[Bug guile/17923] scheme exception about invalid object in a backtrace

["xdje42 at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=17923

--- Comment #7 from Doug Evans <xdje42 at gmail dot com> ---
(In reply to Marcus Daniels from comment #6)
> "for the frame problem, is it possible to set a breakpoint on one of the
> frame functions early on, when f_smob->inferior is not NULL, set a
> watchpoint on it, and then continue?  When the watchpoint triggers a
> backtrace may be sufficient to isolate the culprit."
> 
> Could you clarify if there is ever a situation in which f_smob->inferior
> ought to become NULL, even if the debugged program is continuing?

There are only two situations where f_smob->inferior changes from non-NULL to
NULL:
1) when the inferior goes away,
2) when the frame's Scheme object is garbage collected.

> For now I'm trying to add guards to frame-* calls using the modified
> frame-valid to see what other problems I run into.  If it is an endless
> chain of problems, I can't afford to keep debugging and need to take another
> approach.  If it is a bounded set, then perhaps I can afford to debug these
> issues.

All I'm asking for is to run gdb under gdb with a watchpoint set to catch the
changing of f_smob->inferior from non-NULL to NULL and then sending me a
backtrace. I'm hoping this will be doable.
If you need instructions please let me know.
Basically, run gdb under gdb, and set a breakpoint at the end of
frscm_scm_from_frame *after* this line:

  f_smob->inferior = inferior;

Then in the parent gdb do "watch -l f_smob->inferior",
and then "continue" the child gdb and continue the session
doing whatever tickles this bug.
When the bug is tickled (presumably) the watchpoint will fire
and you will be in the parent gdb.  Then do "backtrace".

Thanks!

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