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I've been reviewing all uses of PyObject_IsInstance, and I believe
that the use of PyObject_IsInstance in py-unwind.c is not entirely
correct. The use of PyObject_IsInstance is in this code in
frame_unwind_python::sniff:
if (PyObject_IsInstance (pyo_unwind_info,
(PyObject *) &unwind_info_object_type) <= 0)
error (_("A Unwinder should return gdb.UnwindInfo instance."));
The problem is that PyObject_IsInstance can return -1 to indicate an
error, in which case a Python error will have been set. Now, the
above code appears to handle this case, it checks for '<= 0', however,
frame_unwind_python::sniff has this near the start:
gdbpy_enter enter_py (gdbarch);
And looking in python.c at 'gdbpy_enter::~gdbpy_enter ()', you'll
notice that if an error is set then the error is printed, but also, we
get a warning about an unhandled Python exception. Clearly, all
exceptions should have been handled by the time the gdbpy_enter
destructor is called.
I've added a test as part of this commit that exposes this problem,
the current output is:
(gdb) backtrace
Python Exception <class 'RuntimeError'>: error in Blah.__class__
warning: internal error: Unhandled Python exception
Python Exception <class 'gdb.error'>: A Unwinder should return gdb.UnwindInfo instance.
#0 corrupt_frame_inner () at /home/andrew/projects/binutils-gdb/build.dev-g/gdb/testsuite/../../../src.dev-g/gdb/test>
(gdb)
An additional observation is that we use PyObject_IsInstance to check
that the return value is a gdb.UnwindInfo, or a sub-class. However,
gdb.UnwindInfo lacks the Py_TPFLAGS_BASETYPE flag, and so cannot be
sub-classed. As such, PyObject_IsInstance is not really needed, we
could use PyObject_TypeCheck instead. The PyObject_TypeCheck function
only returns 0 or 1, there is no -1 error case. Switching to
PyObject_TypeCheck then, fixes the above problem.
There's a new test that exposes the problems that originally existed.
Approved-By: Tom Tromey <tom@tromey.com>