Derive the data folder of a plugin from its settings class

A plugin with a settings class already names itself in `plugin_name`;
passing the name again to plugin_data_folder() repeats it and can drift.
Add a `data_folder()` class method on MVTPluginSettings which returns
plugin_data_folder() for the class's validated plugin name (works on the
class and on an instance); plugin_data_folder(name) stays as the function
underneath for plugins without a settings class.

README.md

@@ -84,7 +84,7 @@ details.
 
 Users can also add top-level commands to `mvt-ios` and `mvt-android` from
 installed Python packages or local files and folders. See the
-[custom CLI command documentation](https://docs.mvt.re/en/latest/custom_commands/)
+[custom CLI command documentation](https://docs.mvt.re/en/latest/development/custom_commands/)
 for the plugin entry points and `--load-command` interface.
 
 

docs/development/custom_commands.md

@@ -1,7 +1,7 @@
 # Custom CLI Commands
 
 MVT can load additional top-level commands into `mvt-ios` and `mvt-android`.
-Custom commands are different from [custom forensic modules](development.md#custom-modules):
+Custom commands are different from [custom forensic modules](index.md#custom-modules):
 commands add new CLI operations, while modules add analysis steps to existing
 `check-*` commands.
 
@@ -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 +1,149 @@
+# 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 server URL or the timestamp of the last
+synchronization. MVT provides a namespaced settings base class so each plugin
+keeps its configuration in its own file, and a data folder for anything else a
+plugin needs to keep on disk.
+
+!!! 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.
+
+## Plugin Data Folder
+
+Everything else a plugin keeps on disk, such as a cache, a downloaded artifact
+or synchronization state, belongs in the folder returned by the `data_folder()`
+class method of the plugin's settings class, or by
+`mvt.common.plugin_config.plugin_data_folder()` called with the plugin name if
+the plugin has no settings class:
+
+```
+~/.local/share/mvt/plugin-data/<plugin name>/                # Linux
+~/Library/Application Support/mvt/plugin-data/<plugin name>/ # macOS
+```
+
+The folder sits beside MVT's own data, such as the downloaded indicators. It is
+created if it is missing, with `0700` permissions. Asking for it again returns
+the same path and leaves the contents alone, so a plugin can ask for it every
+time it needs the folder. `ExamplePluginSettings` below is the settings class
+defined in the next section:
+
+```python
+import os
+
+
+def cache_path() -> str:
+    folder = ExamplePluginSettings.data_folder()
+    return os.path.join(folder, "virustotal_lookups_cache.json")
+```
+
+A plugin which has no settings class calls
+`plugin_data_folder("example-plugin")` instead.
+
+Do not fall back on a path of your own such as `~/.cache/example-plugin`: it
+is a Linux-only convention, and MVT will not create it for you.
+
+## 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
+    MAX_RESULTS: int = 25
+    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_MAX_RESULTS=50
+```
+
+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

