command: implement builder and parser

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

command/unreachable_test.go

@@ -0,0 +1,54 @@
+package command
+
+import (
+	"flag"
+	"testing"
+)
+
+func TestParseUnreachable(t *testing.T) {
+	// top level bypasses name matching and recursive calls to Parse
+	// returns when encountering zero-length args
+	t.Run("zero-length args", func(t *testing.T) {
+		defer checkRecover(t, "Parse", "attempted to parse with zero length args")
+		_ = newNode(panicWriter{}, nil, " ", " ").Parse(nil)
+	})
+
+	// top level must not have siblings
+	t.Run("toplevel siblings", func(t *testing.T) {
+		defer checkRecover(t, "Parse", "invalid toplevel state")
+		n := newNode(panicWriter{}, nil, " ", "")
+		n.append(newNode(panicWriter{}, nil, "  ", " "))
+		_ = n.Parse(nil)
+	})
+
+	// a node with descendents must not have a direct handler
+	t.Run("sub handle conflict", func(t *testing.T) {
+		defer checkRecover(t, "Parse", "invalid subcommand tree state")
+		n := newNode(panicWriter{}, nil, " ", "")
+		n.adopt(newNode(panicWriter{}, nil, " ", " "))
+		n.f = func([]string) error { panic("unreachable") }
+		_ = n.Parse(nil)
+	})
+
+	// this would only happen if a node was matched twice
+	t.Run("parsed flag set", func(t *testing.T) {
+		defer checkRecover(t, "Parse", "invalid set state")
+		n := newNode(panicWriter{}, nil, " ", "")
+		set := flag.NewFlagSet("parsed", flag.ContinueOnError)
+		set.SetOutput(panicWriter{})
+		_ = set.Parse(nil)
+		n.set = set
+		_ = n.Parse(nil)
+	})
+}
+
+type panicWriter struct{}
+
+func (p panicWriter) Write([]byte) (int, error) { panic("unreachable") }
+
+func checkRecover(t *testing.T, name, wantPanic string) {
+	if r := recover(); r != wantPanic {
+		t.Errorf("%s: panic = %v; wantPanic %v",
+			name, r, wantPanic)
+	}
+}