fix(core20): use importlib for local plugins

Avoid random importing from sys.path by using importlib.

This will only ever work with python modules, and not python
packages, we we theoretically do not support per our docs at
https://snapcraft.io/docs/writing-local-plugins

Fixes #5217
Signed-off-by: Sergio Schvezov <[email protected]>

snapcraft_legacy/internal/pluginhandler/_plugin_loader.py

@@ -39,10 +39,10 @@ def load_plugin(
     definitions_schema,
 ) -> plugins.v2.PluginV2:
     local_plugins_dir = project._get_local_plugins_dir()
-    if local_plugins_dir is not None:
-        plugin_class = _get_local_plugin_class(
-            plugin_name=plugin_name, local_plugins_dir=local_plugins_dir
-        )
+
+    plugin_class = _get_local_plugin_class(
+        plugin_name=plugin_name, local_plugins_dir=Path(local_plugins_dir)
+    )
     if plugin_class is None:
         plugin_class = plugins.get_plugin_for_base(
             plugin_name, build_base=project._get_build_base()
@@ -57,57 +57,46 @@ def load_plugin(
     return plugin
 
 
-def _load_compat_x_prefix(plugin_name: str, module_name: str, local_plugin_dir: str):
-    compat_path = Path(local_plugin_dir, f"x-{plugin_name}.py")
-    if not compat_path.exists():
-        return None
-
-    preferred_name = f"{module_name}.py"
-    logger.warning(
-        f"Legacy plugin name detected, please rename the plugin's file name {compat_path.name!r} to {preferred_name!r}."
+def _load_local(plugin_name: str, local_plugin_dir: Path):
+    # The legacy module path is for the case when we allowed the plugin
+    # file name to have '-', this is the first entry
+    module_names = (
+        Path(f"x-{plugin_name}.py"),
+        Path(f"{plugin_name.replace('-', '_')}.py"),
     )
+    module_paths = (local_plugin_dir / m for m in module_names)
+    valid_paths = [m for m in module_paths if m.exists()]
+    # No valid paths means no local plugin to load
+    if not valid_paths:
+        return None
 
-    spec = importlib.util.spec_from_file_location(plugin_name, compat_path)
+    module_path = valid_paths[0]
+    spec = importlib.util.spec_from_file_location(plugin_name, module_path)
     if spec.loader is None:
-        return None
+        raise errors.PluginError(f"unknown plugin: {plugin_name!r}")
 
     # Prevent mypy type complaints by asserting type.
     assert isinstance(spec.loader, importlib.abc.Loader)
 
     module = importlib.util.module_from_spec(spec)
     spec.loader.exec_module(module)
-    return module
-
-
-def _load_local(plugin_name: str, local_plugin_dir: str):
-    module_name = plugin_name.replace("-", "_")
-
-    module = _load_compat_x_prefix(plugin_name, module_name, local_plugin_dir)
-    if module is None:
-        sys.path = [local_plugin_dir] + sys.path
-        logger.debug(
-            f"Loading plugin module {module_name!r} with sys.path {sys.path!r}"
-        )
-        try:
-            module = importlib.import_module(module_name)
-        finally:
-            sys.path.pop(0)
 
     return module
 
 
-def _get_local_plugin_class(*, plugin_name: str, local_plugins_dir: str):
-    with contextlib.suppress(ImportError):
-        module = _load_local(plugin_name, local_plugins_dir)
-        logger.info(f"Loaded local plugin for {plugin_name}")
+def _get_local_plugin_class(*, plugin_name: str, local_plugins_dir: Path):
+    module = _load_local(plugin_name, local_plugins_dir)
+    if not module:
+        return
+    logger.info(f"Loaded local plugin for {plugin_name}")
 
-        # v2 requires plugin implementation to be named "PluginImpl".
-        if hasattr(module, "PluginImpl") and issubclass(
-            module.PluginImpl, plugins.v2.PluginV2
-        ):
-            return module.PluginImpl
+    # v2 requires plugin implementation to be named "PluginImpl".
+    if hasattr(module, "PluginImpl") and issubclass(
+        module.PluginImpl, plugins.v2.PluginV2
+    ):
+        return module.PluginImpl
 
-        raise errors.PluginError(f"unknown plugin: {plugin_name!r}")
+    raise errors.PluginError(f"unknown plugin: {plugin_name!r}")
 
 
 def _validate_pull_and_build_properties(