Add namespaced plugin configuration support

Plugin packages need somewhere to keep their own settings, but MVT
rewrites its config.yaml with only the fields it knows about, so any
foreign section is dropped. Add MVTPluginSettings, a pydantic-settings
base class that gives each plugin its own file under the MVT config
folder and its own MVT_PLUGIN_<NAME>_ environment variable namespace.

Settings resolve from constructor arguments, then the environment, then
the plugin file, then the field defaults. Saving skips the values the
environment currently supplies, so credentials passed as environment
variables are not copied to disk, and writes through a private temporary
file so a settings file is never partially written or briefly readable
by other users.

docs/development/custom_commands.md

@@ -56,6 +56,10 @@ pipx inject mvt my-mvt-plugin
 When MVT is installed in an active virtual environment, install the plugin with
 `pip` in that environment.
 
+Command packages that need their own settings, such as an API key, should store
+them in a namespaced [plugin configuration file](plugin_configuration.md)
+rather than in MVT's own `config.yaml`.
+
 ## Load a Command File
 
 For local commands that are not packaged, create a Python file that exports one

docs/development/index.md

@@ -160,6 +160,10 @@ For a `pipx` installation of MVT, inject the package into MVT's environment:
 pipx inject mvt mvt-plugin-amnesty-custom
 ```
 
+Module packages that need their own settings, such as an API key, should store
+them in a namespaced [plugin configuration file](plugin_configuration.md)
+rather than in MVT's own `config.yaml`.
+
 ### Naming module packages
 
 Name module packages `mvt-plugin-<name>` (import package `mvt_plugin_<name>`),

docs/development/plugin_configuration.md

@@ -0,0 +1,114 @@
+# Plugin Configuration
+
+Plugin packages that add [custom CLI commands](custom_commands.md) or
+[modules](index.md#installed-module-packages) often need to store their
+own settings, such as an API key, a cache folder or the timestamp of the last
+synchronization. MVT provides a namespaced settings base class so each plugin
+keeps its configuration in its own file.
+
+!!! warning
+
+    Do not write plugin settings to MVT's own `config.yaml`. MVT rewrites that
+    file with the settings it knows about every time it starts, so any other
+    section is deleted.
+
+## Where Settings Are Stored
+
+Each plugin gets one YAML file in a `plugins` folder next to MVT's own
+configuration:
+
+```
+~/.config/mvt/plugins/<plugin name>.yaml
+```
+
+The exact parent folder follows the platform convention used for MVT's
+`config.yaml` (for example `~/Library/Application Support/mvt` on macOS). Use
+`mvt.common.plugin_config.plugin_config_path()` instead of building the path by
+hand.
+
+Plugin names must be lowercase and may only contain letters, digits and dashes,
+matching the `mvt-plugin-<name>` package naming convention. MVT creates the
+`plugins` folder with `0700` permissions and writes the settings files with
+`0600` permissions, because they commonly hold credentials. Files are written
+through a temporary file and moved into place, so an interrupted save never
+leaves a partially written settings file behind.
+
+## Defining Plugin Settings
+
+Subclass `MVTPluginSettings`, set `plugin_name` and declare typed fields with
+defaults:
+
+```python
+from typing import Optional
+
+from mvt.common.plugin_config import MVTPluginSettings
+
+
+class ExamplePluginSettings(MVTPluginSettings):
+    plugin_name = "example-plugin"
+
+    API_KEY: Optional[str] = None
+    CACHE_FOLDER: str = "~/.cache/example-plugin"
+    LAST_SYNC: Optional[str] = None
+```
+
+`load()` returns the current settings and `save()` writes them back:
+
+```python
+from datetime import datetime, timezone
+
+import click
+
+
+def sync():
+    settings = ExamplePluginSettings.load()
+    if not settings.API_KEY:
+        raise click.ClickException(
+            "No API key configured. Set MVT_PLUGIN_EXAMPLE_PLUGIN_API_KEY or "
+            "run 'example-plugin configure'."
