Type cache is not correctly invalidated on a class defining mro()

[sjpalt]

BPO	28866
Nosy	@terryjreedy, @vstinner, @encukou, @markshannon, @serhiy-storchaka, @Vgr255, @JulienPalard, @miss-islington
PRs	bpo-28866: Invalidate type cache on every setattr #13117 bpo-28866: No type cache for types with specialized mro, invalidation is hard. #13157 [3.7] bpo-28866: No type cache for types with specialized mro, invalidation is hard. (GH-13157) #13589 bpo-28866: Add regression test #17573
Files	weird.py: Steps to reproduce weird_without_interactive.py issue28866.diff

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-12-04.14:35:45.681>
labels = ['interpreter-core', '3.7', 'type-crash']
title = 'Type cache is not correctly invalidated on a class defining mro()'
updated_at = <Date 2019-12-11.12:36:42.450>
user = 'https://bugs.python.org/sjpalt'

bugs.python.org fields:

activity = <Date 2019-12-11.12:36:42.450>
actor = 'petr.viktorin'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2016-12-04.14:35:45.681>
creator = 'sjpalt'
dependencies = []
files = ['45752', '45756', '45766']
hgrepos = []
issue_num = 28866
keywords = ['patch']
message_count = 22.0
messages = ['282344', '282345', '282349', '282353', '282368', '282375', '282430', '282468', '282485', '282486', '282598', '288072', '341581', '343550', '343581', '343582', '343583', '343585', '343650', '355377', '358257', '358258']
nosy_count = 9.0
nosy_names = ['terry.reedy', 'vstinner', 'petr.viktorin', 'Mark.Shannon', 'serhiy.storchaka', 'abarry', 'mdk', 'sjpalt', 'miss-islington']
pr_nums = ['13117', '13157', '13589', '17573']
priority = 'high'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'crash'
url = 'https://bugs.python.org/issue28866'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6', 'Python 3.7']

[sjpalt]

I ran into a strange bug while experimenting with metaclasses that implement a custom mro() method. It only seems to occur in interactive mode, either returning completely unrelated values or causing a segfault. Python 2 appears unaffected. I have been able to reproduce this with the following code:

# $ python -i weird.py
# >>> proxy.x
# 52011448
# >>> proxy.x
# 6160
# ...

class Foo:
 pass

class Meta(type):
 def mro(cls):
 return (cls, Foo, object)

 def __setattr__(cls, name, value):
 setattr(Foo, name, value)

proxy = Meta('FooProxy', (), {})

proxy.x = 300
proxy.x # don't omit
proxy.x = 0

[serhiy-storchaka]

Python 2 is affected if make Foo a new style class.

[Vgr255]

Additionally, trying to access an attribute before assigning it will result in an AttributeError on all subsequent accesses (even if set).

I didn't manage to get a segfault, however.

[JulienPalard]

FWIW, in _PyType_Lookup I see the "300" PyLong being cached, and later, just before the segfault, I see its address getting out of the cache (a cache hit) but it's no longer a PyLong, it's scrambled, so we're getting a real pointer with scrambled values on the line:

 attribute = _PyType_Lookup(type, name);

segfaulting two lines later in:

descrgetfunc local_get = Py_TYPE(attribute)->tp_descr_get

When I write scrambled value I mean:

(gdb) p *attribute 
$21 = {_ob_next = 0xdbdbdbdbdbdbdbdb, _ob_prev = 0xdbdbdbdbdbdbdbdb, ob_refcnt = -2604246222170760229, ob_type = 0xdbdbdbd\
bdbdbdbdb} 

To debug interactive session in GDB I used:

r -i weird.py < stdin

with "proxy.x" in the stdin file.

[JulienPalard]

I'm able to reproduce the segmentation fault outside the interactive mode, see attached file.

[JulienPalard]

Problem looks mainly due to the __setattr__ done on a different type than the getattr, and the cache of PyType_Lookup:

• type_setattro will be called with Foo, "x", and the value to set
• type_getattro will be called with FooProxy, "x"

The timeline (with my weird_without_interactive.py) is:

• type_setattro is called on Foo for attr "x" with the instance of Bar
• type_getattro is called on FooProxy for attr "x" (the proxy.x line)
  During this type_getattro, the instance of Bar is cached in PyType_Lookup for (FooProxy, "x")
• type_setattro is called on Foo for attr "x" with the value 0
  During this call, insertdict will legitimately call DECREF on the Bar instance, which is deleted
• type_getattro is called on FooProxy with attr "x" for the print()
  During this call, PyType_Lookup will cache hit for (FooProxy, x) with the old reference to the instance of Bar

[vstinner]

If you call sys._clear_type_cache() after the setattr(), weird_without_interactive.py doesn't crash anymore.

