add a helper function to extract the HTTP path from a template

request.go

@@ -5,6 +5,7 @@ import (
 	"net/http"
 	"net/url"
 	"strconv"
+	"strings"
 
 	"github.com/dunglas/httpsfv"
 	"github.com/yosida95/uritemplate/v3"
@@ -89,3 +90,17 @@ func ParseRequest(r *http.Request, template *uritemplate.Template) (*Request, er
 	}
 	return &Request{Target: fmt.Sprintf("%s:%d", targetHost, targetPort)}, nil
 }
+
+// PathFromTemplate extracts the HTTP path from a URI template,
+// such that it can be used in a http.ServeMux.
+func PathFromTemplate(t *uritemplate.Template) (string, error) {
+	u, err := url.Parse(t.Raw())
+	if err != nil {
+		return "", err
+	}
+	path := strings.ReplaceAll(strings.ReplaceAll(u.Path, "/{target_host}", ""), "/{target_port}", "")
+	if path != u.Path && len(path) > 0 && path[len(path)-1] != '/' {
+		path = path + "/"
+	}
+	return path, nil
+}

request_test.go

@@ -65,3 +65,32 @@ func TestRequestParsing(t *testing.T) {
 		require.Equal(t, http.StatusBadRequest, err.(*RequestParseError).HTTPStatus)
 	})
 }
+
+func TestPathFromTemplate(t *testing.T) {
+	for _, tc := range []struct {
+		name, template, expected string
+	}{
+		{
+			"variables as URL parameters",
+			"https://localhost:1234/masque?h={target_host}&p={target_port}",
+			"/masque",
+		},
+		{
+			"variables in URL paths",
+			"https://localhost:1234/masque/{target_host}/{target_port}",
+			"/masque/", // needs to have a trailing /
+		},
+		{
+			"variables in URL paths, no trailing /",
+			"https://localhost:1234/masque/{target_host}/{target_port}/",
+			"/masque/",
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			temp := uritemplate.MustNew(tc.template)
+			path, err := PathFromTemplate(temp)
+			require.NoError(t, err)
+			require.Equal(t, tc.expected, path)
+		})
+	}
+}