Handle legacy index migration errors

src/seedpass/core/vault.py

@@ -43,6 +43,8 @@ class Vault:
 
         legacy_file = self.fingerprint_dir / "seedpass_passwords_db.json.enc"
         self.migrated_from_legacy = False
+        legacy_detected = False
+        backup_dir = None
         if legacy_file.exists() and not self.index_file.exists():
             print(colored("Legacy index detected.", "yellow"))
             resp = (
@@ -78,7 +80,7 @@ class Vault:
             if stray_checksum.exists():
                 stray_checksum.unlink()
 
-            self.migrated_from_legacy = True
+            legacy_detected = True
             print(
                 colored(
                     "Migration complete. Original index backed up to 'legacy_backups'",
@@ -86,10 +88,33 @@ class Vault:
                 )
             )
 
-        data = self.encryption_manager.load_json_data(self.index_file)
-        migration_performed = getattr(
-            self.encryption_manager, "last_migration_performed", False
-        )
+        try:
+            data = self.encryption_manager.load_json_data(self.index_file)
+            migration_performed = getattr(
+                self.encryption_manager, "last_migration_performed", False
+            )
+        except Exception as exc:  # noqa: BLE001 - surface clear error and restore
+            if legacy_detected and backup_dir is not None:
+                backup_file = backup_dir / legacy_file.name
+                legacy_checksum_path = (
+                    self.fingerprint_dir / "seedpass_passwords_db_checksum.txt"
+                )
+                backup_checksum = backup_dir / legacy_checksum_path.name
+                try:
+                    if self.index_file.exists():
+                        self.index_file.unlink()
+                    shutil.copy2(backup_file, legacy_file)
+                    checksum_new = (
+                        self.fingerprint_dir / "seedpass_entries_db_checksum.txt"
+                    )
+                    if checksum_new.exists():
+                        checksum_new.unlink()
+                    if backup_checksum.exists():
+                        shutil.copy2(backup_checksum, legacy_checksum_path)
+                finally:
+                    self.migrated_from_legacy = False
+            raise RuntimeError(f"Migration failed: {exc}") from exc
+
         from .migrations import apply_migrations, LATEST_VERSION
 
         version = data.get("schema_version", 0)
@@ -98,12 +123,36 @@ class Vault:
                 f"File schema version {version} is newer than supported {LATEST_VERSION}"
             )
         schema_migrated = version < LATEST_VERSION
-        data = apply_migrations(data)
-        if schema_migrated:
-            self.encryption_manager.save_json_data(data, self.index_file)
-            self.encryption_manager.update_checksum(self.index_file)
+
+        try:
+            data = apply_migrations(data)
+            if schema_migrated:
+                self.encryption_manager.save_json_data(data, self.index_file)
+                self.encryption_manager.update_checksum(self.index_file)
+        except Exception as exc:  # noqa: BLE001 - surface clear error and restore
+            if legacy_detected and backup_dir is not None:
+                backup_file = backup_dir / legacy_file.name
+                legacy_checksum_path = (
+                    self.fingerprint_dir / "seedpass_passwords_db_checksum.txt"
+                )
+                backup_checksum = backup_dir / legacy_checksum_path.name
+                try:
+                    if self.index_file.exists():
+                        self.index_file.unlink()
+                    shutil.copy2(backup_file, legacy_file)
+                    checksum_new = (
+                        self.fingerprint_dir / "seedpass_entries_db_checksum.txt"
+                    )
+                    if checksum_new.exists():
+                        checksum_new.unlink()
+                    if backup_checksum.exists():
+                        shutil.copy2(backup_checksum, legacy_checksum_path)
+                finally:
+                    self.migrated_from_legacy = False
+            raise RuntimeError(f"Migration failed: {exc}") from exc
+
         self.migrated_from_legacy = (
-            self.migrated_from_legacy or migration_performed or schema_migrated
+            legacy_detected or migration_performed or schema_migrated
         )
         if return_migration_flag:
             return data, self.migrated_from_legacy

src/tests/test_legacy_migration.py

@@ -2,6 +2,8 @@ import json
 import hashlib
 from pathlib import Path
 
+import pytest
+
 from helpers import create_vault, TEST_SEED, TEST_PASSWORD
 from utils.key_derivation import derive_index_key
 from cryptography.fernet import Fernet
@@ -52,6 +54,55 @@ def test_legacy_index_migrates(monkeypatch, tmp_path: Path):
     assert backup.exists()
 
 
+def test_failed_migration_restores_legacy(monkeypatch, tmp_path: Path):
+    vault, enc_mgr = create_vault(tmp_path, TEST_SEED, TEST_PASSWORD)
+
+    key = derive_index_key(TEST_SEED)
+    data = {"schema_version": 4, "entries": {}}
+    enc = Fernet(key).encrypt(json.dumps(data).encode())
+    legacy_file = tmp_path / "seedpass_passwords_db.json.enc"
+    legacy_file.write_bytes(enc)
+    checksum = hashlib.sha256(enc).hexdigest()
+    (tmp_path / "seedpass_passwords_db_checksum.txt").write_text(checksum)
+
+    monkeypatch.setattr("builtins.input", lambda *_a, **_k: "y")
+
+    def bad_load_json(_path):
+        raise ValueError("boom")
+
+    monkeypatch.setattr(enc_mgr, "load_json_data", bad_load_json)
+
+    with pytest.raises(RuntimeError, match="Migration failed:"):
+        vault.load_index()
+
+    # Legacy file restored and new index removed
+    assert legacy_file.exists()
+    assert not (tmp_path / "seedpass_entries_db.json.enc").exists()
+    assert (tmp_path / "seedpass_passwords_db_checksum.txt").read_text() == checksum
+    assert not vault.migrated_from_legacy
+
+
+def test_migrated_index_has_v2_prefix(monkeypatch, tmp_path: Path):
+    vault, _ = create_vault(tmp_path, TEST_SEED, TEST_PASSWORD)
+
+    key = derive_index_key(TEST_SEED)
+    data = {"schema_version": 4, "entries": {}}
+    enc = Fernet(key).encrypt(json.dumps(data).encode())
+    legacy_file = tmp_path / "seedpass_passwords_db.json.enc"
+    legacy_file.write_bytes(enc)
+    (tmp_path / "seedpass_passwords_db_checksum.txt").write_text(
+        hashlib.sha256(enc).hexdigest()
+    )
+
+    monkeypatch.setattr("builtins.input", lambda *_a, **_k: "y")
+
+    vault.load_index()
+
+    new_file = tmp_path / "seedpass_entries_db.json.enc"
+    assert new_file.read_bytes().startswith(b"V2:")
+    assert vault.migrated_from_legacy
+
+
 def test_legacy_index_migration_removes_strays(monkeypatch, tmp_path: Path):
     vault, _ = create_vault(tmp_path, TEST_SEED, TEST_PASSWORD)