[JulienPalard]

I found a way to fix it, but as I'm just discovering cpython internals, I'm not sure it's the right way, review are very welcome.

I fixed it this way because:

The bytecode generated for "proxy.x = 0" (a STORE_ATTR) will call a
PyObject_SetAttr with FooProxy which will soon execute the __setattr__ of Meta, itself calling a PyObject_SetAttr with Foo.

The actual attribute setting is done from the PyObject_SetAttr with Foo (calling in turn type_setattro, and so on), but it's already too late to invalidate the FooProxy type: we no longer have a reference on it and can't guess that FooProxy delegated __setattr__ to Foo.

So the only place when we can invalidate correctly is in the first call of PyObject_SetAttr when we know on which type the attribute will be set, and it make sense: It does not matter what a __setattr__ does and how it does it: invalidation should happen for this attribute on the current type, so invalidating here seems logic.

I did not yet took the time to measure performance loss induced with this patch.

With the patch:

./python -i weird.py
>>> proxy.x
0
>>> proxy.x
0

[vstinner]

The issue is that the Meta class has a reference to the class Foo in its mro() method, but Foo is not aware of Meta. So when Foo is modified, the Foo cache is invalidated, but not Meta cache.

bpo-28866.diff always invalidates the cache, so it works. But it is suboptimal, IMO it defeats the whole purpose of a cache.

I never defined a mro() method. I'm not sure that it's possible to have a type cache and a mro() method?

Options:

• Disable completely the cache on classes defining mro()
• Modify "Meta" (the C code implementing the type, not the Python code) to track the version tag of each class referenced by mro(). Problem: mro() is dynamic!?
• Somehow, notify Foo that Meta has a reference to its cache, so Foo is able to invalidate Meta cache

[vstinner]

• Somehow, notify Foo that Meta has a reference to its cache, so Foo is able to invalidate Meta cache

This was implemented when the type cache was implemented in Python 2.6, but only for explicit subclasses. PyType_Modified() iterates on tp_subclasses.

PyType_Ready() updates tp_subclasses: it stores a weak reference to sub classes in each base class.

I understand that, if we want to implement this feature, type_mro_modified() should be modified to add a backward reference in each base class of the MRO. type_mro_modified() is called when a type is defined, but also when type.__bases__ is explicitly modified.

It would require to add a new slot to types, and so increase a little bit the memory usage, and slow down the creation of a type, and type.__bases__ (slow down: probably negligible, O(1) since the existing tp_subclasses uses a dict).

[JulienPalard]

bpo-28866.diff always invalidates the cache, so it works. But it is suboptimal, IMO it defeats the whole purpose of a cache.

Not sure about defeating the purpose of the cache as I only invalidate in setattr, getattr are still cache hitting. I tried:

$ python3 -m performance run --python=./python --rigorous -b all -o master.json
$ git checkout issue28866; make -j 8
$ python3 -m performance run --python=./python --rigorous -b all -o issue28866.json
$ python3 -m performance compare master.json issue28866.json

And I don't see much differences, probably only noise as I get some faster tests:

### call_method_unknown ###
Median +- Std dev: 56.8 ms +- 3.3 ms -> 52.9 ms +- 1.6 ms: 1.08x faster
Significant (t=12.92)

### pybench.IfThenElse ###
Median +- Std dev: 247 ns +- 3 ns -> 224 ns +- 16 ns: 1.11x faster
Significant (t=15.15)

and some slower:
### pybench.StringPredicates ###
Median +- Std dev: 1.89 us +- 0.05 us -> 2.18 us +- 0.21 us: 1.15x slower
Significant (t=-15.20)

### unpack_sequence ###
Median +- Std dev: 207 ns +- 4 ns -> 231 ns +- 27 ns: 1.12x slower
Significant (t=-11.63)

I'm not yet accustomed to this perf suite, so I may miss something obvious. I'll ran it again on master to measure the noise and should probably fine-tune my system for stability.

I'll also try a benchmark without the cache for comparison.