[Bug 1031065] Re: cloud-init-nonet runs 'start networking' explicitly

[Scott Moser]

So, after much discussion [1] we came to the realization of why 'start networking' was here.
The issue was that the normal case for cloud-images (and ubuntu as a whole) is that / is mounted "ro" when /sbin/init starts.  When this is the case, /run will be mounted before /.  However, if / is mounted "rw" then "mountall first scans for all filesystems that don't require further processing and reports them mounted", so "mounted MOUNTPOINT=/" occurs before virtual-filesystems.

if mounted MOUNTPOINT=/ occurs before virtual-filesystems, then udev
will not start because it depends on virtual-filesystems.  That means
means networking wont get started either, as ifup occurs via udev events
for discovered network adapters.  As a result, we hang in cloud-init-
nonet waiting for networking devices to come up that never will.

Note, slangasek pointed out that while /run is guaranteed to be mounted
before / (except withotu 'ro'), virtual-filesystems is *not* guaranteed.
So, even with 'ro' root, a race condition could occur here.  It would
also, then imply that cloud-init cannot actually use some filesystems in
cloud-init-local (such has /proc or /sys).

slangasek suggested that a fix for bug 643289 would improve the
situation here as the virtual-filesystems could all move forward even
while the 'mounted MOUNTPOINT=/' were processing (currently it blocks
*everything*).  That would mean we wouldn't deadlock here.

--
[1] http://irclogs.ubuntu.com/2012/09/05/%23ubuntu-devel.html#t18:16

** Description changed:

  In development of 'overlayroot' package, I was mounting / as rw in the initramfs.
  This was causing a different order of execution of mounts, and as a result a different order of networking resolvconf and networking bringup.
  
  It exposed a bug in resolvconf, which was being called on boot in this order:
  ==== Mon Jul 30 19:08:58 UTC 2012 /sbin/resolvconf -a lo.inet ====
  ==== Mon Jul 30 19:08:59 UTC 2012 /sbin/dhclient-script  ====
  ==== Mon Jul 30 19:08:59 UTC 2012 /sbin/resolvconf -a eth0.dhclient ====
  ==== Mon Jul 30 19:08:59 UTC 2012 /sbin/resolvconf -a eth0.inet ====
  ==== Mon Jul 30 19:08:59 UTC 2012 /sbin/resolvconf --enable-updates ====
  ==== Mon Jul 30 19:08:59 UTC 2012 /etc/resolvconf/update.d/libc -u ====
  
  The normal order is for resolvconf --enable-updates to be called (from /etc/init/resolvconf.conf) before anything else. As a result, I was seeing errors like:
    resolvconf: Error: /run/resolvconf/interface either does not exist or is not a directory
  
  This may be exposing a more grave issue, in that I believe the reason
  for dhclient coming up before resolvconf.conf started was that it was
  being run as a result of /etc/init/network-interface.conf. I don't
  immediately see how that is guarnateed to have /run ounted at all.
  
  ProblemType: Bug
  DistroRelease: Ubuntu 12.10
  Package: resolvconf 1.67ubuntu1
  ProcVersionSignature: User Name 3.5.0-6.6-generic 3.5.0
  Uname: Linux 3.5.0-6-generic x86_64
  Architecture: amd64
  Date: Mon Jul 30 20:07:04 2012
  PackageArchitecture: all
  ProcEnviron:
   TERM=xterm
   PATH=(custom, no user)
   LANG=en_US.UTF-8
   SHELL=/bin/bash
  SourcePackage: resolvconf
  UpgradeStatus: No upgrade log present (probably fresh install)
  
  Related Bugs:
   * bug 800824: cloud-init-nonet times out in lxc
   * bug 925122: container's udevadm trigger --add affects the host
+  * bug 643289: idmapd does not starts to work after system reboot

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Title:
  cloud-init-nonet runs 'start networking' explicitly

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