cat attachment | python -m json.tool

reliably makes python to SIGSEGV on Arch linux

$ python --version
Python 3.5.0
$ uname -a
Linux kumabox 4.2.2-1-ARCH #1 SMP PREEMPT Tue Sep 29 22:21:33 CEST 2015 x86_64 GNU/Linux

Does not fail on 2.7.10.

Doesn't crash on 3.4.3. Crashes on 3.5.0. The crash is reproduced with Python-only implementation of json, therefore it is not related to json.

Can you post a GDB traceback?

3.6.0a0 (de982d8b7b15)

#0  0x080e62a1 in _PyTrash_thread_destroy_chain () at Objects/object.c:2010
#1  0x080bf8b3 in frame_dealloc (f=f@entry=0x83f06e4) at Objects/frameobject.c:462
#2  0x080e3eef in _Py_Dealloc (op=op@entry=0x83f06e4) at Objects/object.c:1786
#3  0x081909c1 in fast_function (func=func@entry=0xb77f4154, pp_stack=pp_stack@entry=0xbfffe6ac, n=n@entry=0, na=na@entry=0, nk=nk@entry=0) at Python/ceval.c:4752
#4  0x0819102a in call_function (pp_stack=pp_stack@entry=0xbfffe6ac, oparg=oparg@entry=0) at Python/ceval.c:4677
#5  0x0818e1f7 in PyEval_EvalFrameEx (f=f@entry=0xb795f034, throwflag=throwflag@entry=0) at Python/ceval.c:3181
#6  0x081906e7 in _PyEval_EvalCodeWithName (_co=_co@entry=0xb77f00f8, globals=globals@entry=0xb798f734, locals=locals@entry=0xb798f734, args=args@entry=0x0, 
    argcount=argcount@entry=0, kws=kws@entry=0x0, kwcount=kwcount@entry=0, defs=defs@entry=0x0, defcount=defcount@entry=0, kwdefs=kwdefs@entry=0x0, 
    closure=closure@entry=0x0, name=name@entry=0x0, qualname=qualname@entry=0x0) at Python/ceval.c:3962
#7  0x0819080a in PyEval_EvalCodeEx (_co=_co@entry=0xb77f00f8, globals=globals@entry=0xb798f734, locals=locals@entry=0xb798f734, args=args@entry=0x0, 
    argcount=argcount@entry=0, kws=kws@entry=0x0, kwcount=kwcount@entry=0, defs=defs@entry=0x0, defcount=defcount@entry=0, kwdefs=kwdefs@entry=0x0, 
    closure=closure@entry=0x0) at Python/ceval.c:3983
#8  0x0819086d in PyEval_EvalCode (co=co@entry=0xb77f00f8, globals=globals@entry=0xb798f734, locals=locals@entry=0xb798f734) at Python/ceval.c:777
#9  0x08180503 in builtin_exec_impl (module=module@entry=0xb7a1e534, source=0xb77f00f8, globals=0xb798f734, locals=0xb798f734) at Python/bltinmodule.c:942
#10 0x08180688 in builtin_exec (module=module@entry=0xb7a1e534, args=args@entry=0xb78b784c) at Python/clinic/bltinmodule.c.h:275
#11 0x080e025c in PyCFunction_Call (func=func@entry=0xb7a1ecb4, args=args@entry=0xb78b784c, kwds=kwds@entry=0x0) at Objects/methodobject.c:109
#12 0x08190f18 in call_function (pp_stack=pp_stack@entry=0xbfffe97c, oparg=oparg@entry=2) at Python/ceval.c:4651
#13 0x0818e1f7 in PyEval_EvalFrameEx (f=f@entry=0xb79d4cac, throwflag=throwflag@entry=0) at Python/ceval.c:3181
#14 0x081906e7 in _PyEval_EvalCodeWithName (_co=0xb78b9028, globals=<optimized out>, locals=locals@entry=0x0, args=args@entry=0xb7a00e84, argcount=argcount@entry=5, 
    kws=kws@entry=0xb7a00e98, kwcount=kwcount@entry=0, defs=defs@entry=0xb78ba288, defcount=5, kwdefs=kwdefs@entry=0x0, closure=closure@entry=0x0, 
    name=name@entry=0xb78b83e8, qualname=qualname@entry=0xb78b83e8) at Python/ceval.c:3962
#15 0x08190a52 in fast_function (func=func@entry=0xb7925994, pp_stack=pp_stack@entry=0xbfffeb3c, n=n@entry=5, na=na@entry=5, nk=nk@entry=0) at Python/ceval.c:4760
#16 0x0819102a in call_function (pp_stack=pp_stack@entry=0xbfffeb3c, oparg=oparg@entry=5) at Python/ceval.c:4677
#17 0x0818e1f7 in PyEval_EvalFrameEx (f=f@entry=0xb7a00d1c, throwflag=throwflag@entry=0) at Python/ceval.c:3181
#18 0x081906e7 in _PyEval_EvalCodeWithName (_co=_co@entry=0xb78b9298, globals=globals@entry=0xb78b17dc, locals=locals@entry=0x0, args=args@entry=0xb79b83c8, 
    argcount=argcount@entry=2, kws=kws@entry=0x0, kwcount=kwcount@entry=0, defs=defs@entry=0xb78b7eb8, defcount=defcount@entry=1, kwdefs=kwdefs@entry=0x0, 
    closure=closure@entry=0x0, name=name@entry=0x0, qualname=qualname@entry=0x0) at Python/ceval.c:3962
#19 0x0819080a in PyEval_EvalCodeEx (_co=0xb78b9298, globals=globals@entry=0xb78b17dc, locals=locals@entry=0x0, args=args@entry=0xb79b83c8, argcount=2, 
    kws=kws@entry=0x0, kwcount=kwcount@entry=0, defs=defs@entry=0xb78b7eb8, defcount=defcount@entry=1, kwdefs=0x0, closure=0x0) at Python/ceval.c:3983
#20 0x0820be82 in function_call (func=0xb7841454, arg=0xb79b83b4, kw=0x0) at Objects/funcobject.c:632
#21 0x080939ce in PyObject_Call (func=func@entry=0xb7841454, arg=arg@entry=0xb79b83b4, kw=kw@entry=0x0) at Objects/abstract.c:2147
#22 0x0807614a in RunModule (modname=modname@entry=0x8330308 L"json.tool", set_argv0=set_argv0@entry=1) at Modules/main.c:208
#23 0x08076f56 in Py_Main (argc=argc@entry=5, argv=argv@entry=0x832f010) at Modules/main.c:709
#24 0x0805edb4 in main (argc=5, argv=0xbfffee84) at ./Programs/python.c:69

