Gzipping backups suppresses file permissions errors

Here's the relevant code from platform/backup.provision.inc :

  if (substr($backup_file, -2) == 'gz') {
    // same as above: some do not support -z
    $command = 'tar cpf - . | gzip -c > %s';
  } else {
    $command = 'tar cpf %s .';
  }
  $result = drush_shell_exec($command,  $backup_file);

...

  if (!$result && !drush_get_option('force', false)) {
    drush_set_error('PROVISION_BACKUP_FAILED', dt("Could not back up sites directory for drupal"));
  }

(The first bit of code was previously untested without gzip, because there was no option not to gzip. See #2169025 for details.)

There is a difference in behavior between gzipped and non-gzipped backups, in that creating a .tar backup will fail the task when any file in the site's directory can't be read by Aegir. It fails because tar outputs (permission denied) errors (and returns an error code iirc).

Whereas as far as I can tell, gzip simply ignores those errors (or they don't get piped in the first place? not sure), and so doesn't relay them to in the result of drush_shell_exec, silently suppressing them. So we can end up just "losing" files Aegir can't access (created by www-data).

I don't really know how this should be fixed, apart from thinking that piping into gzip is meh, but just a bit higher the code says " // we need to do this because some retarded implementations of tar (e.g. SunOS) don't support -C" and " // same as above: some do not support -z". (Is anyone testing Aegir on SunOS?)

All I know is that ideally the behavior should be the same with and without gzipping, and we should probably test for file permissions somehow so the errors are not suppressed silently.

Feedback/ideas welcome.