command: root early handler func special case

This allows for early initialisation with access to flags on the root node. This can be useful for configuring global state used by subcommands.

Signed-off-by: Ophestra <cat@gensokyo.uk>

command/parse.go

@@ -34,9 +34,6 @@ func (n *node) Parse(arguments []string) error {
 
 match:
 	if n.child != nil {
-		if n.f != nil {
-			panic("invalid subcommand tree state")
-		}
 		// propagate help prefix early: flag set usage dereferences help
 		n.child.prefix = append(n.prefix, n.name)
 	}
@@ -50,6 +47,17 @@ match:
 	args := n.set.Args()
 
 	if n.child != nil {
+		if n.f != nil {
+			if n.usage != "" { // root node early special case
+				panic("invalid subcommand tree state")
+			}
+
+			// special case: root node calls HandlerFunc for initialisation
+			if err := n.f(nil); err != nil {
+				return err
+			}
+		}
+
 		if len(args) == 0 {
 			return n.writeHelp()
 		}