fetch with optional --relay flags.

helpers.go

@@ -2,7 +2,9 @@ package main
 
 import (
 	"bytes"
+	"fmt"
 	"io"
+	"net/url"
 	"os"
 
 	"github.com/urfave/cli/v2"
@@ -27,3 +29,22 @@ func getStdinOrFirstArgument(c *cli.Context) string {
 	}
 	return getStdin()
 }
+
+func validateRelayURLs(wsurls []string) error {
+	for _, wsurl := range wsurls {
+		u, err := url.Parse(wsurl)
+		if err != nil {
+			return fmt.Errorf("invalid relay url '%s': %s", wsurl, err)
+		}
+
+		if u.Scheme != "ws" && u.Scheme != "wss" {
+			return fmt.Errorf("relay url must use wss:// or ws:// schemes, got '%s'", wsurl)
+		}
+
+		if u.Host == "" {
+			return fmt.Errorf("relay url '%s' is missing the hostname", wsurl)
+		}
+	}
+
+	return nil
+}