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bpo-1635741: Fix unicode_dealloc() for mortal interned string #21270

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Merged
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Jul 3, 2020
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bpo-1635741: Fix unicode_dealloc() for mortal interned string #21270

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18 changes: 14 additions & 4 deletions Objects/unicodeobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1943,13 +1943,20 @@ unicode_dealloc(PyObject *unicode)
break;

case SSTATE_INTERNED_MORTAL:
/* revive dead object temporarily for DelItem */
Py_SET_REFCNT(unicode, 3);
#ifdef INTERNED_STRINGS
/* Revive the dead object temporarily. PyDict_DelItem() removes two
references (key and value) which were ignored by
PyUnicode_InternInPlace(). Use refcnt=3 rather than refcnt=2
to prevent calling unicode_dealloc() again. Adjust refcnt after
PyDict_DelItem(). */
assert(Py_REFCNT(unicode) == 0);
Py_SET_REFCNT(unicode, 3);
if (PyDict_DelItem(interned, unicode) != 0) {
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Can we skip PyDict_DelItem() and set interned unicode's refcnt = 0 directly in here?

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We cannot remove the PyDict_DelItem() call.

PyUnicode_InternInPlace() uses black magic: it ignores 2 references (key and value) of the interned dictionary.

We must remove the string from the interned dictionary. Otherwise, the dictionary will contain a dangling pointer when unicode_dealloc() completes: using the dict is likely to crash in this case.

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got it, make sense, thanks.

_PyErr_WriteUnraisableMsg("deletion of interned string failed",
NULL);
}
assert(Py_REFCNT(unicode) == 1);
Py_SET_REFCNT(unicode, 0);
#endif
break;

Expand Down Expand Up @@ -15710,8 +15717,9 @@ PyUnicode_InternInPlace(PyObject **p)
return;
}

/* The two references in interned are not counted by refcnt.
The deallocator will take care of this */
/* The two references in interned dict (key and value) are not counted by
refcnt. unicode_dealloc() and _PyUnicode_ClearInterned() take care of
this. */
Py_SET_REFCNT(s, Py_REFCNT(s) - 2);
_PyUnicode_STATE(s).interned = SSTATE_INTERNED_MORTAL;
#endif
Expand Down Expand Up @@ -15780,6 +15788,8 @@ _PyUnicode_ClearInterned(PyThreadState *tstate)
#endif
break;
case SSTATE_INTERNED_MORTAL:
// Restore the two references (key and value) ignored
// by PyUnicode_InternInPlace().
Py_SET_REFCNT(s, Py_REFCNT(s) + 2);
#ifdef INTERNED_STATS
mortal_size += PyUnicode_GET_LENGTH(s);
Expand Down