Debian package dependency on a backports package

Question

In short, is it possible to define Debian package dependencies directly from the backports repository?

I am trying to craft a custom Debian package which builds itself when installing the package. Building said package however requires the latest linux-headers package which is only available from the backports repository (along with the linux-compiler). This is because the kernel is also installed from backports.

I tried including a custom APT preferences file in the package, but that did not help. Which kind of makes sense in that apt will not re-read config files in the middle of an on-going install. Contents of the preferences file is as follows, and does provide the correct versions when installed manually.

Package: linux-headers-amd64 linux-compiler-gcc-6-x86
Pin: release n=stretch-backports
Pin-Priority: 1000

So is there any way I could make this whole thing work simply by saying apt install my-custom-package?

Answer 1

You can’t specify repositories from which packages are to be installed, as package dependencies in a package itself. The only thing you can do is specify a minimal version:

Depends: linux-headers-amd64 (>= 4.19)

You can ship an apt configuration file in your package, but as you say it won’t be taken into account during the installation. A package installation also can’t request that other packages be installed, except using dependencies, so you can’t set a new repository up then run apt-get install from your postinst.

Perhaps your best approach could be to provide a package which configures the backports repository (if necessary, and without adding pin priorities — pinning in the way you’ve done in your question will end up breaking users’ setups), and tell users to install that then install your real package (they will have to specify -t stretch-backports anyway). Or you could use dkms.

As a side-note, you shouldn’t depend on the kernel compiler packages; specific header packages have the appropriate dependency, so if necessary the compiler package will be pulled in transitively.