[commit/RFA] Add assert inside insert_step_resume_breakpoint_at_frame

[Joel Brobecker <brobecker at adacore dot com>]

Hello,

I just came across a very particular set of conditions that lead
insert_step_resume_breakpoint_at_frame to be passed a NULL frame,
causing the debugger to crash.

Basically, the debugger was doing a "next". During the course of
this operation, it landed inside a function called by the code
I was stepping over. This function was an internally-genearted
function for which no debugging info is generated. So GDB tried
to insert a breakpoint at the return address. Unfortunately,
trying to get the previous frame failed because of a combination
of factors: - The debugging info was stabs. With stabs, the N_FUN
              entries that define a function only provide the function
              start address. At this time of writing, the compiler
              is not generated the unnamed N_FUN entries used by GCC
              to specify the function end address
            - The funtion we called is placed immediately after
              the main (function).
            - This results in GDB thinking that the function we
              stepped over from goes over a range of instruction
              that is too wide. It ends up including our internally
              generated function.
            - So get_prev_frame accidently determines that we're inside
              the main function, and thus stops the backtracing by
              returning a NULL frame.

This is a pretty unlikely situation, but I think we should add an
assert that "return_frame" is not NULL before dereferencing it.
It's a lot cleaner to report an assertion failure rather than crashing.

As for us, we are working on enhancing the compiler to generate that
empty N_FUN stabs entry.

2007-09-14  Joel Brobecker  <brobecker@adacore.com>

        * infrun.c (insert_step_resume_breakpoint_at_frame): Add assertion
        that return_frame is not null.

Tested on x86-linux, no regression. This also prevents the crash
initially observed.

This seems pretty straightforward and safe, but I'll give it a week
for comments before checking in.

-- 
Joel

Attachment: infrun.diff
Description: Text document