revert plugin performance regression (#7589)

Co-authored-by: Serge Matveenko <lig@pydantic.dev>

pydantic/plugin/_loader.py

@@ -18,30 +18,41 @@
 
 PYDANTIC_ENTRY_POINT_GROUP: Final[str] = 'pydantic'
 
-_plugins: dict[str, PydanticPluginProtocol] = {}
+# cache of plugins
+_plugins: dict[str, PydanticPluginProtocol] | None = None
+# return no plugins while loading plugins to avoid recursion and errors while import plugins
+# this means that if plugins use pydantic
+_loading_plugins: bool = False
 
 
 def get_plugins() -> Iterable[PydanticPluginProtocol]:
     """Load plugins for Pydantic.
 
     Inspired by: https://github.com/pytest-dev/pluggy/blob/1.3.0/src/pluggy/_manager.py#L376-L402
     """
-    global _plugins
-
-    for dist in importlib_metadata.distributions():
-        for entry_point in dist.entry_points:
-            if entry_point.group != PYDANTIC_ENTRY_POINT_GROUP:
-                continue
-            if entry_point.value in _plugins:
-                continue
-            try:
-                _plugins[entry_point.value] = entry_point.load()
-            except (ImportError, AttributeError) as e:
-                error_type = e.__class__.__name__
-                warnings.warn(
-                    f'{error_type} while loading the `{entry_point.name}` Pydantic plugin, this could be caused '
-                    f'by a circular import issue (e.g. the plugin importing Pydantic), Pydantic will attempt to '
-                    f'import this plugin again each time a Pydantic validators is created. {e}'
-                )
+    global _plugins, _loading_plugins
+    if _loading_plugins:
+        # this happens when plugins themselves use pydantic, we return no plugins
+        return ()
+    elif _plugins is None:
+        _plugins = {}
+        # set _loading_plugins so any plugins that use pydantic don't themselves use plugins
+        _loading_plugins = True
+        try:
+            for dist in importlib_metadata.distributions():
+                for entry_point in dist.entry_points:
+                    if entry_point.group != PYDANTIC_ENTRY_POINT_GROUP:
+                        continue
+                    if entry_point.value in _plugins:
+                        continue
+                    try:
+                        _plugins[entry_point.value] = entry_point.load()
+                    except (ImportError, AttributeError) as e:
+                        warnings.warn(
+                            f'{e.__class__.__name__} while loading the `{entry_point.name}` Pydantic plugin, '
+                            f'this plugin will not be installed.\n\n{e!r}'
+                        )
+        finally:
+            _loading_plugins = False
 
     return _plugins.values()

tests/plugin/example_plugin.py

@@ -5,6 +5,10 @@ class MyModel(BaseModel):
     x: int
 
 
+m = MyModel(x='10')
+if m.x != 10:
+    raise ValueError('m.x should be 10')
+
 log = []
 
 

tests/plugin/test_plugin.py

@@ -8,11 +8,9 @@
 def test_plugin_usage():
     from pydantic import BaseModel
 
-    with pytest.warns(UserWarning, match='AttributeError while loading the `my_plugin` Pydantic plugin.*'):
-
-        class MyModel(BaseModel):
-            x: int
-            y: str
+    class MyModel(BaseModel):
+        x: int
+        y: str
 
     m = MyModel(x='10', y='hello')
     assert m.x == 10