+        )
+
+    settings.LAST_SYNC = datetime.now(timezone.utc).isoformat()
+    settings.save()
+```
+
+A missing settings file is not an error: the plugin then runs on the field
+defaults and on whatever the environment provides. `save()` only persists the
+values that differ from the defaults, and it never touches MVT's `config.yaml`.
+A settings file that cannot be parsed, or that does not hold a mapping of
+setting names to values, raises a `PluginConfigLoadError` naming the file.
+
+Every subclass that sets its own `plugin_name` gets its own file and its own
+environment namespace. A subclass that does not redefine `plugin_name` inherits
+it, and therefore shares the file and the environment variables of its parent
+class.
+
+## Environment Variables
+
+Every field can also be set with an environment variable. The prefix is
+`MVT_PLUGIN_`, followed by the plugin name upper-cased with dashes replaced by
+underscores, followed by the field name. For the example above:
+
+```bash
+export MVT_PLUGIN_EXAMPLE_PLUGIN_API_KEY=...
+export MVT_PLUGIN_EXAMPLE_PLUGIN_CACHE_FOLDER=/tmp/example-cache
+```
+
+Settings resolve in this order, from highest to lowest priority:
+
+1. Arguments passed to the settings class directly, such as
+   `ExamplePluginSettings(API_KEY="...")`
+2. Environment variables
+3. The plugin's YAML file
+4. The field defaults declared on the settings class
+
+!!! tip
+
+    On shared or multi-user machines, prefer passing API keys through
+    environment variables rather than saving them to the plugin file. `save()`
+    skips every value that the environment currently supplies, so a credential
+    provided that way is not copied into the settings file when a plugin saves
+    an unrelated setting.
+
+Unknown keys in a plugin's YAML file are ignored, so a settings file written by
+a newer version of a plugin does not break an older one.

mkdocs.yml

@@ -52,4 +52,5 @@ nav:
     - Development:
         - Development Instructions: "development/index.md"
         - Custom CLI Commands: "development/custom_commands.md"
+        - Plugin Configuration: "development/plugin_configuration.md"
     - License: "license.md"

src/mvt/common/plugin_config.py

@@ -0,0 +1,256 @@
+# Mobile Verification Toolkit (MVT)
+# Copyright (c) 2021-2026 The MVT Authors.
+# Use of this software is governed by the MVT License 1.1 that can be found at
+#   https://license.mvt.re/1.1/
+
+import contextlib
+import json
+import os
+import re
+import tempfile
+from typing import Any, ClassVar, Dict, List, Tuple, Type, TypeVar
+
+import yaml
+from appdirs import user_config_dir
+from pydantic import ValidationError
+from pydantic_settings import (
+    BaseSettings,
+    EnvSettingsSource,
+    PydanticBaseSettingsSource,
+    SettingsConfigDict,
+    YamlConfigSettingsSource,
+)
+
+PLUGIN_CONFIG_FOLDER_NAME = "plugins"
+PLUGIN_ENV_PREFIX = "MVT_PLUGIN_"
+
+PLUGIN_NAME_PATTERN = re.compile(r"[a-z0-9][a-z0-9-]*")
+
+PluginSettingsType = TypeVar("PluginSettingsType", bound="MVTPluginSettings")
+
+
+class PluginConfigLoadError(Exception):
+    pass
+
+
+def validate_plugin_name(plugin_name: Any) -> str:
+    """
+    Check that a plugin name is safe to use in a file name and an env variable.
+
+    :param plugin_name: Name to validate.
+    :returns: The validated plugin name.
+    """
+    if not isinstance(plugin_name, str):
+        raise TypeError(
+            f"Plugin name must be a string, not {type(plugin_name).__name__}"
+        )
+    if not PLUGIN_NAME_PATTERN.fullmatch(plugin_name):
+        raise ValueError(
+            f"Invalid plugin name {plugin_name!r}: plugin names must start with a "
+            "lowercase letter or a digit and may only contain lowercase letters, "
+            "digits and dashes"
+        )
+    return plugin_name
+
+
+def plugin_config_folder() -> str:
+    """
+    Return the folder where plugins store their configuration files.
