Re: [pve-devel] [RFC container] fix: shutdown: if lxc-stop fails, wait for socket closing with timeout

[Wolfgang Bumiller <w.bumiller@proxmox.com>]

On Thu, Jan 19, 2023 at 01:39:02PM +0100, Friedrich Weber wrote:
> When trying to shutdown a hung container with `forceStop=0` (e.g. via
> the Web UI), the shutdown task may run indefinitely while holding a
> lock on the container config. The reason is that the shutdown
> subroutine waits for the LXC command socket to close, even if the
> `lxc-stop` command has failed due to timeout. This prevents other
> tasks (such as a stop task) from acquiring the lock. In order to stop
> the container, the shutdown task has to be explicitly killed first,
> which is inconvenient. This occurs e.g. when trying to shutdown a hung
> CentOS 7 container (with systemd <v232) in a cgroupv2 environment.
> 
> This fix imposes a timeout on the socket read operation if the
> `lxc-stop` command has failed. Behavior in case `lxc-stop` succeeds is
> unchanged. This reintroduces some code from b1bad293. The timeout
> duration is the given shutdown timeout, meaning that the final task
> duration in the scenario above is twice the shutdown timeout.
> 
> Signed-off-by: Friedrich Weber <f.weber@proxmox.com>
> ---
> 
> I stumbled upon the hanging CentOS 7 container shutdown task while
> looking into #4474. However, it is quite the edge case and only
> slightly inconvenient, so I'm not sure whether it needs to be
> addressed -- and if it needs to be addressed, I'm not sure whether the
> attached fix is the way to go. :) So I'm submitting it as an RFC. Let
> me know what you think.
> 
>  src/PVE/LXC.pm | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/src/PVE/LXC.pm b/src/PVE/LXC.pm
> index ce6d5a5..9b3cd64 100644
> --- a/src/PVE/LXC.pm
> +++ b/src/PVE/LXC.pm
> @@ -2473,11 +2473,21 @@ sub vm_stop {
>      }
>  
>      eval { run_command($cmd, timeout => $shutdown_timeout) };
> +
> +    my $result = 1;
> +    my $wait = sub { $result = <$sock>; };
> +
> +    # Wait until the command socket is closed.
> +    # In case the lxc-stop call failed, reading from the command socket may block forever,
> +    # so read with another timeout to avoid freezing the shutdown task.
>      if (my $err = $@) {
> -	warn $@ if $@;
> -    }
> +	warn $err if $err;
>  
> -    my $result = <$sock>;
> +	eval { PVE::Tools::run_with_timeout($shutdown_timeout, $wait); };

The general approach is fine, but `run_with_timeout` uses SIGALRM and
messes with signal handlers which is rather inelegant for such a thing,
we should limit its use to when we have no other option (mainly
file-locking).

For this case we can just use IO::Poll like:

    my $poll = IO::Poll->new();
    $poll->mask($sock => POLLIN | POLLHUP); # watch for input & EOF
    $poll->poll($shutdown_timeout);

If the socket was closed, then `$poll->mask($sock)` should contain the
`POLLHUP` bits.