cmd/age,plugin: add plugin.NotFoundError and CLI hint

Fixes #486

cmd/age/age.go

@@ -7,6 +7,7 @@ package main
 import (
 	"bufio"
 	"bytes"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
@@ -406,7 +407,11 @@ func encrypt(recipients []age.Recipient, in io.Reader, out io.Writer, withArmor
 		out = a
 	}
 	w, err := age.Encrypt(out, recipients...)
-	if err != nil {
+	if e := new(plugin.NotFoundError); errors.As(err, &e) {
+		errorWithHint(err.Error(),
+			fmt.Sprintf("you might want to install the %q plugin", e.Name),
+			"visit https://age-encryption.org/awesome#plugins for a list of available plugins")
+	} else if err != nil {
 		errorf("%v", err)
 	}
 	if _, err := io.Copy(w, in); err != nil {
@@ -483,7 +488,11 @@ func decrypt(identities []age.Identity, in io.Reader, out io.Writer) {
 	}
 
 	r, err := age.Decrypt(in, identities...)
-	if err != nil {
+	if e := new(plugin.NotFoundError); errors.As(err, &e) {
+		errorWithHint(err.Error(),
+			fmt.Sprintf("you might want to install the %q plugin", e.Name),
+			"visit https://age-encryption.org/awesome#plugins for a list of available plugins")
+	} else if err != nil {
 		errorf("%v", err)
 	}
 	out.Write(nil) // trigger the lazyOpener even if r is empty

cmd/age/testdata/plugin.txt

@@ -19,6 +19,12 @@ cp age-plugin-pwn/pwn $TMPDIR/age-plugin-pwn/pwn
 ! age -d -j pwn/pwn test.age
 ! exists pwn
 
+# check plugin not found hint
+! age -r age1nonexistentplugin1pt5d8z -o test1.age
+stderr /awesome#plugins
+! age -d -i nonexistent-identity.txt test.age
+stderr /awesome#plugins
+
 -- input --
 test
 -- key.txt --
@@ -32,3 +38,5 @@ AGE-PLUGIN-PWN/PWN-19GYK4WLY
 -- age-plugin-pwn/pwn --
 #!/bin/sh
 touch "$WORK/pwn"
+-- nonexistent-identity.txt --
+AGE-PLUGIN-NONEXISTENTPLUGIN-1R4XFW4

plugin/client.go

@@ -9,6 +9,7 @@ package plugin
 import (
 	"bufio"
 	"crypto/rand"
+	"errors"
 	"fmt"
 	"io"
 	mathrand "math/rand/v2"
@@ -402,6 +403,24 @@ type clientConnection struct {
 	close     func()
 }
 
+// NotFoundError is returned by [Recipient.Wrap] and [Identity.Unwrap] when the
+// plugin binary cannot be found.
+type NotFoundError struct {
+	// Name is the plugin (not binary) name.
+	Name string
+	// Err is the underlying error, usually an [exec.Error] wrapping
+	// [exec.ErrNotFound].
+	Err error
+}
+
+func (e *NotFoundError) Error() string {
+	return fmt.Sprintf("%q plugin not found: %v", e.Name, e.Err)
+}
+
+func (e *NotFoundError) Unwrap() error {
+	return e.Err
+}
+
 var testOnlyPluginPath string
 
 func openClientConnection(name, protocol string) (*clientConnection, error) {
@@ -444,6 +463,9 @@ func openClientConnection(name, protocol string) (*clientConnection, error) {
 	cmd.Dir = os.TempDir()
 
 	if err := cmd.Start(); err != nil {
+		if errors.Is(err, exec.ErrNotFound) {
+			return nil, &NotFoundError{Name: name, Err: err}
+		}
 		return nil, err
 	}
 

plugin/client_test.go

@@ -7,8 +7,11 @@
 package plugin
 
 import (
+	"bytes"
+	"errors"
 	"io"
 	"os"
+	"os/exec"
 	"path/filepath"
 	"runtime"
 	"testing"
@@ -108,3 +111,55 @@ func TestLabels(t *testing.T) {
 		t.Errorf("expected one pqc and one normal to fail")
 	}
 }
+
+func TestNotFound(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("Windows support is TODO")
+	}
+
+	r := EncodeRecipient("nonexistentplugin", nil)
+	t.Log(r)
+	testPluginRecipient, err := NewRecipient(r, &ClientUI{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	var e *NotFoundError
+	if _, err := age.Encrypt(io.Discard, testPluginRecipient); err == nil {
+		t.Errorf("expected error for nonexistent plugin")
+	} else if !errors.As(err, &e) {
+		t.Errorf("expected NotFoundError, got %T: %v", err, err)
+	} else if e.Name != "nonexistentplugin" {
+		t.Errorf("expected NotFoundError.Name to be nonexistentplugin, got %q", e.Name)
+	} else if !errors.Is(err, exec.ErrNotFound) {
+		t.Errorf("expected error to wrap exec.ErrNotFound, 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("nonexistentplugin", nil)
+	t.Log(i)
+	testPluginIdentity, err := NewIdentity(i, &ClientUI{})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if _, err := age.Decrypt(buf, testPluginIdentity); err == nil {
+		t.Errorf("expected error for nonexistent plugin")
+	} else if errors.As(err, new(*age.NoIdentityMatchError)) {
+		t.Errorf("expected NotFoundError, got NoIdentityMatchError: %v", err)
+	} else if !errors.As(err, &e) {
+		t.Errorf("expected NotFoundError, got %T: %v", err, err)
+	} else if e.Name != "nonexistentplugin" {
+		t.Errorf("expected NotFoundError.Name to be nonexistentplugin, got %q", e.Name)
+	} else if !errors.Is(err, exec.ErrNotFound) {
+		t.Errorf("expected error to wrap exec.ErrNotFound, got: %v", err)
+	}
+}