secret: Add Fallback stash (#297)

Adds a secret storage that will allow a primary and secondary store.
The secondary store is used only if the primary is broken
for non-expected reasons.

Notably, LoadSecret returning ErrNotFound is not an unexpected breakage.

internal/secret/fallback.go

@@ -0,0 +1,38 @@
+package secret
+
+import "errors"
+
+// FallbackStash is a secret stash that falls back to a secondary stash
+// if the primary stash fails.
+type FallbackStash struct {
+	Primary, Secondary Stash // required
+}
+
+// SaveSecret saves a secret to the primary stash.
+// If the operation fails, it falls back to the secondary stash.
+func (f *FallbackStash) SaveSecret(service, key, secret string) error {
+	if err := f.Primary.SaveSecret(service, key, secret); err != nil {
+		return f.Secondary.SaveSecret(service, key, secret)
+	}
+	return nil
+}
+
+// LoadSecret loads a secret from the primary stash.
+// If the operation fails NOT because the secret is not found,
+// it falls back to the secondary stash.
+func (f *FallbackStash) LoadSecret(service, key string) (string, error) {
+	secret, err := f.Primary.LoadSecret(service, key)
+	if err != nil && !errors.Is(err, ErrNotFound) {
+		secret, err = f.Secondary.LoadSecret(service, key)
+	}
+	return secret, err
+}
+
+// DeleteSecret deletes a secret from the primary stash,
+// and if that fails, from the secondary stash.
+func (f *FallbackStash) DeleteSecret(service, key string) error {
+	if err := f.Primary.DeleteSecret(service, key); err != nil {
+		return f.Secondary.DeleteSecret(service, key)
+	}
+	return nil
+}

internal/secret/stash_test.go

@@ -1,6 +1,7 @@
 package secret_test
 
 import (
+	"errors"
 	"os"
 	"path/filepath"
 	"testing"
@@ -47,6 +48,24 @@ func TestStash(t *testing.T) {
 		}
 		testStash(t, &stash)
 	})
+
+	t.Run("Fallback/PrimaryBroken", func(t *testing.T) {
+		testStash(t, &secret.FallbackStash{
+			Primary: &brokenStash{
+				err: errors.New("great sadness"),
+			},
+			Secondary: new(secret.MemoryStash),
+		})
+	})
+
+	t.Run("Fallback/PrimaryOkay", func(t *testing.T) {
+		testStash(t, &secret.FallbackStash{
+			Primary: new(secret.MemoryStash),
+			Secondary: &brokenStash{
+				err: errors.New("great sadness"),
+			},
+		})
+	})
 }
 
 func testStash(t *testing.T, stash secret.Stash) {
@@ -89,3 +108,20 @@ func testStash(t *testing.T, stash secret.Stash) {
 		require.NoError(t, stash.DeleteSecret(_service, "missing"))
 	})
 }
+
+// brokenStash is a Stash that always returns an error.
+type brokenStash struct {
+	err error
+}
+
+func (b *brokenStash) SaveSecret(service, key, secret string) error {
+	return b.err
+}
+
+func (b *brokenStash) LoadSecret(service, key string) (string, error) {
+	return "", b.err
+}
+
+func (b *brokenStash) DeleteSecret(service, key string) error {
+	return b.err
+}