command: export ParseFlags from Env

This allows an Init hook to emulate the default parse behaviour, which
otherwise is suppressed when CustomFlags is true.

Author: creachadair

command.go

@@ -60,6 +60,7 @@ type Env struct {
 	cancel    context.CancelCauseFunc
 	skipMerge bool      // default: merge flags later in the argument list
 	hflag     HelpFlags // default: no unlisted commands, no private flags
+	didParse  bool      // whether ParseFlags was called
 }
 
 // Context returns the context associated with e. If e does not have its own
@@ -153,13 +154,25 @@ func (e *Env) Write(data []byte) (int, error) {
 	return e.output().Write(data)
 }
 
-// parseFlags parses flags from rawArgs using the flag set from env.Command.
+// ParseFlags parses flags from env.Args using the flag set from env.Command.
 // If parsing succeeds, it updates env.Args.
-// If the command specifies custom flags, this is a no-op without error.
-func (e *Env) parseFlags(rawArgs []string) error {
-	if e.Command.CustomFlags {
+// If flags were already parsed, ParseFlags reports nil.
+//
+// Note: This is done automatically if env.Command does not set CustomFlags.
+// It is safe but unnecessary to call it explicitly, but it is provided to
+// allow an Init hook to use it.
+func (e *Env) ParseFlags() error {
+	if e.didParse {
 		return nil
 	}
+	e.didParse = true
+	return e.parseFlagsInternal(e.Args)
+}
+
+// parseFlagsInternal parses flags from rawArgs using the flag set from env.Command.
+// If parsing succeeds, it updates env.Args.
+// Note this internal helper does NOT mark env.didParse.
+func (e *Env) parseFlagsInternal(rawArgs []string) error {
 	e.Command.Flags.Usage = func() {}
 	e.Command.Flags.SetOutput(io.Discard)
 	toParse := rawArgs
@@ -392,8 +405,11 @@ func Run(env *Env, rawArgs []string) (err error) {
 
 	// Unless this command does custom flag parsing, parse the arguments and
 	// check for errors before passing control to the handler.
-	if err := env.parseFlags(rawArgs); err != nil {
-		return err
+	if !env.Command.CustomFlags {
+		if err := env.parseFlagsInternal(rawArgs); err != nil {
+			return err
+		}
+		env.didParse = true // in case Init calls ParseFlags anyway
 	}
 
 	if cmd.Init != nil {