@@ -50,6 +50,7 @@ nav:
         - Check Android Intrusion Logs: "android/intrusion_logs.md"
     - Indicators of Compromise: "iocs.md"
     - Development:
-        - Development Instructions: "development.md"
-        - Custom CLI Commands: "custom_commands.md"
+        - 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 +1,299 @@
+# 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, user_data_dir
+from pydantic import ValidationError
+from pydantic_settings import (
+    BaseSettings,
+    EnvSettingsSource,
+    PydanticBaseSettingsSource,
+    SettingsConfigDict,
+    YamlConfigSettingsSource,
+)
+
+PLUGIN_CONFIG_FOLDER_NAME = "plugins"
+# Not "plugins": on macOS the configuration and data folders are the same
+# directory, and that name already holds the settings files.
+PLUGIN_DATA_FOLDER_NAME = "plugin-data"
+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_data_folder(plugin_name: str) -> str:
+    """
+    Return the folder where a given plugin stores its data, creating it.
+
+    Plugins should keep whatever they persist, such as caches or downloaded
+    artifacts, in this folder. It is created with owner-only permissions. The
+    path is resolved on every call so it always reflects the current
+    environment. A plugin with a settings class calls
+    `MVTPluginSettings.data_folder()` instead, which passes `plugin_name` here.
+
+    :param plugin_name: Name of the plugin.
+    :returns: The path of the data folder of the plugin.
+    """
+    # Validate the name before anything is created, so an unsafe name cannot
+    # leave a folder behind.
+    name = validate_plugin_name(plugin_name)
+
+    # makedirs() applies its mode only to the last folder of the path, so
+    # MVT's own data folder keeps the default permissions while the two
+    # plugin folders are private.
+    data_folder = os.path.join(user_data_dir("mvt"), PLUGIN_DATA_FOLDER_NAME)
+    os.makedirs(data_folder, mode=0o700, exist_ok=True)
+
+    folder = os.path.join(data_folder, name)
+    os.makedirs(folder, mode=0o700, exist_ok=True)
+    return folder
+
+
+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.
+
+    `data_folder()` returns the folder the plugin keeps its data in.
+    """
+
+    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()
+
+    @classmethod
+    def data_folder(cls) -> str:
+        """
+        Return the data folder of the plugin, creating it.
+
+        The folder is the one plugin_data_folder() returns for `plugin_name`,
+        so a plugin with a settings class does not repeat its name.
+        """
+        return plugin_data_folder(_settings_plugin_name(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 +1,391 @@
+# 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_data_folder,
+    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
+
+
+@pytest.fixture
+def data_folder(tmp_path, monkeypatch):
+    folder = tmp_path / "data"
+    monkeypatch.setattr(
+        "mvt.common.plugin_config.user_data_dir",
+        lambda *args, **kwargs: str(folder),
+    )
+    return folder
+
+
+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)
+
+
+def test_data_folder_is_namespaced_and_created(data_folder):
+    folder = plugin_data_folder("example-plugin")
+
+    assert folder == str(data_folder / "plugin-data" / "example-plugin")
+    assert os.path.isdir(folder)
+
+
+def test_data_folder_can_be_requested_repeatedly(data_folder):
+    folder = plugin_data_folder("example-plugin")
+    with open(os.path.join(folder, "kept.json"), "w") as data_file:
+        data_file.write("{}")
+
+    assert plugin_data_folder("example-plugin") == folder
+    assert os.listdir(folder) == ["kept.json"]
+
+
+@pytest.mark.skipif(
+    sys.platform == "win32", reason="POSIX file permissions are not available"
+)
+def test_data_folder_is_only_accessible_by_the_user(data_folder):
+    folder = plugin_data_folder("example-plugin")
+
+    assert stat.S_IMODE(os.stat(folder).st_mode) & 0o077 == 0
+    parent_mode = stat.S_IMODE(os.stat(os.path.dirname(folder)).st_mode)
+    assert parent_mode & 0o077 == 0
+
+
+def test_plugins_get_their_own_data_folder(data_folder):
+    example_folder = plugin_data_folder("example-plugin")
+    other_folder = plugin_data_folder("other-plugin")
+
+    assert example_folder != other_folder
+    assert sorted(os.listdir(data_folder / "plugin-data")) == [
+        "example-plugin",
+        "other-plugin",
+    ]
+
+
+def test_data_folder_does_not_touch_the_configuration_folder(
+    config_folder, data_folder
+):
+    plugin_data_folder("example-plugin")
+
+    assert not os.path.exists(plugin_config_folder())
+
+
+@pytest.mark.parametrize(
+    "plugin_name", ["../escape", "folder/name", "UPPER", "-dash", ""]
+)
+def test_unsafe_plugin_names_have_no_data_folder(data_folder, plugin_name):
+    with pytest.raises(ValueError, match="Invalid plugin name"):
+        plugin_data_folder(plugin_name)
+
+    assert not os.path.exists(data_folder)
+
+
+def test_settings_class_knows_its_data_folder(data_folder):
+    folder = ExamplePluginSettings.data_folder()
+
+    assert folder == plugin_data_folder("example-plugin")
+    assert os.path.isdir(folder)
+    assert OtherPluginSettings.data_folder() != folder
+
+
+def test_settings_instance_uses_the_same_data_folder(config_folder, data_folder):
+    settings = ExamplePluginSettings.load()
+
+    assert settings.data_folder() == ExamplePluginSettings.data_folder()
+
+
+def test_subclass_without_its_own_name_shares_the_data_folder(data_folder):
+    class InheritingSettings(ExamplePluginSettings):
+        pass
+
+    assert InheritingSettings.data_folder() == ExamplePluginSettings.data_folder()