Multiple extensions using `get_base_class_hook` interfere with each other

[MadLittleMods]

Bug Report

The first plugin that uses get_base_class_hook and returns a callback appears to take exclusive ownership of that hook, preventing other plugins from intercepting the same class definitions.

I specifically noticed this with the mypy_zope plugin which uses get_base_class_hook to analyze every class. And then writing my own custom mypy plugin where I also want to use get_base_class_hook but it wasn't getting called at all.

To Reproduce

1. Setup two plugins using get_base_class_hook. To notice the effect, they should return a callback instead of None.
   mypy_plugin1.py

   from typing import Callable, Optional, Type
   
   from mypy.plugin import (
       ClassDefContext,
       Plugin,
   )
   
   
   class Plugin1(Plugin):
       def get_base_class_hook(
           self, fullname: str
       ) -> Optional[Callable[[ClassDefContext], None]]:
           def _analyze_class(classdef_ctx: ClassDefContext) -> None:
               print("plugin1 get_base_class_hook", fullname)
   
           return _analyze_class
   
   
   def plugin(version: str) -> Type[Plugin1]:
       return Plugin1

   mypy_plugin2.py

   from typing import Callable, Optional, Type
   
   from mypy.plugin import (
       ClassDefContext,
       Plugin,
   )
   
   
   class Plugin2(Plugin):
       def get_base_class_hook(
           self, fullname: str
       ) -> Optional[Callable[[ClassDefContext], None]]:
           def _analyze_class(classdef_ctx: ClassDefContext) -> None:
               print("plugin2 get_base_class_hook", fullname)
   
           return _analyze_class
   
   
   def plugin(version: str) -> Type[Plugin2]:
       return Plugin2

2. Configure mypy to use those plugins. Edit mypy.ini and add plugins = scripts-dev/mypy_plugin1.py, scripts-dev/mypy_plugin2.py
3. Lint some files, poetry run mypy
4. Notice that there is only logs from plugin1

Expected Behavior

mypy calls get_base_class_hook in every plugin (logs for plugin1 and plugin2 show up)

Actual Behavior

plugin1 blocks other plugins from hooking into get_base_class_hook (only logs from plugin1 show up).

Your Environment

• Mypy version used: v1.16.1
• Mypy command-line flags: (none)
• Mypy configuration options from mypy.ini (and other config files): plugins = scripts-dev/mypy_plugin1.py, scripts-dev/mypy_plugin2.py (real-life use case)
• Python version used: 3.13.3

[brianschubert]

Note that this is consistent with the documented behavior of plugin hooks:

Mypy calls a method once for each plugin in the list until one of the methods returns a non-None value. This callback will be then used to customize the corresponding aspect of analyzing/checking the current abstract syntax tree node.

[MadLittleMods]

Ahh, thank you @brianschubert! (known-limitation vs bug)

Somehow, this didn't stand out to me when I read it just a moment ago. I guess it could also depend on what "further processing" means in the paragraph just below:

The callback returned by the get_xxx method will be given a detailed current context and an API to create new nodes, new types, emit error messages, etc., and the result will be used for further processing.

[MadLittleMods]

We were able to workaround this problem by putting our custom mypy plugin first and then importing the other plugins that use the same conflicting hooks and calling them manually (see element-hq/synapse#18733).

Is making the plugin situation better something that mypy maintainers want to track with this issue or should this be closed as a known-limitation?