Autarchy of the Private Cave

Drush is awesome, especially for updating multisite Drupal installations.
I had only started using it a few days ago, and I’ve immediately hit a problem, to which I did find a workaround.

Symptoms

• running drush @sites pm-update results in normal execution up to after answering ‘y[es]‘; then drush seems to hang indefinitely (haven’t waited beyond about 10 minutes, maybe it does produce an error after a long while);
• running the same command with --debug shows that drush hangs when trying to untar the downloaded module.tar.gz archive; there are no errors/warnings, it just hangs with no CPU usage;
• trying to untar any of the modules downloaded from drupal.org manually is also unsuccessful: tar -xzvf module.tar.gz seems to do nothing, it also hangs with zero CPU usage/time and no warnings/errors;
• interestingly, if I create some test.tar.gz locally, tar does happily extract that;
• finally, running strace tar -xzvf module.tar.gz shows a number of unexpected lines, such as references to NSS and libnss files (I am only showing some of the lines of strace output, including the last line):
  

  open(“/etc/nsswitch.conf”, O_RDONLY) = 4
  read(4, “# /etc/nsswitch.conf\n#\n# Example”…, 4096) = 683
  open(“/lib/x86_64-linux-gnu/libnss_nis.so.2″, O_RDONLY) = 4
  open(“/lib/x86_64-linux-gnu/libnss_files.so.2″, O_RDONLY) = 4
  open(“/etc/passwd”, O_RDONLY|O_CLOEXEC) = 4
  open(“/usr/lib/x86_64-linux-gnu/libnss_mysql.so.2″, O_RDONLY) = 4
  open(“/etc/group”, O_RDONLY|O_CLOEXEC) = 4
  open(“/etc/libnss-mysql.cfg”, O_RDONLY) = -1 EACCES (Permission denied)
  open(“/etc/libnss-mysql-root.cfg”, O_RDONLY) = -1 EACCES (Permission denied)
  futex(0x7fd0816e8c48, FUTEX_WAIT_PRIVATE, 2, NULL

Analysis
strace output provided enough information to understand the issue and generate a workaround. Briefly, we see tar querying users and groups information. On the system where this problem was identified, MySQL is used as a name-service back-end. This is why we see references to mysql libraries in the trace. Apparently, tar is trying to resolve some user/groups information, but for some reason does not get what it is asking in a timely manner, or possibly never gets it and will only fail/proceed when the request times out.

Workaround
Not a solution, but works: tar -xzv --numeric-owner -f module.tar.gz. The --numeric-owner switch asks tar to use numeric file/directory owner information as-is, without trying to resolve the name of the owner. This works. I have not checked strace for the workaround, but I expect to see no MySQL/NSS references in it with the switch.

To actually be able to use drush with this workaround, I had to edit drush.inc somewhere under /usr/share/drush/; look for ‘tar ‘ string, and add --numeric-owner where necessary. Do not forget that -f has to be just in front of the archive filename, otherwise your edits will not work.