[Bug python/12425] New: Confusing python backtrace on iterator end

["mjw at redhat dot com" <sourceware-bugzilla@sourceware.org>]

http://sourceware.org/bugzilla/show_bug.cgi?id=12425

           Summary: Confusing python backtrace on iterator end
           Product: gdb
           Version: archer
            Status: NEW
          Severity: normal
          Priority: P2
         Component: python
        AssignedTo: unassigned@sourceware.org
        ReportedBy: mjw@redhat.com


Take the following silly example:

#include <map>
#include <string>

struct foo
{
  int a;
  int b;
};

int main(int argc, char **argv)
{
  std::map<std::string, foo> map;

  foo f = { 1, 2 };
  map.insert (std::pair<std::string, foo> ("first", f));

  std::map<std::string, foo>::iterator it = map.begin ();
  it++;

  return 0;
}

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<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /tmp/foo...done.
(gdb) break main
Breakpoint 1 at 0x400b5d: file foo.cpp, line 12.
(gdb) n
The program is not being run.
(gdb) r
Starting program: /tmp/foo 

Breakpoint 1, main (argc=1, argv=0x7fffffffe278) at foo.cpp:12
12      std::map<std::string, foo> map;
(gdb) n
14      foo f = { 1, 2 };
(gdb) n
15      map.insert (std::pair<std::string, foo> ("first", f));
(gdb) n
17      std::map<std::string, foo>::iterator it = map.begin ();
(gdb) n
18      it++;
(gdb) p it
$1 = {first = "first", second = {a = 1, b = 2}}
(gdb) n
20      return 0;
(gdb) p it
$2 = {first = Traceback (most recent call last):
  File "/usr/lib64/../share/gcc-4.5.1/python/libstdcxx/v6/printers.py", line
549, in to_string
    return self.val['_M_dataplus']['_M_p'].lazy_string (length = len)
RuntimeError: Cannot access memory at address 0xffffffffffffffe9
, second = {a = -7944, b = 32767}}

It isn't so much surprising that the printing fails, since we are now at it ==
map.end(). But you might see this sometimes when you forgot, or when taking an
iterator when a map is empty so map.begin () == map.end ().

It would be nice to not get that RuntimeError and partial python backtrace.
It doesn't really add any information, except that the string couldn't be
shown.

Ideally it would detect the iterator ran out and just report that. If that
isn't possible then please don't print these internal python printer errors
that are just confusing and don't add any useful information.

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