Re: [PATCH] MI: Allow non-raw varobj evaluation

["Leszek Swirski via gdb-patches" <gdb-patches@sourceware.org>]

On Thu, Jan 25, 2018 at 2:53 AM, Simon Marchi <simon.marchi@polymtl.ca> wrote:
>
> Could you give an example to reproduce this problem?  I tried with a vector of vector, but -var-evaluate-expression just prints "{...}":
>
>   -var-evaluate-expression var1
>   ^done,value="{...}"
>
> so I am not sure what problem you are after.

Hi Simon,

So, this is a bit of an edge case, which I encountered with the
chromium pretty printers. Effectively, it's when a pretty printer
returns another object in its to_string, rather than a string.

Consider the following code:

        struct Foo { int val; };
        struct Wrapper { Foo foo; };

        int main() {
                Wrapper w;
                w.foo.val = 23;
        }

and this pretty printer file:

        import gdb.printing

        class FooPrinter:
          def __init__(self, val):
            self.val = val
          def to_string(self):
            return "Foo" + str(self.val["val"])

        class WrapperPrinter:
          def __init__(self, val):
            self.val = val
          def to_string(self):
            return self.val["foo"]

        test_printer = gdb.printing.RegexpCollectionPrettyPrinter("test")
        test_printer.add_printer('Foo', '^Foo$', FooPrinter)
        test_printer.add_printer('Wrapper', '^Wrapper$', WrapperPrinter)

        gdb.printing.register_pretty_printer(None, test_printer)

Setting a breakpoint at the end of the function, we call the following commands:

        -enable-pretty-printing
        ^done

        -var-create var_w @ w
        ^done,name="var_w",numchild="0",value="{val =
23}",type="Wrapper",dynamic="1",has_more="0"
        -var-create var_w_foo @ w.foo
        ^done,name="var_w_foo",numchild="0",value="Foo23",type="Foo",dynamic="1",has_more="0"

        -var-evaluate-expression var_w
        ^done,value="{val = 23}"
        -var-evaluate-expression var_w_foo
        ^done,value="Foo23"

        -data-evaluate-expression w
        ^done,value="Foo23"
        -data-evaluate-expression w.foo
        ^done,value="Foo23"

So, in the -var-evaluate-expression var_w case, we print the "raw"
value of w.foo, while in the -data-evaluate-expression w case, we
print the pretty printed w.foo value. After my patch, all of the above
print "Foo23".

Note that this isn't encountered very often, probably because it
disappears if I replace `return self.val["foo"]` with `return
str(self.val["foo"])`. But, it does feel like the wrong behaviour.

- Leszek