plugin: allow empty prompt responses

Reported by Joe Doyle of Trail of Bits.

plugin/client_test.go

@@ -7,6 +7,7 @@
 package plugin
 
 import (
+	"bufio"
 	"bytes"
 	"errors"
 	"io"
@@ -19,6 +20,7 @@ import (
 
 	"filippo.io/age"
 	"filippo.io/age/internal/bech32"
+	"filippo.io/age/internal/format"
 )
 
 func TestMain(m *testing.M) {
@@ -104,6 +106,22 @@ func TestPluginNameCase(t *testing.T) {
 	}
 }
 
+func TestRequestValueEmpty(t *testing.T) {
+	p, err := New("test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	p.SetIO(nil, io.Discard, io.Discard)
+	p.sr = format.NewStanzaReader(bufio.NewReader(strings.NewReader("-> ok\n\n")))
+	value, err := p.RequestValue("prompt", true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if value != "" {
+		t.Errorf("value = %q", value)
+	}
+}
+
 func TestLabels(t *testing.T) {
 	if runtime.GOOS == "windows" {
 		t.Skip("Windows support is TODO")

plugin/plugin.go

@@ -537,7 +537,7 @@ func (p *Plugin) RequestValue(prompt string, secret bool) (string, error) {
 	if s.Type == "fail" {
 		return "", fmt.Errorf("client failed to request value")
 	}
-	if err := expectStanzaWithBody(s, 0); err != nil {
+	if err := expectStanzaWithAnyBody(s, 0); err != nil {
 		return "", p.fatalInteractf("%v", err)
 	}
 	return string(s.Body), nil
@@ -596,8 +596,8 @@ func expectStanzaWithNoBody(s *format.Stanza, wantArgs int) error {
 }
 
 func expectStanzaWithBody(s *format.Stanza, wantArgs int) error {
-	if len(s.Args) != wantArgs {
-		return fmt.Errorf("%s stanza has %d arguments, want %d", s.Type, len(s.Args), wantArgs)
+	if err := expectStanzaWithAnyBody(s, wantArgs); err != nil {
+		return err
 	}
 	if len(s.Body) == 0 {
 		return fmt.Errorf("%s stanza has 0 bytes of body, want >0", s.Type)
@@ -605,6 +605,13 @@ func expectStanzaWithBody(s *format.Stanza, wantArgs int) error {
 	return nil
 }
 
+func expectStanzaWithAnyBody(s *format.Stanza, wantArgs int) error {
+	if len(s.Args) != wantArgs {
+		return fmt.Errorf("%s stanza has %d arguments, want %d", s.Type, len(s.Args), wantArgs)
+	}
+	return nil
+}
+
 func (p *Plugin) recipientError(idx int, err error) int {
 	if err := p.writeError([]string{"recipient", fmt.Sprint(idx)}, err); err != nil {
 		return p.fatalf("%v", err)