Re: RFH: failed assert debugging threaded+fork program over gdbserver

[Don Breazeal <donb at codesourcery dot com>]

On 5/12/2016 10:16 AM, Joel Brobecker wrote:
> Hello,
> 
> I have noticed the following problem, when debugging a program which
> uses both threads and fork. The program is attached in copy, and
> it was compiled by simply doing:
> 
>     % gnatmake -g a_test
> 
> The issue appears only randomly, but it seems to show up fairly
> reliably when using certain versions of GNU/Linux such as RHES7,
> or WRSLinux. I also see it on Ubuntu, but less reliably. Here is
> what I have found, debugging on WRSLinux (we set it up as a cross,
> but it should be the same with native GNU/Linux distros):
> 
>     % gdb a_test
>     (gdb) break a_test.adb:30
>     (gdb) break a_test.adb:39
>     (gdb) target remote my_board:4444
>     (gdb) continue
>     Continuing.
>     [...]
>     [New Thread 866.868]
>     [New Thread 866.869]
>     [New Thread 870.870]
>     /[...]/gdb/thread.c:89: internal-error: thread_info* inferior_thread(): Assertion `tp' failed.
>     A problem internal to GDB has been detected,
>     further debugging may prove unreliable.
>     Quit this debugging session? (y or n) 
> 
> The error happens because GDBserver returns a list of threads
> to GDB where a new thread as a different PID (870 in the case
> above, instead of 866).

Hi Joel,
This is not supposed to happen.  In remote.c:remote_update_thread_list
there is a call to remove_new_fork_children that is explicitly supposed
to prevent this scenario.  The new fork child thread should be deleted
from the thread list ("context") before we call remote_notice_new_inferior.

We don't want the remote to report new threads related to the fork child
until after we have handled the fork using infrun.c:follow_fork. (Note:
it looks like the function comment for remove_new_fork_children is
stale, since there are scenarios other than being stopped at a fork
catchpoint where this can occur.)

> Any advice on how I should be fixing the issue?

It looks like there is a case that remove_new_fork_children isn't
handling correctly with your test+target(s). I'd start there to find out
how the new thread is getting through.
Thanks
--Don