Leave no partial unlocker directory when adding an unlocker fails (closes #48)

CreatePGPUnlocker looked up the GPG key's fingerprint, and the keychain
unlocker got the long-term key, only after writing part of the unlocker,
so a failure there left a directory with no metadata. Both now do every
step that can fail before writing anything. `secret unlocker add pgp`
looks the fingerprint up once, for its duplicate check, and passes it to
CreatePGPUnlocker to record. All four unlocker types write their files
through the new secret.WriteDir, which builds a new directory in a
temporary directory, renames it into place when complete and removes it
on a failure. A directory that already exists, as when an unlocker
replaces one of the same name, is written in place and never removed.

Model: opus-5-5

internal/secret/pgpunlocker_test.go

@@ -0,0 +1,67 @@
+package secret_test
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"git.eeqj.de/sneak/secret/internal/secret"
+	"git.eeqj.de/sneak/secret/internal/vault"
+	"github.com/spf13/afero"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// The GPG key ID and fingerprint passed to CreatePGPUnlocker.
+const (
+	testGPGKeyID       = "0123456789ABCDEF"
+	testGPGFingerprint = "0123456789ABCDEF0123456789ABCDEF01234567"
+)
+
+// fakeGPGScript is a gpg for which `gpg --version` succeeds and anything
+// else fails.
+const fakeGPGScript = `#!/bin/sh
+[ "$*" = --version ]
+`
+
+// installFakeGPG makes fakeGPGScript the only gpg on PATH for the test.
+func installFakeGPG(t *testing.T) {
+	t.Helper()
+
+	dir := t.TempDir()
+
+	//nolint:gosec // G306: the script must be executable
+	err := os.WriteFile(filepath.Join(dir, "gpg"), []byte(fakeGPGScript), 0o700)
+	require.NoError(t, err)
+
+	t.Setenv("PATH", dir)
+}
+
+// TestCreatePGPUnlockerFailureWritesNothing makes CreatePGPUnlocker fail at
+// getting the vault's long-term key, which used to come after part of the
+// unlocker was written, and asserts that nothing is written. Getting the key
+// fails because on macOS there is no mnemonic and no current unlocker, and
+// on every other platform it always fails
+// (https://git.eeqj.de/sneak/secret/issues/88).
+func TestCreatePGPUnlockerFailureWritesNothing(t *testing.T) {
+	installFakeGPG(t)
+	t.Setenv(secret.EnvMnemonic, "")
+
+	base := afero.NewMemMapFs()
+	vlt, err := vault.CreateVault(base, testVaultStateDir, testVaultName)
+	require.NoError(t, err)
+
+	fs := hookFs{Fs: base, before: func(_, path string) error {
+		t.Errorf("changed %s", path)
+
+		return nil
+	}}
+
+	_, err = secret.CreatePGPUnlocker(
+		fs, testVaultStateDir, testGPGKeyID, testGPGFingerprint)
+	require.Error(t, err)
+
+	vaultDir, err := vlt.GetDirectory()
+	require.NoError(t, err)
+	assert.Empty(t, dirNames(t, base, filepath.Join(vaultDir, "unlockers.d")))
+}