Bug#1060700: Requesting advice regarding the impact of problems caused by aliasing on declared Conflicts

[Sam Hartman <hartmans@debian.org>]

>>>>> "Helmut" == Helmut Grohne <helmut@subdivi.de> writes:

    Helmut> Package: tech-ctte Given our discussion at the last CTTE
    Helmut> meeting, I am turning my request for advice into a formal
    Helmut> one.

    Helmut> Most of the /usr-move that is happening via DEP17 seems to
    Helmut> be working out, but the effects of Conflicts raise the
    Helmut> question of what kinds of interactions with a package
    Helmut> manager are considered supported.

    Helmut> A naive reading of Debian policy 7.4 suggests that declaring
    Helmut> a conflict reliably prevents concurrent unpack:

    Helmut> | When one binary package declares a conflict with another
    Helmut> using a | Conflicts field, dpkg will refuse to allow them to
    Helmut> be unpacked on | the system at the same time.

    Helmut> If you account for the effects of aliasing, this turns out
    Helmut> to be a too naive reading as dpkg actually allows unpacking
    Helmut> a conflicting package if the other package is scheduled for
    Helmut> removal. Normally, this exception should not have observable
    Helmut> consequences, but aliasing makes it observable in the form
    Helmut> of file loss. I have filed #1057199 to clarify
    Helmut> debian-policy.

I'd really like to understand why this is desired dpkg behavior.
I appreciate that even if it is not desired behavior, we might not be
able to get it fixed in ways that matter for this discussion.
However my intuition is that it  will help me at least think about the
situation.
As an example if the reason that behavior is needed has to do with some
situation involving essential packages and conflicts, I'd like to
understand that situation and how common it is.
It would not be the first time in this discussion that we have
discovered a new complexity.

--Sam