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[Steve Langasek <steve.langasek@xxxxxxxxxxxxx>]

Public bug reported:

The phased update implementation in ./systemimage/helpers.py does:

def phased_percentage(*, reset=False):
    global _pp_cache
    if _pp_cache is None:
        with open(UNIQUE_MACHINE_ID_FILE, 'rb') as fp:
            data = fp.read()
        now = str(time.time()).encode('us-ascii')
        r = random.Random()
        r.seed(data + now)
        _pp_cache = r.randint(0, 100)
    try:
        return _pp_cache
    finally:
        if reset:
            _pp_cache = None


Since the current time is used as a seed, this function will return a different value each time it's called.

The phased update model requires that, for each given machine and each
given update, the calculated percentage persists.  With this
implementation, since the seed is different for each invocation and the
result is not cached, phasing will happen much more quickly than
intended.

system-image should avoid using the time as a seed, and instead use a
predictable (but well-distributed) seed of the machine ID plus an image
identifier (perhaps full channel name + image ID).

** Affects: system-image (Ubuntu)
     Importance: Undecided
     Assignee: Barry Warsaw (barry)
         Status: New

** Changed in: system-image (Ubuntu)
     Assignee: (unassigned) => Barry Warsaw (barry)

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https://bugs.launchpad.net/bugs/1383539

Title:
  phased update support does not give idempotent answer for each
  (machine,update)

Status in “system-image” package in Ubuntu:
  New

Bug description:
  The phased update implementation in ./systemimage/helpers.py does:

  def phased_percentage(*, reset=False):
      global _pp_cache
      if _pp_cache is None:
          with open(UNIQUE_MACHINE_ID_FILE, 'rb') as fp:
              data = fp.read()
          now = str(time.time()).encode('us-ascii')
          r = random.Random()
          r.seed(data + now)
          _pp_cache = r.randint(0, 100)
      try:
          return _pp_cache
      finally:
          if reset:
              _pp_cache = None

  
  Since the current time is used as a seed, this function will return a different value each time it's called.

  The phased update model requires that, for each given machine and each
  given update, the calculated percentage persists.  With this
  implementation, since the seed is different for each invocation and
  the result is not cached, phasing will happen much more quickly than
  intended.

  system-image should avoid using the time as a seed, and instead use a
  predictable (but well-distributed) seed of the machine ID plus an
  image identifier (perhaps full channel name + image ID).

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