refuse to migrate an invalid configuration

pre_commit/commands/migrate_config.py

@@ -2,6 +2,7 @@ import re
 
 import yaml
 
+from pre_commit.clientlib import load_config
 from pre_commit.util import yaml_load
 
 
@@ -43,6 +44,9 @@ def _migrate_sha_to_rev(contents: str) -> str:
 
 
 def migrate_config(config_file: str, quiet: bool = False) -> int:
+    # ensure that the configuration is a valid pre-commit configuration
+    load_config(config_file)
+
     with open(config_file) as f:
         orig_contents = contents = f.read()
 

tests/commands/migrate_config_test.py

@@ -1,6 +1,7 @@
 import pytest
 
 import pre_commit.constants as C
+from pre_commit.clientlib import InvalidConfigError
 from pre_commit.commands.migrate_config import _indent
 from pre_commit.commands.migrate_config import migrate_config
 
@@ -147,10 +148,10 @@ def test_migrate_config_sha_to_rev(tmpdir):
 
 
 @pytest.mark.parametrize('contents', ('', '\n'))
-def test_empty_configuration_file_user_error(tmpdir, contents):
+def test_migrate_config_invalid_configuration(tmpdir, contents):
     cfg = tmpdir.join(C.CONFIG_FILE)
     cfg.write(contents)
-    with tmpdir.as_cwd():
-        assert not migrate_config(C.CONFIG_FILE)
+    with tmpdir.as_cwd(), pytest.raises(InvalidConfigError):
+        migrate_config(C.CONFIG_FILE)
     # even though the config is invalid, this should be a noop
     assert cfg.read() == contents