contrib/gorilla/mux: allow custom resource naming (DataDog#617)

Adds a new `WithResourceNamer` that allows customizing how resources are named based on information from the router and the request.

contrib/gorilla/mux/mux.go

@@ -97,18 +97,27 @@ func (r *Router) ServeHTTP(w http.ResponseWriter, req *http.Request) {
 	var (
 		match    mux.RouteMatch
 		spanopts []ddtrace.StartSpanOption
-		route    = "unknown"
 	)
 	// get the resource associated to this request
 	if r.Match(req, &match) && match.Route != nil {
-		if r, err := match.Route.GetPathTemplate(); err == nil {
-			route = r
-		}
 		if h, err := match.Route.GetHostTemplate(); err == nil {
 			spanopts = append(spanopts, tracer.Tag("mux.host", h))
 		}
 	}
 	spanopts = append(spanopts, r.config.spanOpts...)
-	resource := req.Method + " " + route
+	resource := r.config.resourceNamer(r, req)
 	httputil.TraceAndServe(r.Router, w, req, r.config.serviceName, resource, spanopts...)
 }
+
+// defaultResourceNamer attempts to quantize the resource for an HTTP request by
+// retrieving the path template associated with the route from the request.
+func defaultResourceNamer(router *Router, req *http.Request) string {
+	var match mux.RouteMatch
+	// get the resource associated with the given request
+	if router.Match(req, &match) && match.Route != nil {
+		if r, err := match.Route.GetPathTemplate(); err == nil {
+			return req.Method + " " + r
+		}
+	}
+	return req.Method + " unknown"
+}

contrib/gorilla/mux/mux_test.go

@@ -182,6 +182,26 @@ func TestAnalyticsSettings(t *testing.T) {
 	})
 }
 
+func TestResourceNamer(t *testing.T) {
+	staticName := "static resource name"
+	staticNamer := func(*Router, *http.Request) string {
+		return staticName
+	}
+
+	assert := assert.New(t)
+	mt := mocktracer.Start()
+	defer mt.Stop()
+	mux := NewRouter(WithResourceNamer(staticNamer))
+	mux.Handle("/200", okHandler()).Host("localhost")
+	r := httptest.NewRequest("GET", "http://localhost/200", nil)
+	w := httptest.NewRecorder()
+	mux.ServeHTTP(w, r)
+
+	spans := mt.FinishedSpans()
+	assert.Equal(1, len(spans))
+	assert.Equal(staticName, spans[0].Tag(ext.ResourceName))
+}
+
 func router() http.Handler {
 	mux := NewRouter(WithServiceName("my-service"))
 	mux.Handle("/200", okHandler())

contrib/gorilla/mux/option.go

@@ -7,6 +7,7 @@ package mux
 
 import (
 	"math"
+	"net/http"
 
 	"gopkg.in/DataDog/dd-trace-go.v1/ddtrace"
 	"gopkg.in/DataDog/dd-trace-go.v1/internal/globalconfig"
@@ -16,6 +17,7 @@ type routerConfig struct {
 	serviceName   string
 	spanOpts      []ddtrace.StartSpanOption // additional span options to be applied
 	analyticsRate float64
+	resourceNamer func(*Router, *http.Request) string
 }
 
 // RouterOption represents an option that can be passed to NewRouter.
@@ -24,6 +26,7 @@ type RouterOption func(*routerConfig)
 func defaults(cfg *routerConfig) {
 	cfg.analyticsRate = globalconfig.AnalyticsRate()
 	cfg.serviceName = "mux.router"
+	cfg.resourceNamer = defaultResourceNamer
 }
 
 // WithServiceName sets the given service name for the router.
@@ -63,3 +66,11 @@ func WithAnalyticsRate(rate float64) RouterOption {
 		}
 	}
 }
+
+// WithResourceNamer specifies a quantizing function which will be used to
+// obtain the resource name for a given request.
+func WithResourceNamer(namer func(router *Router, req *http.Request) string) RouterOption {
+	return func(cfg *routerConfig) {
+		cfg.resourceNamer = namer
+	}
+}