Re: Fix foll-fork.exp foll-vfork.exp fork-child-threads.exp

[Michael Snyder <msnyder@vmware.com>]

Pedro Alves wrote:
> Long story short:
> 
> After following a child, detaching from the parent,
> 
> ('set follow-fork-mode child' + 'set detach-on-fork on')
> 
> here in this bit,
> 
>  infrun.c:resume():
>   {
>     ....
>     follow_fork ();
>     ...
>     tp->stop_signal = TARGET_SIGNAL_0;
>   }
> 
> ... `tp' is no longer in the thread list (it was pointing at a thread
> of the parent process, which we've detached from, hence no longer
> in the thread list), so if the assignment above doesn't crash, it ends
> up writing to who-knows-where.
> 
> With some local changes I was making, sometimes, `tp' happened to be left pointing
> at linux_nat.c:lwp_list, and so that assignment above ended up clearing
> lp->waitstatus.kind (of the first lwp in the list), which resulted in
> GDB considering that the child process had
> exited (because TARGET_SIGNAL_0 == TARGET_WAITKIND_EXITED).
> 
> This should fix intermittent foll-fork.exp foll-vfork.exp
> fork-child-threads.exp failures.
> 
> Checked in.

Pedro,

I'm not sure if this change goes far enough.
If a multi-threaded program forks, only the currently
executing thread survives in the child.  All others are
left behind (and its not unlikely that the thread library
is left in an inconsistant state, possibly leading to
deadlocks).

We can't do anything about that, but we could, eg.,
invalidate all known debugger state having to do with
other threads.  Clear the gdb thread list and preserve
only the current thread.

What do you think?




> ------------------------------------------------------------------------
> 
> 2008-11-20  Pedro Alves  <pedro@codesourcery.com>
> 
> 	* infrun.c (resume): If following a fork, reread the current
> 	thread.  Avoid dereferencing a possibly dangling pointer.
> 
> ---
>  gdb/infrun.c |    7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> Index: src/gdb/infrun.c
> ===================================================================
> --- src.orig/gdb/infrun.c	2008-11-20 05:37:35.000000000 +0000
> +++ src/gdb/infrun.c	2008-11-20 12:30:26.000000000 +0000
> @@ -1053,6 +1053,9 @@ a command like `return' or `jump' to con
>        pending_follow.kind = TARGET_WAITKIND_SPURIOUS;
>        if (follow_fork ())
>  	should_resume = 0;
> +
> +      /* Following a fork may change inferior_ptid.  */
> +      tp = inferior_thread ();
>        break;
>  
>      case TARGET_WAITKIND_EXECD:
> @@ -1148,11 +1151,11 @@ a command like `return' or `jump' to con
>            displaced_step_dump_bytes (gdb_stdlog, buf, sizeof (buf));
>          }
>  
> -      target_resume (resume_ptid, step, sig);
> -
>        /* Avoid confusing the next resume, if the next stop/resume
>  	 happens to apply to another thread.  */
>        tp->stop_signal = TARGET_SIGNAL_0;
> +
> +      target_resume (resume_ptid, step, sig);
>      }
>  
>    discard_cleanups (old_cleanups);