[Bug python/13111] New: "-var-update" command incorrect when using pretty printers

["developer at audioranger dot com" <sourceware-bugzilla@sourceware.org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=13111

             Bug #: 13111
           Summary: "-var-update" command incorrect when using pretty
                    printers
           Product: gdb
           Version: 7.3
            Status: NEW
          Severity: minor
          Priority: P2
         Component: python
        AssignedTo: unassigned@sourceware.org
        ReportedBy: developer@audioranger.com
    Classification: Unclassified


The gdb/mi command "-var-update" may return an incorrect value for
"type_changed" when pretty-printers are used.

Consider the following example. The struct "printMePretty" should be
pretty-printed by using the Python TestPrinter class from below. It should be
printed as an integer "array" (it doesn't actually makes sense here, it's just
for testing) if the structure's "printAsInt" member is true, and as a floating
point "array" otherwise.


****************** File: main.cpp *****************

struct printMePretty
{
    bool printAsInt;
    int intMember;
    float floatMember;

    printMePretty() :
        printAsInt(true),
        intMember(0xFF),
        floatMember(3.1415)
    { }
};

void main()
{
    printMePretty pretty;
    pretty.printAsInt = false;
}


****************** File: printer.py *****************

[...]

class TestPrinter:
    class _iterator:
        def __init__(self, val):
            self.val = val
            self.index = 0

        def __iter__(self):
            return self

        def next(self):
            if self.index == 1:
                raise StopIteration

            if self.val['printAsInt']:
                childValue = self.val['intMember']
            else:
                childValue = self.val['floatMember']

            self.index += 1
            return ('Value', childValue)

    def __init__(self, val):
        self.val = val

    def children(self):
        return self._iterator(self.val)

    def display_hint(self):
        return 'array'


****************** End of file *****************


Using Eclipse CDT to trace gdb/mi commands, one encounters the following until
the line "pretty.printAsInt = false;":


-var-create --thread 1 --frame 0 - * pretty
^done,name="var1",numchild="0",value="{...}",type="printMePretty",thread-id="1",displayhint="array",dynamic="1",has_more="1"

-var-list-children var1 0 1
^done,numchild="1",displayhint="array",children=[child={name="var1.Value",exp="Value",numchild="0",type="int",thread-id="1"}],has_more="0"


OK, so "var1.Value" has type "int". Now, stepping over the line
"pretty.printAsInt = false;" results in:


-var-update 1 var1
^done,changelist=[{name="var1.Value",value="3.1415",in_scope="true",type_changed="false",displayhint="array",dynamic="1",has_more="0"}]


It now says that "var1.Value" would have a "value" of "3.1415" - that's
obviously a strange value for an "int" (since "type_changed" reports "false" -
I suppose it should actually report that the type has changed to "float").

This seems like a bug within gdb to me. The same problem may also apply if a
pretty-printer's toString() method returns gdb.Values which have different
gdb.Types, depending on the data members of the structure which should be
pretty-printed.

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