[Bug threads/21117] Occasional wrong thread numbering when following an exec

["palves at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=21117

Pedro Alves <palves at redhat dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |palves at redhat dot com

--- Comment #1 from Pedro Alves <palves at redhat dot com> ---
>From the kernel's perspective, it should be the leader lwp that hits the
breakpoint too, so either a kernel bug, or a gdb bug.  From your logs, looks
like the latter.

>From the logs, we can tell that the leader/process pid is "19676":

> process 19676 is executing new program: /home/simark/build/binutils-
>         ^^^^^

I think that there's a good hint here, just before the first time we see a
thread != 1:

> [New Thread 0x7ffff751a700 (LWP 19697)]
> [Thread 0x7ffff7f8b700 (LWP 19676) exited] <<<< LEADER DETECTED AS EXITED
> [New LWP 19676]                            <<<< LEADER RE-ADDED
> process 19676 is executing new program:    <<<< STILL SAME PID
> 
> Thread 10 "mi-nsthrexec" hit Breakpoint 1, main (argc=1, argv=0x7fffffffda98)

So we detected the leader as exited, deleted it (most probably in
linux-nat.c:check_zombie_leaders), and then when we get
the exec event out of the kernel, we re-add the leader thread.
This will give it a new gdb thread number.

A "set debug lin-lwp 1" log probably makes that clearer.

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