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gh-105927: PyImport_AddModule() uses _PyWeakref_GET_REF() #106001

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vstinner merged 1 commit into from
Jun 22, 2023
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gh-105927: PyImport_AddModule() uses _PyWeakref_GET_REF() #106001

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gh-105927: PyImport_AddModule() uses _PyWeakref_GET_REF() 
It now raises an exception if sys.modules doesn't hold a strong
reference to the module.

Elaborate the comment explaining why a weak reference is used to
create a borrowed reference.
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@vstinner
vstinner committed Jun 22, 2023
commit 341d2a004ce535e815c76d2a7c46def6a5b22604
22 changes: 19 additions & 3 deletions Python/import.c
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Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,7 @@
#include "pycore_pymem.h" // _PyMem_SetDefaultAllocator()
#include "pycore_pystate.h" // _PyInterpreterState_GET()
#include "pycore_sysmodule.h" // _PySys_Audit()
#include "pycore_weakref.h" // _PyWeakref_GET_REF()
#include "marshal.h" // PyMarshal_ReadObjectFromString()
#include "importdl.h" // _PyImport_DynLoadFiletab
#include "pydtrace.h" // PyDTrace_IMPORT_FIND_LOAD_START_ENABLED()
Expand Down Expand Up @@ -373,15 +374,30 @@ PyImport_AddModuleObject(PyObject *name)
return NULL;
}

// gh-86160: PyImport_AddModuleObject() returns a borrowed reference
// gh-86160: PyImport_AddModuleObject() returns a borrowed reference.
// Create a weak reference to produce a borrowed reference, since it can
// become NULL. sys.modules type can be different than dict and it is not
// guaranteed that it keeps a strong reference to the module. It can be a
// custom mapping with __getitem__() which returns a new object or removes
// returned object, or __setitem__ which does nothing. There is so much
// unknown. With weakref we can be sure that we get either a reference to
// live object or NULL.
//
// Use PyImport_AddModuleRef() to avoid these issues.
PyObject *ref = PyWeakref_NewRef(mod, NULL);
Py_DECREF(mod);
if (ref == NULL) {
return NULL;
}

mod = PyWeakref_GetObject(ref);
mod = _PyWeakref_GET_REF(ref);
Py_DECREF(ref);
Py_XDECREF(mod);

if (mod == NULL && !PyErr_Occurred()) {
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@serhiy-storchaka serhiy-storchaka Jun 23, 2023

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I am not sure that comparing mod with NULL after Py_XDECREF does not have an undefined behavior. AFAIK, you can do nothing with a pointer to released memory, not even compare it with NULL.

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@vstinner vstinner Jun 23, 2023

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See comments of my PR #105998: IMO most of this code is useless and can be removed.

PyErr_SetString(PyExc_RuntimeError,
"sys.modules does not hold a strong reference "
"to the module");
}
return mod; /* borrowed reference */
}

Expand Down