Monthly Updates versus Monthly Images

[Robie Basak]

On Wed, Mar 06, 2013 at 09:58:41PM +0000, Dmitrijs Ledkovs wrote:
> Everything gets uploaded into -proposed, FTBFS / installability /
> components mismatches are sorted out [*] / autopkgtests [*] are run
> and then the packages are finally migrated by britney into rolling as
> it is currently done.
> 
> At this point all of these packages are phased at 0%. [1]
> Over the period of time (e.g. 4 weeks) we gradually phase those
> packages to 100%.

>From the spec:
    A computer is in the testing set if
              Phased-Update-Percentage ✕ 2^128
                               ≥
              md5(machine id + package name + package version). 

With precise-updates (for example), ordering of phasing between packages
doesn't matter, since the dependency tree generally remains the same.

With rolling updates, won't phasing ordering start to matter? What
happens if the results of my md5 cause an update to package A to be
phased in before NEW package B, and the updated A now depends on B?
Presumably dist-upgrade can hold that specific case automatically, but:

  1) Are there more complex cases that will break things?

  2) Is holding going to cause any other problems? User confusion? More
     invalid bugs as people wonder why they can't install things or "fully
     update"?