When a breakpoint is set in one of the frames of the frame stack, Pdb
may not stop at that breakpoint when the frame does not have a trace
function. This problem is closely related to bpo-13183 and issue
14728. The following scenario demonstrates this problem.

==== main.py ================================

import bar

def foo():
    bar.bar()
    x = 1

foo()
====   bar.py  ==================================
def bar():
    pass

=================================================

$ python3 -m pdb main.py 
> /path_to/main.py(1)<module>()
-> import bar
(Pdb) import sys; print(sys.version)
3.2.2 (default, Dec 27 2011, 17:35:55) 
[GCC 4.3.2]
(Pdb) break bar.bar
Breakpoint 1 at /path_to/bar.py:1
(Pdb) continue
> /path_to/bar.py(2)bar()
-> pass
(Pdb) break main.py:5
Breakpoint 2 at /path_to/main.py:5
(Pdb) continue
The program finished and will be restarted
> /path_to/main.py(1)<module>()
-> import bar
(Pdb) quit

=================================================

The attached patch fixes this problem. A test case is included in the
patch. The patch is made against the proposed fix of bpo-14728 (i.e.
assumes this patch is applied), the reason being that self._curframe
must be correctly set. Actually this issue and bpo-14728 should
probably be merged.

Note that the trace function does not need anymore to be set in all
the frames of the frame stack in set_trace(), so setting the trace
function has been removed from the while loop.

Uploaded pdb_default_2.patch that corrects the initial patch: the
trace function must be set in all frames whose co_filename is the
breakpoint file name and not just the first frame of this set.

Fixed in #124454