contrib/dimfeld/httptreemux.v5: fix route and name for 30X redirects

The httptreemux router has a redirect behaviour that is invoked when
a request URL and matched route only differs in a trailing slash. The
default behaviour is to respond with a 301 (moved permanently) to
redirect the client to the exact path of the matched handler. We
previously patched one of the two scenarios where this occurs in DataDog#2332.
The changes in this commit addresses the other scenario for the 301
redirect, but also the 307 and 308 redirect options. Several tests
were included to cover the various scenarios.

We also standardize the resource name in the event that there is a
trailing slash missmatch between request URL and matched handler. We
always truncate the trailing slash in the resource name.

Fixes DataDog#2663

contrib/dimfeld/httptreemux.v5/httptreemux.go

@@ -112,18 +112,26 @@ func getRoute(router *httptreemux.TreeMux, w http.ResponseWriter, req *http.Requ
 	if !found {
 		return "", false
 	}
+	routeLen := len(route)
+	trailingSlash := route[routeLen-1] == '/' && routeLen > 1
 
-	// Check for redirecting route due to trailing slash for parameters.
-	// The redirecting behaviour originates from httptreemux router.
-	if lr.StatusCode == http.StatusMovedPermanently && strings.HasSuffix(route, "/") {
+	// Retry the population of lookup result parameters.
+	// If the initial attempt to populate the parameters fails, clone the request and modify the URI and URL Path.
+	// Depending on whether the route has a trailing slash or not, it will either add or remove the trailing slash and retry the lookup.
+	if routerRedirectEnabled(router) && isSupportedRedirectStatus(lr.StatusCode) && lr.Params == nil {
 		rReq := req.Clone(req.Context())
-		rReq.RequestURI = strings.TrimSuffix(rReq.RequestURI, "/")
-		rReq.URL.Path = strings.TrimSuffix(rReq.URL.Path, "/")
-
-		lr, found = router.Lookup(w, rReq)
-		if !found {
-			return "", false
+		if trailingSlash {
+			// if the route has a trailing slash, remove it
+			rReq.RequestURI = strings.TrimSuffix(rReq.RequestURI, "/")
+			rReq.URL.Path = strings.TrimSuffix(rReq.URL.Path, "/")
+		} else {
+			// if the route does not have a trailing slash, add one
+			rReq.RequestURI = rReq.RequestURI + "/"
+			rReq.URL.Path = rReq.URL.Path + "/"
 		}
+		// no need to check found again
+		// we already matched a route and we are only trying to populate the lookup result params
+		lr, _ = router.Lookup(w, rReq)
 	}
 
 	for k, v := range lr.Params {
@@ -139,5 +147,31 @@ func getRoute(router *httptreemux.TreeMux, w http.ResponseWriter, req *http.Requ
 		newP = "/:" + k
 		route = strings.Replace(route, oldP, newP, 1)
 	}
+
+	// remove trailing slash from route to standardize returned value
+	// the router does not allow you to register two matching routes with the only difference being a trailing slash
+	// this only affects the resulting returned value and not the actual request URL set on tag http.url
+	if trailingSlash {
+		route = strings.TrimSuffix(route, "/")
+	}
+
 	return route, true
 }
+
+// isSupportedRedirectStatus checks if the given HTTP status code is a supported redirect status.
+// It returns true if the status code is either StatusMovedPermanently, StatusTemporaryRedirect, or StatusPermanentRedirect.
+// Otherwise, it returns false.
+func isSupportedRedirectStatus(status int) bool {
+	return status == http.StatusMovedPermanently ||
+		status == http.StatusTemporaryRedirect ||
+		status == http.StatusPermanentRedirect
+}
+
+// routerRedirectEnabled checks if the redirection is enabled on the router.
+// It returns true if either RedirectCleanPath or RedirectTrailingSlash is enabled,
+// and the RedirectBehavior is not set to UseHandler. Otherwise, it returns false.
+// This function is used to determine whether to perform redirections based on the router's configuration.
+func routerRedirectEnabled(router *httptreemux.TreeMux) bool {
+	return (router.RedirectCleanPath || router.RedirectTrailingSlash) &&
+		router.RedirectBehavior != httptreemux.UseHandler
+}