockSpan struct {
 	sc     core.SpanContext
 	tracer apitrace.Tracer
+	Name   string
 }
 
 var _ apitrace.Span = (*MockSpan)(nil)
@@ -66,8 +67,9 @@ func (ms *MockSpan) End(options ...apitrace.EndOption) {
 func (ms *MockSpan) RecordError(ctx context.Context, err error, opts ...apitrace.ErrorOption) {
 }
 
-// SetName does nothing.
+// SetName sets the span name.
 func (ms *MockSpan) SetName(name string) {
+	ms.Name = name
 }
 
 // Tracer returns MockTracer implementation of Tracer.

internal/trace/mock_tracer.go

@@ -37,6 +37,9 @@ type MockTracer struct {
 
 	// Sampled specifies if the new span should be sampled or not.
 	Sampled bool
+
+	// OnSpanStarted is called every time a new trace span is started
+	OnSpanStarted func(span *MockSpan)
 }
 
 var _ apitrace.Tracer = (*MockTracer)(nil)
@@ -77,6 +80,10 @@ func (mt *MockTracer) Start(ctx context.Context, name string, o ...apitrace.Star
 	span = &MockSpan{
 		sc:     sc,
 		tracer: mt,
+		Name:   name,
+	}
+	if mt.OnSpanStarted != nil {
+		mt.OnSpanStarted(span)
 	}
 
 	return apitrace.ContextWithSpan(ctx, span), span

plugin/othttp/handler.go

@@ -53,12 +53,13 @@ type Handler struct {
 	operation string
 	handler   http.Handler
 
-	tracer           trace.Tracer
-	props            propagation.Propagators
-	spanStartOptions []trace.StartOption
-	readEvent        bool
-	writeEvent       bool
-	filters          []Filter
+	tracer            trace.Tracer
+	props             propagation.Propagators
+	spanStartOptions  []trace.StartOption
+	readEvent         bool
+	writeEvent        bool
+	filters           []Filter
+	spanNameFormatter func(string, *http.Request) string
 }
 
 // Option function used for setting *optional* Handler properties
@@ -140,10 +141,27 @@ func WithMessageEvents(events ...event) Option {
 	}
 }
 
+// WithSpanNameFormatter takes a function that will be called on every
+// incoming request and the returned string will become the Span Name
+func WithSpanNameFormatter(f func(operation string, r *http.Request) string) Option {
+	return func(h *Handler) {
+		h.spanNameFormatter = f
+	}
+}
+
+func defaultFormatter(operation string, _ *http.Request) string {
+	return operation
+}
+
 // NewHandler wraps the passed handler, functioning like middleware, in a span
 // named after the operation and with any provided HandlerOptions.
 func NewHandler(handler http.Handler, operation string, opts ...Option) http.Handler {
-	h := Handler{handler: handler, operation: operation}
+	h := Handler{
+		handler:           handler,
+		operation:         operation,
+		spanNameFormatter: defaultFormatter,
+	}
+
 	defaultOpts := []Option{
 		WithTracer(global.Tracer("go.opentelemetry.io/plugin/othttp")),
 		WithPropagators(global.Propagators()),
@@ -169,7 +187,7 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	opts := append([]trace.StartOption{}, h.spanStartOptions...) // start with the configured options
 
 	ctx := propagation.ExtractHTTP(r.Context(), h.props, r.Header)
-	ctx, span := h.tracer.Start(ctx, h.operation, opts...)
+	ctx, span := h.tracer.Start(ctx, h.spanNameFormatter(h.operation, r), opts...)
 	defer span.End()
 
 	readRecordFunc := func(int64) {}

plugin/othttp/handler_test.go

@@ -100,3 +100,63 @@ func TestBasicFilter(t *testing.T) {
 		t.Fatalf("got %q, expected %q", got, expected)
 	}
 }
+
+func TestSpanNameFormatter(t *testing.T) {
+	var testCases = []struct {
+		name      string
+		formatter func(s string, r *http.Request) string
+		operation string
+		expected  string
+	}{
+		{
+			name:      "default formatter",
+			formatter: defaultFormatter,
+			operation: "test_operation",
+			expected:  "test_operation",
+		},
+		{
+			name: "custom formatter",
+			formatter: func(s string, r *http.Request) string {
+				return r.URL.Path
+			},
+			operation: "",
+			expected:  "/hello",
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			rr := httptest.NewRecorder()
+			var id uint64
+			var spanName string
+			tracer := mocktrace.MockTracer{
+				StartSpanID: &id,
+				OnSpanStarted: func(span *mocktrace.MockSpan) {
+					spanName = span.Name
+				},
+			}
+			handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				if _, err := io.WriteString(w, "hello world"); err != nil {
+					t.Fatal(err)
+				}
+			})
+			h := NewHandler(
+				handler,
+				tc.operation,
+				WithTracer(&tracer),
+				WithSpanNameFormatter(tc.formatter),
+			)
+			r, err := http.NewRequest(http.MethodGet, "http://localhost/hello", nil)
+			if err != nil {
+				t.Fatal(err)
+			}
+			h.ServeHTTP(rr, r)
+			if got, expected := rr.Result().StatusCode, http.StatusOK; got != expected {
+				t.Fatalf("got %d, expected %d", got, expected)
+			}
+			if got, expected := spanName, tc.expected; got != expected {
+				t.Fatalf("got %q, expected %q", got, expected)
+			}
+		})
+	}
+}