+
+    The path is resolved on every call so it always reflects the current
+    environment.
+    """
+    return os.path.join(user_config_dir("mvt"), PLUGIN_CONFIG_FOLDER_NAME)
+
+
+def plugin_config_path(plugin_name: str) -> str:
+    """
+    Return the path of the configuration file of a given plugin.
+
+    :param plugin_name: Name of the plugin.
+    """
+    return os.path.join(
+        plugin_config_folder(), f"{validate_plugin_name(plugin_name)}.yaml"
+    )
+
+
+def plugin_env_prefix(plugin_name: str) -> str:
+    """
+    Return the environment variable prefix used by a given plugin.
+
+    Dashes are replaced by underscores. Plugin names cannot contain underscores,
+    so two different plugin names never share an environment namespace.
+
+    :param plugin_name: Name of the plugin.
+    """
+    name = validate_plugin_name(plugin_name).upper().replace("-", "_")
+    return f"{PLUGIN_ENV_PREFIX}{name}_"
+
+
+def _settings_plugin_name(settings_cls: Type[BaseSettings]) -> str:
+    plugin_name = getattr(settings_cls, "plugin_name", None)
+    if plugin_name is None:
+        raise TypeError(
+            f"{settings_cls.__name__} must set a 'plugin_name' class attribute to "
+            "namespace its configuration file and environment variables"
+        )
+    return validate_plugin_name(plugin_name)
+
+
+def _plugin_yaml_source(
+    settings_cls: Type[BaseSettings], config_path: str
+) -> YamlConfigSettingsSource:
+    """
+    Build the YAML settings source of a plugin, reporting unusable files.
+
+    A missing file is not an error, but a file which cannot be parsed or which
+    does not hold a mapping of setting names is reported with its path.
+    """
+    try:
+        return YamlConfigSettingsSource(settings_cls, config_path)
+    except yaml.YAMLError as exc:
+        raise PluginConfigLoadError(
+            f"Invalid plugin configuration file {config_path}: {exc}"
+        ) from exc
+    except (TypeError, ValueError) as exc:
+        raise PluginConfigLoadError(
+            f"Invalid plugin configuration file {config_path}: the file must "
+            "contain a mapping of setting names to values"
+        ) from exc
+
+
+class MVTPluginSettings(BaseSettings):
+    """
+    Base class for plugin-namespaced settings.
+
+    Subclass with typed fields and set `plugin_name`. Values resolve from
+    constructor arguments, then environment variables (MVT_PLUGIN_<NAME>_*),
+    then the plugin's YAML file (~/.config/mvt/plugins/<name>.yaml), then field
+    defaults.
+
+    Plugins must not store their settings in MVT's own configuration file: MVT
+    rewrites it with the fields it knows about, dropping anything else.
+    """
+
+    model_config = SettingsConfigDict(extra="ignore")
+
+    plugin_name: ClassVar[str]
+
+    def __init_subclass__(cls, **kwargs: Any) -> None:
+        super().__init_subclass__(**kwargs)
+        plugin_name = _settings_plugin_name(cls)
+        # Namespace the environment variables of this plugin. Each pydantic
+        # model gets its own configuration dictionary, so this does not leak
+        # into other plugins.
+        cls.model_config["env_prefix"] = plugin_env_prefix(plugin_name)
+
+    @classmethod
+    def settings_customise_sources(
+        cls,
+        settings_cls: Type[BaseSettings],
+        init_settings: PydanticBaseSettingsSource,
+        env_settings: PydanticBaseSettingsSource,
+        dotenv_settings: PydanticBaseSettingsSource,
+        file_secret_settings: PydanticBaseSettingsSource,
+    ) -> Tuple[PydanticBaseSettingsSource, ...]:
+        config_path = plugin_config_path(_settings_plugin_name(settings_cls))
+        yaml_source = _plugin_yaml_source(settings_cls, config_path)
+        # Explicit arguments take precedence over environment variables, which
+        # in turn take precedence over the configuration file.
