trace: tighten the check for duplicate registration

From comment at @santsai at https://github.com/golang/go/issues/24137#issuecomment-452983000

Change-Id: Icf0aff2172811752b240d94e709550b0da353360
Reviewed-on: https://go-review.googlesource.com/c/157378
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: JBD <jbd@google.com>

trace/trace.go

@@ -86,6 +86,12 @@ import (
 // FOR DEBUGGING ONLY. This will slow down the program.
 var DebugUseAfterFinish = false
 
+// HTTP ServeMux paths.
+const (
+	debugRequestsPath = "/debug/requests"
+	debugEventsPath   = "/debug/events"
+)
+
 // AuthRequest determines whether a specific request is permitted to load the
 // /debug/requests or /debug/events pages.
 //
@@ -112,8 +118,8 @@ var AuthRequest = func(req *http.Request) (any, sensitive bool) {
 }
 
 func init() {
-	_, pat := http.DefaultServeMux.Handler(&http.Request{URL: &url.URL{Path: "/debug/requests"}})
-	if pat != "" {
+	_, pat := http.DefaultServeMux.Handler(&http.Request{URL: &url.URL{Path: debugRequestsPath}})
+	if pat == debugRequestsPath {
 		panic("/debug/requests is already registered. You may have two independent copies of " +
 			"golang.org/x/net/trace in your binary, trying to maintain separate state. This may " +
 			"involve a vendored copy of golang.org/x/net/trace.")
@@ -121,8 +127,8 @@ func init() {
 
 	// TODO(jbd): Serve Traces from /debug/traces in the future?
 	// There is no requirement for a request to be present to have traces.
-	http.HandleFunc("/debug/requests", Traces)
-	http.HandleFunc("/debug/events", Events)
+	http.HandleFunc(debugRequestsPath, Traces)
+	http.HandleFunc(debugEventsPath, Events)
 }
 
 // NewContext returns a copy of the parent context