cmd/utils: fix bootnodes config priority (ethereum#28095)

This fixes an issue where the --bootnodes flag was overridden by the config file.

---------

Co-authored-by: NathanBSC <Nathan.l@nodereal.io>
Co-authored-by: Felix Lange <fjl@twurst.com>

cmd/utils/flags.go

@@ -1032,35 +1032,45 @@ func setNodeUserIdent(ctx *cli.Context, cfg *node.Config) {
 
 // setBootstrapNodes creates a list of bootstrap nodes from the command line
 // flags, reverting to pre-configured ones if none have been specified.
+// Priority order for bootnodes configuration:
+//
+// 1. --bootnodes flag
+// 2. Config file
+// 3. Network preset flags (e.g. --goerli)
+// 4. default to mainnet nodes
 func setBootstrapNodes(ctx *cli.Context, cfg *p2p.Config) {
 	urls := params.MainnetBootnodes
-	switch {
-	case ctx.IsSet(BootnodesFlag.Name):
+	if ctx.IsSet(BootnodesFlag.Name) {
 		urls = SplitAndTrim(ctx.String(BootnodesFlag.Name))
-	case ctx.Bool(HoleskyFlag.Name):
-		urls = params.HoleskyBootnodes
-	case ctx.Bool(SepoliaFlag.Name):
-		urls = params.SepoliaBootnodes
-	case ctx.Bool(GoerliFlag.Name):
-		urls = params.GoerliBootnodes
-	}
-
-	// don't apply defaults if BootstrapNodes is already set
-	if cfg.BootstrapNodes != nil {
-		return
+	} else {
+		if cfg.BootstrapNodes != nil {
+			return // Already set by config file, don't apply defaults.
+		}
+		switch {
+		case ctx.Bool(HoleskyFlag.Name):
+			urls = params.HoleskyBootnodes
+		case ctx.Bool(SepoliaFlag.Name):
+			urls = params.SepoliaBootnodes
+		case ctx.Bool(GoerliFlag.Name):
+			urls = params.GoerliBootnodes
+		}
 	}
+	cfg.BootstrapNodes = mustParseBootnodes(urls)
+}
 
-	cfg.BootstrapNodes = make([]*enode.Node, 0, len(urls))
+func mustParseBootnodes(urls []string) []*enode.Node {
+	nodes := make([]*enode.Node, 0, len(urls))
 	for _, url := range urls {
 		if url != "" {
 			node, err := enode.Parse(enode.ValidSchemes, url)
 			if err != nil {
 				log.Crit("Bootstrap URL invalid", "enode", url, "err", err)
-				continue
+				return nil
 			}
-			cfg.BootstrapNodes = append(cfg.BootstrapNodes, node)
+			nodes = append(nodes, node)
 		}
 	}
+	return nodes
 }
 
 // setBootstrapNodesV5 creates a list of bootstrap nodes from the command line