gh-127773: Disable attribute cache on incompatible MRO entries #127924
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colesbury
approved these changes
Dec 13, 2024
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This makes sense to me, but @markshannon is probably more familiar with the MRO cache and tp_versions_used.
| @@ -1208,6 +1214,8 @@ _PyType_GetVersionForCurrentState(PyTypeObject *tp) | |||
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| #define MAX_VERSIONS_PER_CLASS 1000 | |||
| _Static_assert(MAX_VERSIONS_PER_CLASS < _Py_ATTR_CACHE_UNUSED, | |||
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I think _freeze_module in the Windows build is not compiling this as C11.
Eclips4
reviewed
Dec 13, 2024
| @@ -0,0 +1 @@ | |||
| Do not use the type attribute cache for types with incompatible MRO. | |||
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Suggested change
| Do not use the type attribute cache for types with incompatible MRO. | |
| Do not use the type attribute cache for types with incompatible :term:`MRO`. |
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This brings back the functionality of
Py_TPFLAGS_HAVE_VERSION_TAG, which should have been named “use version tag”, as the field itself is always present -- see #27260 where I mistakenly removed it.The type attribute cache relies on a class being able to invalidate caches of all of its subclasses. If a type's MRO contains an entry that's not that type's base, the cache needs to be disabled.
This requires a single bit, which was in
tp_flagsin 3.9 and below.I'd like to avoid using
tp_flagsfor an internal flag (even one that's currently reserved for ABI compatibility).We already have a mechanism for disabling the cache: it's disabled after 1000 modifications to a single type, using
tp_versions_usedas a counter. It would be possible to set this to 1000 (MAX_VERSIONS_PER_CLASS) when an incompatible MRO is found. To make the code & logic more understandable, this PR uses a new bigger constant,_Py_ATTR_CACHE_UNUSED. The existingtp_versions_usedcheck is reused.If the
tp_versions_usedmechanism is dropped in the future,_Py_ATTR_CACHE_UNUSEDcan become a single bit stored elsewhere inPyTypeObject. Ideally in a private (underscore-prefixed) field; not intp_flags.@markshannon @colesbury Does this sound right? I'm not much of an attribute cache expert and I don't know about future plans in this area.