[gdb/python] Fix gdb.python/py-finish-breakpoint2.exp for -m32

[gdb/python] Fix gdb.python/py-finish-breakpoint2.exp for -m32

[ Partial resubmission of an earlier submission by Andrew (
https://sourceware.org/pipermail/gdb-patches/2012-September/096347.html ), so
listing him as co-author. ]

With x86_64-linux and target board unix/-m32, we have:
...
(gdb) continue^M
Continuing.^M
Exception #10^M
^M
Breakpoint 3, throw_exception_1 (e=10) at py-finish-breakpoint2.cc:23^M
23        throw new int (e);^M
(gdb) FAIL: gdb.python/py-finish-breakpoint2.exp: \
  check FinishBreakpoint in catch()
...

The following scenario happens:
- set breakpoint in throw_exception_1, a function that throws an exception
- continue
- hit breakpoint, with call stack main.c:38 -> throw_exception_1
- set a finish breakpoint
- continue
- hit the breakpoint again, with call stack main.c:48 -> throw_exception
  -> throw_exception_1

Due to the exception, the function call did not properly terminate, and the
finish breakpoint didn't trigger.  This is expected behaviour.

However, the intention is that gdb detects this situation at the next stop
and calls the out_of_scope callback, which would result here in this test-case
in a rather confusing "exception did not finish" message.  So the problem is
that this message doesn't show up, in other words, the out_of_scope callback
is not called.

[ Note that the fact that the situation is detected only at the next stop
(wherever that happens to be) could be improved upon, and the earlier
submission did that by setting a longjmp breakpoint.  But I'm considering this
problem out-of-scope for this patch. ]

Note that the message does show up later, at thread exit:
...
[Inferior 1 (process 20046) exited with code 0236]^M
exception did not finish ...^M
...

The decision on whether to call the out_of_scope call back is taken in
bpfinishpy_detect_out_scope_cb, and the interesting bit is here:
...
             if (b->pspace == current_inferior ()->pspace
                 && (!target_has_registers ()
                     || frame_find_by_id (b->frame_id) == NULL))
               bpfinishpy_out_of_scope (finish_bp);
...

In the case of the thread exit, the callback triggers because
target_has_registers () == 0.

So why doesn't the callback trigger in the case of the breakpoint?

Well, the b->frame_id is the frame_id of the frame of main (the frame
in which the finish breakpoint is supposed to trigger), so AFAIU
frame_find_by_id (b->frame_id) == NULL will only be true once we've
left main, at which point I guess we don't stop till thread exit.

Fix this by saving the frame in which the finish breakpoint was created, and
using frame_find_by_id () == NULL on that frame instead, such that we have:
...
(gdb) continue^M
Continuing.^M
Exception #10^M
^M
Breakpoint 3, throw_exception_1 (e=10) at py-finish-breakpoint2.cc:23^M
23        throw new int (e);^M
exception did not finish ...^M
(gdb) FAIL: gdb.python/py-finish-breakpoint2.exp: \
  check FinishBreakpoint in catch()
...

Still, the test-case is failing because it's setup to match the behaviour that
we get on x86_64-linux with target board unix/-m64:
...
(gdb) continue^M
Continuing.^M
Exception #10^M
stopped at ExceptionFinishBreakpoint^M
(gdb) PASS: gdb.python/py-finish-breakpoint2.exp: \
  check FinishBreakpoint in catch()
...

So what happens here?  Again, due to the exception, the function call did not
properly terminate, but the finish breakpoint still triggers.  This is somewhat
unexpected.  This happens because it just so happens to be that the frame
return address at which the breakpoint is set, is also the first instruction
after the exception has been handled.  This is a know problem, filed as
PR29909, so KFAIL it, and modify the test-case to expect the out_of_scope
callback.

Also add a breakpoint after setting the finish breakpoint but before throwing
the exception, to check that we don't call the out_of_scope callback too early.

Tested on x86_64-linux, with target boards unix/-m32.

Co-Authored-By: Andrew Burgess <aburgess@redhat.com>
PR python/27247
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=27247