From f3b008d1b8d367ab5f87164916901cc9d29b0c41 Mon Sep 17 00:00:00 2001
From: Filippo Valsorda <hi@filippo.io>
Date: Tue, 23 Dec 2025 12:35:06 +0100
Subject: [PATCH] plugin: fix returning in-protocol errors from plugins

Fixes Foxboron/age-plugin-tpm#31
---
 plugin/client_test.go | 62 +++++++++++++++++++++++++++++++++++++++++++
 plugin/plugin.go      |  2 +-
 2 files changed, 63 insertions(+), 1 deletion(-)

diff --git a/plugin/client_test.go b/plugin/client_test.go
index 35b0eca..2a82fc3 100644
--- a/plugin/client_test.go
+++ b/plugin/client_test.go
@@ -14,6 +14,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"runtime"
+	"strings"
 	"testing"
 
 	"filippo.io/age"
@@ -34,6 +35,15 @@ func TestMain(m *testing.M) {
 			return testPQCRecipient{}, nil
 		})
 		os.Exit(p.Main())
+	case "age-plugin-error":
+		p, _ := New("error")
+		p.HandleRecipient(func(data []byte) (age.Recipient, error) {
+			return nil, errors.New("oh my, an error occurred")
+		})
+		p.HandleIdentity(func(data []byte) (age.Identity, error) {
+			return nil, errors.New("oh my, an error occurred")
+		})
+		os.Exit(p.Main())
 	default:
 		os.Exit(m.Run())
 	}
@@ -163,3 +173,55 @@ func TestNotFound(t *testing.T) {
 		t.Errorf("expected error to wrap exec.ErrNotFound, got: %v", err)
 	}
 }
+
+func TestPluginError(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("Windows support is TODO")
+	}
+	temp := t.TempDir()
+	testOnlyPluginPath = temp
+	t.Cleanup(func() { testOnlyPluginPath = "" })
+	ex, err := os.Executable()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Link(ex, filepath.Join(temp, "age-plugin-error")); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Chmod(filepath.Join(temp, "age-plugin-error"), 0755); err != nil {
+		t.Fatal(err)
+	}
+
+	r := EncodeRecipient("error", nil)
+	testPluginRecipient, err := NewRecipient(r, &ClientUI{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := age.Encrypt(io.Discard, testPluginRecipient); err == nil {
+		t.Errorf("expected error from plugin")
+	} else if !strings.Contains(err.Error(), "oh my, an error occurred") {
+		t.Errorf("expected plugin error, got: %v", err)
+	}
+
+	buf := &bytes.Buffer{}
+	id, err := age.GenerateHybridIdentity()
+	if err != nil {
+		t.Fatal(err)
+	}
+	w, err := age.Encrypt(buf, id.Recipient())
+	if err != nil {
+		t.Fatal(err)
+	}
+	w.Close()
+
+	i := EncodeIdentity("error", nil)
+	testPluginIdentity, err := NewIdentity(i, &ClientUI{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := age.Decrypt(buf, testPluginIdentity); err == nil {
+		t.Errorf("expected error from plugin")
+	} else if !strings.Contains(err.Error(), "oh my, an error occurred") {
+		t.Errorf("expected plugin error, got: %v", err)
+	}
+}
diff --git a/plugin/plugin.go b/plugin/plugin.go
index 9ec3a45..d0ca76c 100644
--- a/plugin/plugin.go
+++ b/plugin/plugin.go
@@ -654,7 +654,7 @@ func expectUnsupported(sr *format.StanzaReader) error {
 func (p *Plugin) writeError(args []string, err error) error {
 	s := &format.Stanza{Type: "error", Args: args}
 	s.Body = []byte(err.Error())
-	if err := s.Marshal(p.stderr); err != nil {
+	if err := s.Marshal(p.stdout); err != nil {
 		return fmt.Errorf("failed to write error stanza: %v", err)
 	}
 	if err := expectOk(p.sr); err != nil {