[CRIU] [PATCH] criu: Add exec-cmd option (v2)

[Deyan Doychev]

On 03/21/2014 11:14 AM, Pavel Emelyanov wrote:
>>> ...
>>>> >>>  		wait(NULL);
>>>> >>>  
>>>> >>>  	return 0;
>>>> >>> @@ -1600,6 +1600,9 @@ int cr_restore_tasks(void)
>>>> >>>  {
>>>> >>>  	int ret = -1;
>>>> >>>  
>>>> >>> +	if (opts.exec_cmd && opts.restore_detach && daemon(1, 0))
>>> >> In that case we will lose the ability to get error code from failed restore.
>>> >> Can we move the daemonizing to the very end of the restore procedure?
>> > 
>> > We want the new process to be the father of the restored processes so
>> > the last place I see is right before the fork_with_pid(init); call in
>> > restore_root_task.
> Hm... Indeed. The daemon will create a child process and make the parent exit.
> Thus doing it after restore is not what we want :(
>
>> > Is this ok or do you have something else in mind?
> This is OK, but this brings a problem -- we lose the ability to check whether
> the restore failed or not.
>
> I think that making --exec-cmd work together with --restore-detached is quite
> tricky. We have to fork another task, make this child do restore, then execv()
> and then the parent should somehow check, that restore succeeded and exit. Or
> propagate the error code upwards.

I have an Idea of how to implement this check. We can use two signal
handlers in the parent process to catch CHLD and some other signal (USR1
seems OK). The child process can then send USR1 before calling execvp.
This way, receiving CHLD means the restore failure and USR1 - restore
success. However this has the following two problems:

1. we cannot check the exec call
 
2. there is some chance of concurrency issue occurring if the restored
process finishes too fast

I am open to other ideas.

>
> Deyan, can we do it in two steps -- first you make --exec-cmd work without the -d
> option, i.e. -- the crtools process will just call execv at the end and that's
> it. It's parent will wait for it to finish. And with a check that --exec-cmd and
> -d are not used together (for sanity). I will commit this patch.
>
> Then you implement the support for -d and --exec-cmd together, so that we correctly
> and synchronously handle the failed restore code.
>
> Is that OK for you?
>

Yes, it is OK.

I will have some time between the two patches to work on the LXC daemon
we will execute to restore containers and the console plugin.

Thanks,
Deyan

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