PyModuleDef for an extension/builtin module is usually stored in a static variable and (with immortal objects, see gh-101755) is mostly immutable. The exception is m_copy, which is problematic in some cases for modules imported in multiple interpreters.

Note that m_copy is only relevant for legacy (single-phase init) modules, whether builtin and an extension, and only if the module does not support repeated initialization³. It is never relevant for multi-phase init (PEP 489) modules.

• initialization
  • m_copy is only set by _PyImport_FixupExtensionObject() (and thus indirectly _PyImport_FixupBuiltin() and _imp.create_builtin())
  • _PyImport_FixupExtensionObject() is called by _PyImport_LoadDynamicModuleWithSpec()` when a legacy (single-phase init) extension module is loaded
• usage
  • m_copy is only used in import_find_extension(), which is only called by _imp.create_builtin() and _imp.create_dynamic() (via the respective importers)

When such a legacy module is imported for the first time, m_copy is set to a new copy of the just-imported module's __dict__, which is "owned" by the current interpreter (the one importing the module). Whenever the module is loaded again (e.g. reloaded or deleted from sys.modules and then imported), a new empty module is created and m_copy is [shallow] copied into that object's __dict__.

When m_copy is originally initialized, normally that will be the first time the module is imported. However, that code can be triggered multiple times for that module if it is imported under a different name (an unlikely case but apparently a real one). In that case the m_copy from the previous import is replaced with the new one right after it is released (decref'ed). This isn't the ideal approach but it's also been the behavior for quite a while.

The tricky problem here is that the same code is triggered for each interpreter that imports the legacy module. Things are fine when a module is imported for the first time in any interpreter. However, currently, any subsequent import of that module in another interpreter will trigger that replacing code. The second interpreter decref's the old m_copy, but that object is "owned" by the first interpreter. This is a problem¹.

Furthermore, even if the decref-in-the-wrong-interpreter problem was gone. When m_copy is copied into the new module's __dict__ on subsequent imports, it's only a shallow copy. Thus such a legacy module, imported in other interpreters than the first one, would end up with its __dict__ filled with objects not owned by the correct interpreter.