Likely this issue is related to C implementation of OrderedDict. json.tool doesn't crash with --sort-keys and doesn't crash with Python implementation of OrderedDict.

The patch for similar bpo-25406 doesn't fix this issue.

The following simplified code produces the crash:

from collections import OrderedDict
obj = []
for _ in range(33):
    obj = OrderedDict(((None, obj),))
for _ in range(17):
    obj = [obj]
print("Still alive, crash happens at interpreter finalization")

This crashes at the same line as in Serhiy’s backtrace. In _PyTrash_thread_destroy_chain(), Py_TYPE(op) is a bad pointer (0xdbdbdbdbdbdbdbdb); I have enabled --with-pydebug.

Here is a patch that seems to fix the problem for me. Can someone who knows more about tp_dealloc() methods and the Py_TRASHCAN_SAFE stuff have a look? All I know is that the body of the trashcan stuff can be deferred until an outer nested body is finished, so I though it might be best to defer that PyDict_Type.tp_dealloc() call as well.

Is it worth packaging up my simplified code in a test case, perhaps in an assert_python_ok() child process?

I think there is no need to run the test in a subprocess. You can trigger the crash by "del obj".

from collections import OrderedDict
obj = None
for _ in range(1000):
    obj = OrderedDict([(None, obj)])

del obj
support.gc_collect()

Unfortunately this test is crashed with the patch.

Fatal Python error: Objects/odictobject.c:1443 object at 0xb749ddfc has negative ref count -1

Current thread 0xb7580700 (most recent call first):
File "<stdin>", line 1 in <module>
Aborted (core dumped)

Here is a new patch which uses Py_CLEAR() rather than Py_XDECREF(), which seems to cure the negative reference count problem. This change was only a guess based on looking at namespaceobject.c and the Py_CLEAR() documentation, so it would be good for someone else to carefully review it.

Using Py_CLEAR() fixes symptoms in this test, but the real issue is that
deallocating code is executed twice for some objects (for every 25th
OrderedDict).

PyDict_Type.tp_dealloc() can deposit an object in the _PyTrash_delete_later
list if trash_delete_nesting exceeds PyTrash_UNWIND_LEVEL. Later Py_TYPE(op)-

tp_dealloc is called for objects in the _PyTrash_delete_later list. But this
is odict_dealloc that already was called.

One of ways to avoid repeated deallocation is to change the type of the object
before calling PyDict_Type.tp_dealloc(). This looks as a hack, but correct
hack. May be there is more straightforward solution.

Thanks for solving this!

odict-trashcan.v3.patch LGTM

There is a problem with odict-trashcan.v3.patch. PyDict_Type.tp_dealloc() can put the object to the freelist if it's type is dict. Since odict_dealloc() fakes object's type, it alows deallocated OrderedDict to be later used as dict. But the size of OrderedDict object is larger than the size of plain dict, this creates a leak.

odict-trashcan.v2.patch is not good too. I found new test that crashes with it.

Here is a patch that solves all known issues with OrderedDict deallocation. Please correct my wording in the comment if needed. Similar bad scenario is described in the comment at the end of subtype_dealloc() in Objects/typeobject.c, but the patch uses different method to solve this.

Left a comment about a minor English grammar problem.

The existing comment Serhiy mentioned was added way back in 2003 for bpo-668433. It appears to use the same underlying technique, reverting the nesting level before calling the base class dealloc. One paragraph talks about the extra increment used there compared to Serhiy’s method:

'''
But now it's possible that a chain of objects consisting solely of objects whose deallocator is subtype_dealloc() will defeat the trashcan mechanism completely: the decremented level means that the effective level never reaches the limit. Therefore, we *increment* the level *before* entering the trashcan block, and matchingly decrement it after leaving. This means the trashcan code will trigger a little early, but that's no big deal.
'''

I think we may not have to worry about this for ordered dict, assuming that PyDict_Type.tp_dealloc() can only recursively invoke odict_dealloc() from within its own trashcan block.

So as far as I can tell, this patch is good to apply, unless someone with more knowledge of garbage collection has any comments.

New changeset ce8c850a52d4 by Serhiy Storchaka in branch '3.5':
Issue bpo-25395: Fixed crash when highly nested OrderedDict structures were
https://hg.python.org/cpython/rev/ce8c850a52d4

New changeset bd6bfa5fe203 by Serhiy Storchaka in branch 'default':
Issue bpo-25395: Fixed crash when highly nested OrderedDict structures were
https://hg.python.org/cpython/rev/bd6bfa5fe203

Thank you Martin for your review and for initial investigation.