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.

docs/development/plugin_configuration.md

@@ -37,29 +37,34 @@ 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
-`mvt.common.plugin_config.plugin_data_folder()`:
+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.
-`plugin_data_folder()` creates it if it is missing, with `0700` permissions,
-and returns its path. Calling it again returns the same path and leaves the
-contents alone, so a plugin can call it every time it needs the folder:
+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
 
-from mvt.common.plugin_config import plugin_data_folder
-
 
 def cache_path() -> str:
-    return os.path.join(plugin_data_folder("example-plugin"), "results.json")
+    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.
 

src/mvt/common/plugin_config.py

@@ -84,7 +84,8 @@ def plugin_data_folder(plugin_name: str) -> str:
     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.
+    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.
@@ -160,6 +161,8 @@ class MVTPluginSettings(BaseSettings):
 
     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")
@@ -199,6 +202,16 @@ class MVTPluginSettings(BaseSettings):
         """
         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.

tests/common/test_plugin_config.py

@@ -368,3 +368,24 @@ def test_unsafe_plugin_names_have_no_data_folder(data_folder, 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()