Better error reporting for missing gpg binary

The error returned by `gpgExec` has just been swallowed. Now it is
stringified and returned together with any output to stderr.

Signed-off-by: Max Jonas Werner <mail@makk.es>

pgp/keysource.go

@@ -9,6 +9,7 @@ package pgp //import "github.com/getsops/sops/v3/pgp"
 import (
 	"bytes"
 	"encoding/hex"
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -131,9 +132,21 @@ func (d GnuPGHome) Import(armoredKey []byte) error {
 	}
 
 	args := []string{"--batch", "--import"}
-	_, stderr, err := gpgExec(d.String(), args, bytes.NewReader(armoredKey))
+	_, stderrBuf, err := gpgExec(d.String(), args, bytes.NewReader(armoredKey))
 	if err != nil {
-		return fmt.Errorf("failed to import armored key data into GnuPG keyring: %s", strings.TrimSpace(stderr.String()))
+		stderr := stderrBuf.String()
+		errStr := err.Error()
+		var sb strings.Builder
+		sb.WriteString("failed to import armored key data into GnuPG keyring")
+		if len(stderr) > 0 {
+			fmt.Fprintf(&sb, ": %s", stderr)
+			if len(errStr) > 0 {
+				fmt.Fprintf(&sb, ": %s", errStr)
+			}
+		} else if len(errStr) > 0 {
+			fmt.Fprintf(&sb, ": %s", errStr)
+		}
+		return errors.New(sb.String())
 	}
 	return nil
 }

pgp/keysource_test.go

@@ -66,10 +66,27 @@ func TestGnuPGHome_Import(t *testing.T) {
 	_, stderr, err = gpgExec(gnuPGHome.String(), []string{"--list-secret-keys", mockFingerprint}, nil)
 	assert.NoErrorf(t, err, stderr.String())
 
-	assert.Error(t, gnuPGHome.Import([]byte("invalid armored data")))
+	err = gnuPGHome.Import([]byte("invalid armored data"))
+	assert.Error(t, err)
+	assert.ErrorContains(t, err, "gpg: no valid OpenPGP data found.\ngpg: Total number processed: 0\n: exit status 2")
 	assert.Error(t, GnuPGHome("").Import(b))
 }
 
+func TestGnuPGHome_Import_With_Missing_Binary(t *testing.T) {
+	t.Setenv(SopsGpgExecEnv, "/does/not/exist")
+
+	gnuPGHome, err := NewGnuPGHome()
+	assert.NoError(t, err)
+	t.Cleanup(func() {
+		_ = os.RemoveAll(gnuPGHome.String())
+	})
+
+	b, err := os.ReadFile(mockPublicKey)
+	assert.NoError(t, err)
+	err = gnuPGHome.Import(b)
+	assert.ErrorContains(t, err, "failed to import armored key data into GnuPG keyring: fork/exec /does/not/exist: no such file or directory")
+}
+
 func TestGnuPGHome_ImportFile(t *testing.T) {
 	gnuPGHome, err := NewGnuPGHome()
 	assert.NoError(t, err)