+        return (init_settings, env_settings, yaml_source)
+
+    @classmethod
+    def load(cls: Type[PluginSettingsType]) -> PluginSettingsType:
+        """
+        Load the settings of the plugin.
+
+        A missing configuration file is not an error: the settings then come
+        from the environment and from the field defaults.
+        """
+        return cls()
+
+    def _environment_values(self) -> Dict[str, Any]:
+        """
+        Return the settings values currently supplied by the environment.
+
+        Values are validated by the model, so they can be compared with the
+        values held by this instance.
+        """
+        settings_cls = type(self)
+        raw_values = EnvSettingsSource(settings_cls)()
+        names = [name for name in raw_values if name in settings_cls.model_fields]
+        if not names:
+            return {}
+
+        # Fall back on the current values for the fields the environment does
+        # not set, so that required fields do not fail validation here.
+        current_values = json.loads(self.model_dump_json())
+        try:
+            from_environment = settings_cls.model_validate(
+                {**current_values, **raw_values}
+            )
+        except ValidationError:
+            # An environment variable which the model cannot validate must not
+            # stop the other environment values from being recognised, or a
+            # credential would be written to the configuration file.
+            return self._environment_values_by_field(current_values, raw_values, names)
+        return {name: getattr(from_environment, name) for name in names}
+
+    def _environment_values_by_field(
+        self,
+        current_values: Dict[str, Any],
+        raw_values: Dict[str, Any],
+        names: List[str],
+    ) -> Dict[str, Any]:
+        """
+        Validate each environment value on its own, skipping unusable ones.
+
+        :param current_values: Serialized values held by this instance.
+        :param raw_values: Values supplied by the environment.
+        :param names: Names of the fields set by the environment.
+        """
+        settings_cls = type(self)
+        values = {}
+        for name in names:
+            try:
+                from_environment = settings_cls.model_validate(
+                    {**current_values, name: raw_values[name]}
+                )
+            except ValidationError:
+                continue
+            values[name] = getattr(from_environment, name)
+        return values
+
+    def save(self) -> None:
+        """
+        Save the current settings to the configuration file of the plugin.
+
+        Only values which differ from the field defaults are persisted. Values
+        which come from the environment are not written to disk, so credentials
+        passed as environment variables stay out of the configuration file.
+        MVT's own configuration file is never modified.
+        """
+        config_folder = plugin_config_folder()
+        if not os.path.isdir(config_folder):
+            os.makedirs(config_folder, mode=0o700, exist_ok=True)
+
+        values = json.loads(self.model_dump_json(exclude_defaults=True))
+        for name, environment_value in self._environment_values().items():
+            if name in values and getattr(self, name, None) == environment_value:
+                del values[name]
+
+        # Settings files can hold credentials, so write them through a private
+        # temporary file and move it in place. The file is then never partially
+        # written and never briefly readable by other users.
+        config_path = plugin_config_path(self.plugin_name)
+        descriptor, temporary_path = tempfile.mkstemp(
+            dir=config_folder, prefix=f".{self.plugin_name}-", suffix=".yaml"
+        )
+        try:
+            with os.fdopen(descriptor, "w") as config_file:
+                config_file.write(yaml.dump(values, default_flow_style=False))
+            os.replace(temporary_path, config_path)
+        except BaseException:
+            with contextlib.suppress(OSError):
+                os.unlink(temporary_path)
+            raise

tests/common/test_plugin_config.py

@@ -0,0 +1,303 @@
+# Mobile Verification Toolkit (MVT)
+# Copyright (c) 2021-2026 The MVT Authors.
