[RFC] dh_ifexists: support for Depends-If-Exists

[Geoffrey Thomas <geofft@MIT.EDU>]

Hi all,

I'd like some feedback on a Debhelper command I wrote, dh_ifexists, to 
parse lines of the form
  Depends-If-Exists: foo, bar, baz (>= 2.5)
and add all the satisfiable dependencies in ${misc:Depends}. So, if a 
release has foo 1.0-1 and baz 1.0-1 and no bar, then ${misc:Depends} will 
expand to just foo.

The intended use case is to allow using the same source package to build 
on multiple releases, and depend on packages as appropriate. For instance, 
if we want a metapackage to depend on whichever soname of libthing exists 
on the current release (because we have non-Debian network-installed 
software, compiled for each release, that uses libthing), we could write
  Depends-If-Exists: libthing4, libthing5, libthing6
run sbuild once for each upstream Debian or Ubuntu release, and have the 
resulting binary packages depend on whichever of those is available.

I figured I'd ask debian-derivatives for feedback on this, since this is 
likely to be useful for people who build on top of the other apt 
repositories (such as Debian's or Ubuntu's) and useless for packages in 
Debian itself. Debathena builds for all supported Debian and Ubuntu 
releases, and has so far managed not to need to branch source packages for 
multiple releases; I imagine this is not an uncommon desire.

You can get dh-ifexists from my git repo:
  git clone git://geofft.mit.edu/dh-ifexists.git
You can also poke around my checkout on the web via
  http://web.mit.edu/geofft/debathena/dh-ifexists/
If this looks reasonable, my plan is to see about getting it integrated 
into Debhelper upstream. The current packaging inspired by dh-autoreconf.

In order to use it, add Build-Depends: dh-ifexists and the appropriate 
Foo-If-Exists: lines to debian/control, and make sure that you have Foo: 
${misc:Foo} also in debian/control. (For the common case, you probably 
already depend on ${misc:Depends}.) Then make sure dh_ifexists gets 
called. If using CDBS, add "include /usr/share/cdbs/1/rules/ifexists.mk" 
to debian/rules. If using Debhelper >= 7 pass "--with=ifexists" to dh, and 
if using traditional Debhelper call dh_ifexists somewhere before 
dh_gencontrol.

--
Geoffrey Thomas
geofft@mit.edu