Bug#128868: debian-policy: Depends semantics unclear re circular depends

[Peter Moulder <pmoulder@csse.monash.edu.au>]

Package: debian-policy
Version: 3.5.6.0
Severity: wishlist

 From section 7.2 `Binary Dependencies' of debian-policy:

#    `Depends'
#         This declares an absolute dependency.  A package will not be
#         configured unless all of the packages listed in its `Depends'
#         field have been correctly configured.

Suppose that package A Depends on B, and package B Depends on A.

My reading of the above policy excerpt is that is that circular
dependencies are not allowed, since it would be impossible to configure
either A or B without first configuring the other.[*1]

Whereas, in fact, a number of dependency cycles do occur in Debian; a
quick-and-dirty script (appended to this message) lists 65 cycles, of
which I've manually verified 4 (cpp <> cpp-2.95; perl <> perl-modules;
python <> python2.1; kdelibs3 <> kdelibs3-bin).

Given that the most natural reading of current policy forbids this
practice that occurs within some fairly high-profile packages, the
current wording of policy should be reworded: either it should make clear
that packages may be configured before their dependencies are configured,
or else explicitly state that dependency cycles are not allowed.

pjm.


[*1]: A notational shortcut is used in the A <-> B example, in that with
`x Depends: y', x is a "package instance", whereas y is a package
specification that can potentially be met by any of several package
instances, e.g.  different version numbers.  Where the above says "A
Depends on B" above, assume that B is the only package instance that
meets A's actual dependency specification.  This is the usual case when
installing for the first time.


Quick-and-dirty cycle-finding script (results require manual
verification, see the above comments):

Shell part:

  cat /var/lib/apt/lists/*_dists_unstable_main_*Packages |
   egrep '^(Package|(Pre-)?Depends):' |
   sed 's,([^)]*),,g' |
   tr -s ',| ' ' ' |
   perl to-tsort-in.pl |
   tsort 2>&1 > /dev/null |
   grep -v '^tsort: input contains a loop:$' |
   sed 's,^tsort: ,,' > cycles

Perl part (to-tsort-in.pl):

#! /usr/bin/perl -w
use strict;
my $pkg;
while(<>) {
    chomp;
    if(s/^Package: //) {
	$pkg = $_;
	next;
    }
    if(s/^(?:Pre-)?Depends: //) {
	my @deps = split(' ', $_);
	for my $i (@deps) {
	    print "$pkg $i\n";
	}
	next;
    }
    die("unrecognized line");
}