+# Use of this software is governed by the MVT License 1.1 that can be found at
+#   https://license.mvt.re/1.1/
+
+import os
+import stat
+import sys
+from typing import Optional
+
+import pytest
+import yaml
+
+from mvt.common.plugin_config import (
+    MVTPluginSettings,
+    PluginConfigLoadError,
+    plugin_config_folder,
+    plugin_config_path,
+    plugin_env_prefix,
+)
+
+
+class ExamplePluginSettings(MVTPluginSettings):
+    plugin_name = "example-plugin"
+
+    API_KEY: Optional[str] = None
+    CACHE_FOLDER: str = "cache"
+    MAX_RESULTS: int = 25
+
+
+class OtherPluginSettings(MVTPluginSettings):
+    plugin_name = "other-plugin"
+
+    API_KEY: Optional[str] = None
+
+
+@pytest.fixture
+def config_folder(tmp_path, monkeypatch):
+    monkeypatch.setattr(
+        "mvt.common.plugin_config.user_config_dir",
+        lambda *args, **kwargs: str(tmp_path),
+    )
+    return tmp_path
+
+
+def _write_plugin_file(plugin_name, values):
+    config_path = plugin_config_path(plugin_name)
+    os.makedirs(os.path.dirname(config_path), exist_ok=True)
+    content = values if isinstance(values, str) else yaml.dump(values)
+    with open(config_path, "w") as config_file:
+        config_file.write(content)
+    return config_path
+
+
+def test_plugin_paths_and_prefixes_are_namespaced(config_folder):
+    assert plugin_config_folder() == str(config_folder / "plugins")
+    assert plugin_config_path("example-plugin") == str(
+        config_folder / "plugins" / "example-plugin.yaml"
+    )
+    assert plugin_env_prefix("example-plugin") == "MVT_PLUGIN_EXAMPLE_PLUGIN_"
+    assert plugin_env_prefix("other-plugin") == "MVT_PLUGIN_OTHER_PLUGIN_"
+
+
+def test_defaults_are_used_without_file_or_environment(config_folder):
+    settings = ExamplePluginSettings.load()
+
+    assert settings.API_KEY is None
+    assert settings.CACHE_FOLDER == "cache"
+    assert settings.MAX_RESULTS == 25
+    assert not os.path.exists(plugin_config_path("example-plugin"))
+
+
+def test_values_are_loaded_from_the_plugin_file(config_folder):
+    _write_plugin_file("example-plugin", {"API_KEY": "from-file", "MAX_RESULTS": 5})
+
+    settings = ExamplePluginSettings.load()
+
+    assert settings.API_KEY == "from-file"
+    assert settings.MAX_RESULTS == 5
+    assert settings.CACHE_FOLDER == "cache"
+
+
+def test_environment_overrides_the_plugin_file(config_folder, monkeypatch):
+    _write_plugin_file("example-plugin", {"API_KEY": "from-file", "MAX_RESULTS": 5})
+    monkeypatch.setenv("MVT_PLUGIN_EXAMPLE_PLUGIN_API_KEY", "from-environment")
+
+    settings = ExamplePluginSettings.load()
+
+    assert settings.API_KEY == "from-environment"
+    assert settings.MAX_RESULTS == 5
+
+
+def test_arguments_override_the_environment_and_the_plugin_file(
+    config_folder, monkeypatch
+):
+    _write_plugin_file("example-plugin", {"API_KEY": "from-file", "MAX_RESULTS": 5})
+    monkeypatch.setenv("MVT_PLUGIN_EXAMPLE_PLUGIN_API_KEY", "from-environment")
+    monkeypatch.setenv("MVT_PLUGIN_EXAMPLE_PLUGIN_MAX_RESULTS", "10")
+
+    settings = ExamplePluginSettings(API_KEY="from-argument")
+
+    assert settings.API_KEY == "from-argument"
+    assert settings.MAX_RESULTS == 10
+
+
+def test_save_and_load_round_trip(config_folder):
+    settings = ExamplePluginSettings.load()
+    settings.API_KEY = "saved-key"
+    settings.MAX_RESULTS = 100
+
+    settings.save()
