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

request.go

@@ -2,12 +2,12 @@ package masque
 
 import (
 	"fmt"
+	"github.com/dunglas/httpsfv"
+	"github.com/yosida95/uritemplate/v3"
 	"net/http"
 	"net/url"
 	"strconv"
-
-	"github.com/dunglas/httpsfv"
-	"github.com/yosida95/uritemplate/v3"
+	"strings"
 )
 
 // Request is the parsed CONNECT-UDP request returned from ParseRequest.
@@ -89,3 +89,19 @@ 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.
+// This is useful when the template variables are parts of the path, e.g.
+// when using the default template (.well-known/masque/udp/{target_host}/{target_port}/).
+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 +65,39 @@ 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, errContains string }{
+		{
+			name:        "invalid URL",
+			template:    "://localhost:1234",
+			errContains: "missing protocol scheme",
+		},
+		{
+			name:     "variables as URL parameters",
+			template: "https://localhost:1234/masque?h={target_host}&p={target_port}",
+			expected: "/masque",
+		},
+		{
+			name:     "variables in URL paths",
+			template: "https://localhost:1234/masque/{target_host}/{target_port}",
+			expected: "/masque/", // needs to have a trailing /
+		},
+		{
+			name:     "variables in URL paths, no trailing /",
+			template: "https://localhost:1234/masque/{target_host}/{target_port}/",
+			expected: "/masque/",
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			temp := uritemplate.MustNew(tc.template)
+			path, err := PathFromTemplate(temp)
+			if tc.errContains != "" {
+				require.ErrorContains(t, err, tc.errContains)
+			} else {
+				require.NoError(t, err)
+				require.Equal(t, tc.expected, path)
+			}
+		})
+	}
+}