fix: Raise exception when meltano.yml is empty (meltano#6627)

* Raise exception when empty meltano.yml

* tests for empty meltano.yml

* Update tests/meltano/cli/test_cli.py

Co-authored-by: Florian Hines <syn@ronin.io>

* remove unnecessary tests

Co-authored-by: Edgar R. M <edgarrm358@gmail.com>
Co-authored-by: Florian Hines <syn@ronin.io>

src/meltano/core/error.py

@@ -7,7 +7,7 @@
 from enum import Enum
 
 
-class ExitCode(int, Enum):
+class ExitCode(int, Enum):  # noqa: D101
     OK = 0
     FAIL = 1
     NO_RETRY = 2
@@ -16,14 +16,14 @@ class ExitCode(int, Enum):
 class Error(Exception):
     """Base exception for ELT errors."""
 
-    def exit_code(self):
+    def exit_code(self):  # noqa: D102
         return ExitCode.FAIL
 
 
 class ExtractError(Error):
     """Error in the extraction process, like API errors."""
 
-    def exit_code(self):
+    def exit_code(self):  # noqa: D102
         return ExitCode.NO_RETRY
 
 
@@ -35,7 +35,7 @@ def __init__(
         message: str,
         process: Process,
         stderr: str | None = None,
-    ):
+    ):  # noqa: DAR101
         """Initialize AsyncSubprocessError."""
         self.process = process
         self._stderr: str | StreamReader | None = stderr or process.stderr
@@ -44,7 +44,7 @@ def __init__(
     @property
     async def stderr(self) -> str | None:
         """Return the output of the process to stderr."""
-        if not self._stderr:
+        if not self._stderr:  # noqa: DAR201
             return None
         elif not isinstance(self._stderr, str):
             stream = await self._stderr.read()
@@ -59,3 +59,7 @@ class PluginInstallError(Exception):
 
 class PluginInstallWarning(Exception):
     """Exception for when a plugin optional optional step fails to install."""
+
+
+class EmptyMeltanoFileException(Exception):
+    """Exception for empty meltano.yml file."""

src/meltano/core/project.py

@@ -19,7 +19,7 @@
 from meltano.core.plugin.base import PluginRef
 
 from .behavior.versioned import Versioned
-from .error import Error
+from .error import EmptyMeltanoFileException, Error
 from .project_files import ProjectFiles
 from .utils import makedirs, truthy
 
@@ -227,6 +227,9 @@ def project_files(self):
     def meltano(self) -> MeltanoFileHint:
         """Return a copy of the current meltano config.
 
+        Raises:
+            EmptyMeltanoFileException: For empty meltano.yml
+
         Returns:
             the current meltano config
         """
@@ -236,6 +239,8 @@ def meltano(self) -> MeltanoFileHint:
 
         with open(self.meltanofile) as melt_ff:
             meltano_ff: dict = configure_yaml().load(melt_ff)
+        if meltano_ff is None:
+            raise EmptyMeltanoFileException
 
         uvicorn_enabled = (
             meltano_ff.get(f"{FEATURE_FLAG_PREFIX}.{FeatureFlags.ENABLE_UVICORN}")

tests/fixtures/fs.py

@@ -36,6 +36,13 @@ def test_dir(tmp_path_factory) -> Path:
         os.chdir(cwd)
 
 
+@pytest.fixture(scope="session")
+def test_empty_meltano_yml(test_dir):
+    meltano_file = test_dir / "meltano.yml"
+    meltano_file.write_text("")
+    return test_dir
+
+
 @pytest.fixture
 def pushd(request):
     def _pushd(path):

tests/meltano/cli/test_cli.py

@@ -6,6 +6,7 @@
 
 import meltano
 from meltano.cli import cli
+from meltano.core.error import EmptyMeltanoFileException
 from meltano.core.project import PROJECT_READONLY_ENV, Project
 from meltano.core.project_settings_service import ProjectSettingsService
 
@@ -28,6 +29,14 @@ def project(self, test_dir, project_init_service):
     def deactivate_project(self):
         Project.deactivate()
 
+    @pytest.fixture()
+    def empty_project(self, test_empty_meltano_yml, pushd):
+        project = Project(test_empty_meltano_yml)
+        try:
+            yield project
+        finally:
+            Project.deactivate()
+
     def test_activate_project(self, project, cli_runner, pushd):
         assert Project._default is None
 
@@ -38,6 +47,11 @@ def test_activate_project(self, project, cli_runner, pushd):
         assert Project._default.root == project.root
         assert Project._default.readonly is False
 
+    def test_empty_meltano_yml_project(self, empty_project, cli_runner, pushd):
+        pushd(empty_project.root)
+        with pytest.raises(EmptyMeltanoFileException):
+            cli_runner.invoke(cli, ["config"], catch_exceptions=False)
+
     def test_activate_project_readonly_env(
         self, project, cli_runner, pushd, monkeypatch
     ):