+
+    config_path = plugin_config_path("example-plugin")
+    assert os.path.isfile(config_path)
+    with open(config_path) as config_file:
+        assert yaml.safe_load(config_file) == {
+            "API_KEY": "saved-key",
+            "MAX_RESULTS": 100,
+        }
+
+    reloaded = ExamplePluginSettings.load()
+    assert reloaded.API_KEY == "saved-key"
+    assert reloaded.MAX_RESULTS == 100
+    assert reloaded.CACHE_FOLDER == "cache"
+
+
+@pytest.mark.skipif(
+    sys.platform == "win32", reason="POSIX file permissions are not available"
+)
+def test_saved_file_is_only_readable_by_the_user(config_folder):
+    settings = ExamplePluginSettings.load()
+    settings.API_KEY = "saved-key"
+
+    settings.save()
+
+    config_path = plugin_config_path("example-plugin")
+    assert stat.S_IMODE(os.stat(config_path).st_mode) == 0o600
+    folder_mode = stat.S_IMODE(os.stat(plugin_config_folder()).st_mode)
+    assert folder_mode & 0o077 == 0
+
+
+@pytest.mark.skipif(
+    sys.platform == "win32", reason="POSIX file permissions are not available"
+)
+def test_save_restricts_the_permissions_of_an_existing_file(config_folder):
+    config_path = _write_plugin_file("example-plugin", {"API_KEY": "from-file"})
+    os.chmod(config_path, 0o644)
+
+    settings = ExamplePluginSettings.load()
+    settings.MAX_RESULTS = 100
+    settings.save()
+
+    assert stat.S_IMODE(os.stat(config_path).st_mode) == 0o600
+    assert os.listdir(plugin_config_folder()) == ["example-plugin.yaml"]
+
+
+def test_save_only_persists_non_default_values(config_folder):
+    settings = ExamplePluginSettings.load()
+    settings.CACHE_FOLDER = "another-cache"
+
+    settings.save()
+
+    with open(plugin_config_path("example-plugin")) as config_file:
+        assert yaml.safe_load(config_file) == {"CACHE_FOLDER": "another-cache"}
+
+
+def test_save_does_not_persist_values_coming_from_the_environment(
+    config_folder, monkeypatch
+):
+    monkeypatch.setenv("MVT_PLUGIN_EXAMPLE_PLUGIN_API_KEY", "environment-secret")
+
+    settings = ExamplePluginSettings.load()
+    assert settings.API_KEY == "environment-secret"
+    settings.MAX_RESULTS = 100
+    settings.save()
+
+    with open(plugin_config_path("example-plugin")) as config_file:
+        assert yaml.safe_load(config_file) == {"MAX_RESULTS": 100}
+
+
+def test_an_invalid_environment_variable_still_protects_the_other_values(
+    config_folder, monkeypatch
+):
+    monkeypatch.setenv("MVT_PLUGIN_EXAMPLE_PLUGIN_API_KEY", "environment-secret")
+    monkeypatch.setenv("MVT_PLUGIN_EXAMPLE_PLUGIN_MAX_RESULTS", "not-an-int")
+
+    settings = ExamplePluginSettings(MAX_RESULTS=100)
+    assert settings.API_KEY == "environment-secret"
+    settings.save()
+
+    with open(plugin_config_path("example-plugin")) as config_file:
+        saved_values = yaml.safe_load(config_file)
+    assert saved_values == {"MAX_RESULTS": 100}
+    assert "API_KEY" not in saved_values
+
+
+def test_save_persists_values_which_differ_from_the_environment(
+    config_folder, monkeypatch
+):
+    monkeypatch.setenv("MVT_PLUGIN_EXAMPLE_PLUGIN_API_KEY", "environment-secret")
+
+    settings = ExamplePluginSettings.load()
+    settings.API_KEY = "chosen-key"
+    settings.save()
+
+    with open(plugin_config_path("example-plugin")) as config_file:
+        assert yaml.safe_load(config_file) == {"API_KEY": "chosen-key"}
+
