plugin handler: fix local plugin class detection with direct imports

If a local plugin imports a snapcraft in-tree plugin directly,
e.g. `from snapcraft.plugins.v1 import AutotoolsPlugin`,
_get_local_plugin_class() will find an return the AutotoolsPlugin
attribute, not the desired plugin class, which inherits from
AutotoolsPlugin.

This change filters out any attributes that has a __module__ from
snapcraft.plugins.v1.  This allows us to remove the direct check for
PluginV1, which was likely the first case of filtering for this problem.

Spread tests:
- add another x-local test with this logic for PluginV1.
- modify from-nilplugin to use the direct import of NilPlugin.

Signed-off-by: Chris Patterson <chris.patterson@canonical.com>

Author: Chris Patterson

snapcraft/internal/pluginhandler/_plugin_loader.py

@@ -124,7 +124,12 @@ def _get_local_plugin_class(*, plugin_name: str, local_plugins_dir: str):
                 continue
             if not issubclass(attr, plugins.v1.PluginV1):
                 continue
-            if attr == plugins.v1.PluginV1:
+            if not hasattr(attr, "__module__"):
+                continue
+            logger.debug(
+                f"Plugin attribute {attr!r} has __module__: {attr.__module__!r}"
+            )
+            if attr.__module__.startswith("snapcraft.plugins"):
                 continue
             return attr
         else:

tests/spread/plugins/v1/x-local/local-plugin/task.yaml

@@ -3,6 +3,7 @@ summary: Use local plugins to build a snap
 environment:
   SNAP_DIR/baseplugin: ../snaps/from-baseplugin
   SNAP_DIR/nilplugin: ../snaps/from-nilplugin
+  SNAP_DIR/pluginv1: ../snaps/from-pluginv1
 
 prepare: |
   #shellcheck source=tests/spread/tools/snapcraft-yaml.sh

tests/spread/plugins/v1/x-local/snaps/from-nilplugin/snap/plugins/x_local_plugin.py

@@ -14,10 +14,10 @@
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-from snapcraft.plugins import nil
+from snapcraft.plugins.v1 import NilPlugin
 
 
-class LocalPlugin(nil.NilPlugin):
+class LocalPlugin(NilPlugin):
     @classmethod
     def schema(cls):
         schema = super().schema()

tests/spread/plugins/v1/x-local/snaps/from-pluginv1/snap/plugins/x_local_plugin.py

@@ -0,0 +1,38 @@
+# -*- Mode:Python; indent-tabs-mode:nil; tab-width:4 -*-
+#
+# Copyright (C) 2015 Canonical Ltd
+#
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 3 as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+from snapcraft.plugins.v1 import PluginV1
+
+
+class LocalPlugin(PluginV1):
+    @classmethod
+    def schema(cls):
+        schema = super().schema()
+
+        schema["properties"]["foo"] = {"type": "string"}
+
+        return schema
+
+    @classmethod
+    def get_pull_properties(cls):
+        return ["foo", "stage-packages"]
+
+    @classmethod
+    def get_build_properties(cls):
+        return ["foo", "stage-packages"]
+
+    def build(self):
+        return self.run(["touch", "build-stamp"], self.installdir)

tests/spread/plugins/v1/x-local/snaps/from-pluginv1/snap/snapcraft.yaml

@@ -0,0 +1,12 @@
+name: test-local-plugins
+base: core18
+version: "0.1"
+summary: local plugin in snap/plugins
+description: Tests if local plugins load and can build
+confinement: strict
+grade: devel
+
+parts:
+    x-local-plugin:
+      plugin: x-local-plugin
+      source: .