I found a segfault in pickle.load when you overload __getattr__ and
create yourself a infinite loop in the latest svn checkout of python 3:

########################################
import pickle

class Foo:
    def __getattr__(self, key):
        self.foo

with open('foo.db', 'wb') as f:
    foo = Foo()
    pickle.dump(foo, f)

with open('foo.db', 'rb') as f:
    pickle.load(f)
########################################

This results in this stack trace on my mac:

Reason: KERN_PROTECTION_FAILURE at address: 0x0000000c
0x0000dc6b in PyObject_Call (func=0x0, arg=0x44cd58, kw=0x0) at
Objects/abstract.c:2174
2174 if ((call = func->ob_type->tp_call) != NULL) {
(gdb) bt
#0 0x0000dc6b in PyObject_Call (func=0x0, arg=0x44cd58, kw=0x0) at
Objects/abstract.c:2174
#1 0x004c1b4d in unpickler_call (self=0x4a6240, func=0x0, arg=0x4b66c8)
at /Users/Shared/erickt/Projects/py3k.svn/Modules/_pickle.c:413
#2 0x004cac9a in load_build (self=0x4a6240) at
/Users/Shared/erickt/Projects/py3k.svn/Modules/_pickle.c:3844
#3 0x004cbb4f in load (self=0x4a6240) at
/Users/Shared/erickt/Projects/py3k.svn/Modules/_pickle.c:4047
#4 0x004cbe71 in Unpickler_load (self=0x4a6240) at
/Users/Shared/erickt/Projects/py3k.svn/Modules/_pickle.c:4119
#5 0x000f2fef in call_function (pp_stack=0xbfffea84, oparg=0) at
Python/ceval.c:3387
#6 0x000edfdb in PyEval_EvalFrameEx (f=0x326cd8, throwflag=0) at
Python/ceval.c:2205
#7 0x000f157e in PyEval_EvalCodeEx (co=0x4a9628, globals=0x487f50,
locals=0x0, args=0x32593c, argcount=1, kws=0x325940, kwcount=0,
defs=0x0, defcount=0, kwdefs=0x4b6428, closure=0x0) at
Python/ceval.c:2840
#8 0x000f39e5 in fast_function (func=0x4b4ab8, pp_stack=0xbfffee54,
n=1, na=1, nk=0) at Python/ceval.c:3501
#9 0x000f35cf in call_function (pp_stack=0xbfffee54, oparg=1) at
Python/ceval.c:3424
#10 0x000edfdb in PyEval_EvalFrameEx (f=0x3257f8, throwflag=0) at
Python/ceval.c:2205
#11 0x000f157e in PyEval_EvalCodeEx (co=0x444c28, globals=0x255818,
locals=0x255818, args=0x0, argcount=0, kws=0x0, kwcount=0, defs=0x0,
defcount=0, kwdefs=0x0, closure=0x0) at Python/ceval.c:2840
#12 0x000e564f in PyEval_EvalCode (co=0x444c28, globals=0x255818,
locals=0x255818) at Python/ceval.c:519
#13 0x00122a96 in run_mod (mod=0x872c80, filename=0xbffff228 "foo.py",
globals=0x255818, locals=0x255818, flags=0xbffff628, arena=0x322020) at
Python/pythonrun.c:1553
#14 0x00122884 in PyRun_FileExFlags (fp=0xa00dcde0, filename=0xbffff228
"foo.py", start=257, globals=0x255818, locals=0x255818, closeit=1,
flags=0xbffff628) at Python/pythonrun.c:1510
#15 0x00120e39 in PyRun_SimpleFileExFlags (fp=0xa00dcde0,
filename=0xbffff228 "foo.py", closeit=1, flags=0xbffff628) at
Python/pythonrun.c:1048
#16 0x001202f9 in PyRun_AnyFileExFlags (fp=0xa00dcde0,
filename=0xbffff228 "foo.py", closeit=1, flags=0xbffff628) at
Python/pythonrun.c:845
#17 0x00134d1c in Py_Main (argc=2, argv=0x227028) at Modules/main.c:592
#18 0x00002574 in main (argc=2, argv=0xbffff748) at python.c:57

It seems that this isn't just for infinite loops. If you replace the
class with this:

class Foo:
    def __init__(self):
        self.foo = {}

    def __getattr__(self, key):
        self.foo[5]

It still errors out. So I'm guessing pickle is just not handling
exceptions properly.

Does this occur in 2.6 or 2.5?

Guido van Rossum <guido@python.org> added the comment:

Does this occur in 2.6 or 2.5?

It doesn't in python 2.5. The RuntimeError manages to get printed out.
I don't have 2.6 installed to test against at the moment. Here's the
equivalent code:

########################
import pickle

class Foo:
    def __getattr__(self, key):
        self.foo

f = open('foo.db', 'w')
foo = Foo()
pickle.dump(foo, f)
f.close()

f = open('foo.db', 'r')
pickle.load(f)
f.close()
########################

This also happens with cPickle.

This is a bug in the C implementation of pickle (i.e., the _pickle
module). I think you're right about the missing exception check. At
first glance, it looks like the missing else-if case for "setstate ==
NULL", in load_build(), is the cause of the problem:

static int
load_build(UnpicklerObject *self)
{
...
    setstate = PyObject_GetAttrString(inst, "__setstate__");
    if (setstate == NULL && PyErr_ExceptionMatches(PyExc_AttributeError)) {
        PyErr_Clear();
    }
/*---missing else-if case---------
    else if (setstate == NULL) {
        return NULL;
    }
----------------------------------*/
    else {
        PyObject *result;

        /* The explicit __setstate__ is responsible for everything. */
        result = unpickler_call(self, setstate, state);
        Py_DECREF(setstate);
        if (result == NULL)
            return -1;
        Py_DECREF(result);
        return 0;
    }
...

Good find Alexandre! Can you work up a patch? Would be even better if
it had a unittest.

Committed fix in r65689. Thanks!