[Bug 1577844] Re: don't run ifup at .service before networking.service

[Martin Pitt]

Adding cloud-init task as once we fix ifup at .service we can drop the udev
rule blocking.

** Description changed:

  ifup at .service can (and often does) run for a particular interface before
  networking.service runs. This is brittle as during early boot ifup is
  prone to fail: / might still be read-only, /var might not yet exist or
  be writable, dhclient-enter-hooks.d/ or if-up.d/ hooks might silently
  fail, etc. It is also unnecessary as networking.service will bring up
  all "auto" and all present "allow-hotplug" interfaces anyway, and it
  runs at the right time.
  
  We should make either 80-ifupdown.rules or ifup at .service ignore events
  until networking.service is active, or wait until after it has run
  (slower, but avoids race conditions when hotplug events happen while
  networking.service is running). Ideally adding After=networking.service
  ought to suffice (need to check that activating the unit is properly
  postponed until after networking.service), or we need to do that
  waiting/ignoring in the ExecStart= shell code.
  
+ This also affects cloud-init's setup of networking: this currently jumps
+ through a lot of bad hoops to make sure that ifup@ does not run until
+ after cloud-init-local.service. This includes blocking udev rules for an
+ indefinite time, which is racy, a potential deadlock, and highly non-
+ elegant.
  
  https://bugs.debian.org/752919 is related to this issue.

** Also affects: cloud-init (Ubuntu)
   Importance: Undecided
       Status: New

** Changed in: ifupdown (Ubuntu)
       Status: New => In Progress

** Changed in: ifupdown (Ubuntu)
     Assignee: (unassigned) => Martin Pitt (pitti)

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https://bugs.launchpad.net/bugs/1577844

Title:
  don't run ifup at .service before networking.service

Status in cloud-init package in Ubuntu:
  New
Status in ifupdown package in Ubuntu:
  In Progress

Bug description:
  ifup at .service can (and often does) run for a particular interface
  before networking.service runs. This is brittle as during early boot
  ifup is prone to fail: / might still be read-only, /var might not yet
  exist or be writable, dhclient-enter-hooks.d/ or if-up.d/ hooks might
  silently fail, etc. It is also unnecessary as networking.service will
  bring up all "auto" and all present "allow-hotplug" interfaces anyway,
  and it runs at the right time.

  We should make either 80-ifupdown.rules or ifup at .service ignore events
  until networking.service is active, or wait until after it has run
  (slower, but avoids race conditions when hotplug events happen while
  networking.service is running). Ideally adding
  After=networking.service ought to suffice (need to check that
  activating the unit is properly postponed until after
  networking.service), or we need to do that waiting/ignoring in the
  ExecStart= shell code.

  This also affects cloud-init's setup of networking: this currently
  jumps through a lot of bad hoops to make sure that ifup@ does not run
  until after cloud-init-local.service. This includes blocking udev
  rules for an indefinite time, which is racy, a potential deadlock, and
  highly non-elegant.

  https://bugs.debian.org/752919 is related to this issue.

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