DEP 17: Improve support for directory aliasing in dpkg

[Helmut Grohne <helmut@subdivi.de>]

Hi,

I am hereby proposing a new DEP and thus allocating the number 17 for
it. The title is "Improve support for directory aliasing in dpkg". As
per DEP 0, I am announcing this on debian-project@l.d.o. The actual
discussion will start at the same time on debian-devel@l.d.o and
debian-dpkg@l.d.o. Please do not reply to debian-project with technical
matters.

I'm including the introduction for reference here:

    At its core, dpkg assumes that every filename uniquely refers to a
    file on disk.  The situation where two distinct filenames refer to
    the same file on disk is referred to as aliasing.  Violating this
    assumption leads to undefined behaviour such as file loss.  The
    assumption is commonly violated when a leading directory component
    contains a symbolic link.  A situation where this is known to cause
    file loss happens when a file is moved from one binary package to
    another binary package while at the same time changing the filename
    in a way that retains its final location.  In this situation, dpkg
    may first unpack the new replacing location and then remove the
    replaced package thus unknowingly remove the aliased file.  Other
    components such as dpkg-divert or update-alternatives are likely
    affected in similar ways.

    The purpose of this DEP is selecting and implementing a change to
    dpkg to improve the way it handles such situations that affect
    typical Debian installations.

Helmut