Re: [Qemu-devel] [PATCH] io/channel-command: Do not kill the child process after closing the pipe

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Feb 14, 2018 at 05:09:35PM +0100, Thomas Huth wrote:
> We are currently facing some migration failure on s390x when running
> certain avocado-vt tests, e.g. when running the test
> type_specific.io-github-autotest-qemu.migrate.with_reboot.exec.gzip_exec.
> This test is using 'migrate -d "exec:nc localhost 5200"' for the migration.
> The problem is detected at the receiving side, where the migration stream
> apparently ends too early. However, the cause for the problem is at the
> sending side: After writing the migration stream into the pipe to netcat,
> the source QEMU calls qio_channel_command_close() which closes the pipe
> and immediately (!) kills the child process afterwards (via the function
> qio_channel_command_abort()). So if the  sending netcat did not read the
> final bytes from the pipe yet, or  if it did not manage to send out all
> its buffers yet, it is killed before the whole migration stream is passed
> to the destination side.
> 
> QEMU can not know how much time is required by the child process to send
> over all migration data, so we should not kill it, neither directly nor
> after a delay. Let's simply wait for the child process to exit gracefully
> instead (this was also the behaviour of pclose() that was used in "exec:"
> migration before the QIOChannel rework).
> 
> Signed-off-by: Thomas Huth <thuth@redhat.com>
> ---
>  io/channel-command.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/io/channel-command.c b/io/channel-command.c
> index 319c5ed..3e7eb17 100644
> --- a/io/channel-command.c
> +++ b/io/channel-command.c
> @@ -301,6 +301,9 @@ static int qio_channel_command_close(QIOChannel *ioc,
>  {
>      QIOChannelCommand *cioc = QIO_CHANNEL_COMMAND(ioc);
>      int rv = 0;
> +#ifndef WIN32
> +    pid_t wp;
> +#endif
>  
>      /* We close FDs before killing, because that
>       * gives a better chance of clean shutdown
> @@ -315,11 +318,18 @@ static int qio_channel_command_close(QIOChannel *ioc,
>          rv = -1;
>      }
>      cioc->writefd = cioc->readfd = -1;
> +
>  #ifndef WIN32
> -    if (qio_channel_command_abort(cioc, errp) < 0) {
> +    do {
> +        wp = waitpid(cioc->pid, NULL, 0);
> +    } while (wp == (pid_t)-1 && errno == EINTR);
> +    if (wp == (pid_t)-1) {
> +        error_setg_errno(errp, errno, "Failed to wait for pid %llu",
> +                         (unsigned long long)cioc->pid);
>          return -1;
>      }
>  #endif
> +
>      if (rv < 0) {
>          error_setg_errno(errp, errno, "%s",
>                           "Unable to close command");

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

I'll queue this as i had a PR pending...

Regards,
Daniel
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