use a better error message for missing sigs on clearsigned piped scripts

main.go

@@ -38,6 +38,7 @@ func main() {
 	// still happen.
 	defer func() {
 		if r := recover(); r != nil {
+			flag.Usage()
 			os.Exit(1)
 		}
 	}()
@@ -91,7 +92,10 @@ func main() {
 			log.Panic(err)
 		}
 
-		signature := NewSignature(key, script, *sigSource)
+		signature, err := NewSignature(key, script, *sigSource)
+		if err != nil {
+			log.Panic(err)
+		}
 		defer os.Remove(signature.Name())
 
 		if err := signature.Verify(); err != nil {

script.go

@@ -205,3 +205,9 @@ func (s Script) Inspect(inspect bool, editor string) bool {
 
 	return strings.ToLower(runScript) == "y"
 }
+
+// IsClearsigned returns true if the script and signature are attached,
+// and false otherwise.
+func (s Script) IsClearsigned() bool {
+	return s.clearsigned
+}

signature.go

@@ -27,11 +27,15 @@ type Signature struct {
 }
 
 // NewSignature loads a key ring and Script into a new Signature.
-func NewSignature(key openpgp.KeyRing, script *Script, source string) *Signature {
+func NewSignature(key openpgp.KeyRing, script *Script, source string) (*Signature, error) {
+	if script.IsPiped() && !script.IsClearsigned() && source == "" {
+		return nil, errors.New("Your script is not clearsigned and you're piping from STDIN, so I need -signature.")
+	}
+
 	sig := &Signature{key: key, script: script, source: source}
 	sig.filename = script.Name() + ".sig"
 
-	return sig
+	return sig, nil
 }
 
 // Name is the name of the temporary file holding the signature.
@@ -42,7 +46,7 @@ func (s Signature) Name() string {
 // Source is the original location of the signature file. It defaults to
 // <script source>.sig.
 func (s *Signature) Source() string {
-	if s.source != "" || s.script == nil || s.script.clearsigned {
+	if s.source != "" || s.script == nil || s.script.IsClearsigned() {
 		return s.source
 	}
 
@@ -53,7 +57,7 @@ func (s *Signature) Source() string {
 
 // Download saves the signature to a temporary file.
 func (s *Signature) Download() error {
-	if s.script != nil && s.script.clearsigned {
+	if s.script != nil && s.script.IsClearsigned() {
 		return nil
 	}