list_plugin_distributions() raises KeyError for entry points that register() declined (regression from #767) #807

🤖 Written by Claude Opus 5.5 via Claude Code for the pluggy maintainers; I prompted it, it did the work, I read it.

Since #767 (merged, unreleased), load_setuptools_entrypoints() records every loaded entry point in _name2dist keyed by ep.name, whether or not register() actually registered it:

plugin = ep.load()
self.register(plugin, name=ep.name)
self._name2dist[ep.name] = dist
count += 1

list_plugin_distributions() then looks each name up with self._name2plugin[name], and list_plugin_distinfo() calls it. When register() declines a plugin (it returns None for a None plugin, and pytest's PytestPluginManager.register() returns None for the plugins merged into pytest such as pytest_faulthandler and pytest_catchlog), the name is recorded but never registered. The lookup then fails with KeyError.

Impact: with pytest-faulthandler installed, a pytest run built on pluggy main crashes with INTERNALERROR ... KeyError: 'pytest_faulthandler' while printing the header, because the header lists plugins through list_plugin_distinfo(). 1.7.0 is not affected because it stored (plugin, dist) pairs.

Fix: use the return value of register(). Skip the bookkeeping and the count when it is None, and otherwise key _name2dist by the returned name. The test should cover a None plugin and a PluginManager subclass whose register() returns None.

Found while reviewing #804.