+
+def test_save_does_not_write_the_mvt_configuration_file(config_folder):
+    settings = ExamplePluginSettings.load()
+    settings.API_KEY = "saved-key"
+
+    settings.save()
+
+    assert os.listdir(config_folder) == ["plugins"]
+
+
+def test_unknown_keys_in_the_plugin_file_are_ignored(config_folder):
+    _write_plugin_file(
+        "example-plugin",
+        {"API_KEY": "from-file", "UNKNOWN_SETTING": "ignored"},
+    )
+
+    settings = ExamplePluginSettings.load()
+
+    assert settings.API_KEY == "from-file"
+    assert not hasattr(settings, "UNKNOWN_SETTING")
+
+
+def test_unparsable_plugin_file_is_reported_with_its_path(config_folder):
+    config_path = _write_plugin_file("example-plugin", "API_KEY: [unclosed\n")
+
+    with pytest.raises(PluginConfigLoadError) as raised:
+        ExamplePluginSettings.load()
+
+    assert config_path in str(raised.value)
+
+
+def test_plugin_file_which_is_not_a_mapping_is_reported_with_its_path(config_folder):
+    config_path = _write_plugin_file("example-plugin", "- one\n- two\n")
+
+    with pytest.raises(PluginConfigLoadError) as raised:
+        ExamplePluginSettings.load()
+
+    assert config_path in str(raised.value)
+    assert "mapping of setting names" in str(raised.value)
+
+
+def test_plugins_do_not_interfere_with_each_other(config_folder, monkeypatch):
+    _write_plugin_file("other-plugin", {"API_KEY": "other-file-key"})
+    monkeypatch.setenv("MVT_PLUGIN_EXAMPLE_PLUGIN_API_KEY", "example-environment-key")
+
+    example_settings = ExamplePluginSettings.load()
+    other_settings = OtherPluginSettings.load()
+
+    assert example_settings.API_KEY == "example-environment-key"
+    assert other_settings.API_KEY == "other-file-key"
+
+    example_settings.MAX_RESULTS = 100
+    example_settings.save()
+    assert sorted(os.listdir(plugin_config_folder())) == [
+        "example-plugin.yaml",
+        "other-plugin.yaml",
+    ]
+    with open(plugin_config_path("other-plugin")) as config_file:
+        assert yaml.safe_load(config_file) == {"API_KEY": "other-file-key"}
+
+
+def test_subclass_without_plugin_name_is_rejected():
+    with pytest.raises(TypeError, match="plugin_name"):
+
+        class MissingNameSettings(MVTPluginSettings):
+            API_KEY: Optional[str] = None
+
+
+def test_subclass_with_invalid_plugin_name_is_rejected():
+    with pytest.raises(ValueError, match="Invalid plugin name"):
+
+        class InvalidNameSettings(MVTPluginSettings):
+            plugin_name = "Bad/Name"
+
+
+def test_underscores_are_not_allowed_in_plugin_names():
+    # Underscores are replaced by dashes in the environment prefix, so allowing
+    # both would let two plugin names share one environment namespace.
+    with pytest.raises(ValueError, match="Invalid plugin name"):
+
+        class UnderscoreNameSettings(MVTPluginSettings):
+            plugin_name = "under_score"
+
+    with pytest.raises(ValueError, match="Invalid plugin name"):
+        plugin_config_path("under_score")
+    with pytest.raises(ValueError, match="Invalid plugin name"):
+        plugin_env_prefix("under_score")
+
+
+@pytest.mark.parametrize(
+    "plugin_name", ["../escape", "folder/name", "UPPER", "-dash", ""]
+)
+def test_unsafe_plugin_names_have_no_configuration_path(plugin_name):
+    with pytest.raises(ValueError, match="Invalid plugin name"):
+        plugin_config_path(plugin_name)