Re: Python: fetch value when building gdb.Value object

Phil Muldoon
2011-09-28T19:25:04Z

Paul Koning <paulkoning@comcast.net> writes:

> GDB sometimes lazily evaluates operations on values, and py-value.c wasn't taking that into account. The result was that assigning a Value object to a Python variable could assign a lazy value, so that any errors in accessing the data would occur at a later time, and sometimes would not be handled right. (For example, the "nonzero" operation would fail without a Python traceback.)
>
> The attached patch cures this by fetching any lazy values when the gdb.Value object is built, and adds a test in the testcases to verify this.
>
> Ok to submit?
>
> paul
>
> ChangeLog:
>
> 2011-09-21 Paul Koning <paul_koning@dell.com>
>
> * python/py-value.c (valpy_get_address): Use Py_XINCREF.
> (value_to_value_object): Fetch value if it was lazy.
>
> testsuite/ChangeLog:
>
> 2011-09-21 Paul Koning <paul_koning@dell.com>
>
> * gdb.python/py-value.exp: Add test for null pointer reference
> assigned to a variable.
>
> Index: python/py-value.c
> ===================================================================
> RCS file: /cvs/src/src/gdb/python/py-value.c,v
> retrieving revision 1.25
> diff -u -r1.25 py-value.c
> --- python/py-value.c 27 Jun 2011 19:21:51 -0000 1.25
> +++ python/py-value.c 21 Sep 2011 15:45:12 -0000
> @@ -209,7 +209,7 @@
> val_obj->address = value_to_value_object (res_val);
> }
>
> - Py_INCREF (val_obj->address);
> + Py_XINCREF (val_obj->address);
>
> return val_obj->address;
> }

This seems an unrelated change?

> @@ -1045,7 +1045,15 @@
> value_to_value_object (struct value *val)
> {
> value_object *val_obj;
> + volatile struct gdb_exception except;
>
> + TRY_CATCH (except, RETURN_MASK_ALL)
> + {

Something that Jan pointed out a few weeks ago, is our exception net is
too wide, and asked me to review usage of REVIEW_MASK_ALL. In this
case, this should probably be RETURN_MASK_ERROR. I understand there are
many many usages of RETURN_MASK_ALL used incorrectly already.

> # Test memory error.
> gdb_test "python print gdb.parse_and_eval('*(int*)0')" "gdb.MemoryError: Cannot access memory at address 0x0.*"
> + gdb_test "python inval = gdb.parse_and_eval('*(int*)0')" "gdb.MemoryError: Cannot access memory at address 0x0.*"

What scenario will this test catch that the previous test won't? I'm
not saying you are incorrect, I just don't understand. What
error-trigger does the assignment to "inval" trigger?

Cheers,

Phil