[UBUNTU 20.04] apt build-dep qemu can't resolve dependencies

[bugproxy]

I was trying to re-build the QEMU package from source, but the build dependencies can't be resolved.

Steps to reproduce:
1. Install Ubuntu 20.04 System on an LPAR|z/VM guest|KVM guest from ports.ubuntu.com
2. Uncomment all #deb-src lines in /etc/apt/sources.list
3. Run the following commands:
   $ sudo apt update
   $ sudo apt install build-essential
   $ apt source qemu
   $ sudo apt build-dep qemu
4. The last command produces the following error output
---
$ sudo apt build-dep qemu
Reading package lists... Done
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 builddeps:qemu : Depends: libncursesw5-dev
                  Depends: gcc-alpha-linux-gnu but it is not installable
                  Depends: gcc-powerpc64-linux-gnu but it is not installable
E: Unable to correct problems, you have held broken packages.

[Steve Langasek (vorlon)]

These packages are only Build-Depends-Indep, meaning they are only needed when you are building the arch: all packages from the qemu source package.

The following command succeeds on s390x with an up-to-date focal system:

# apt build-dep --arch-only qemu

If you need to rebuild qemu-system-data, the only arch: all binary package from the qemu source, you will want to do that rebuild on an x86 system. Official arch: all builds for the Ubuntu archive are always done on amd64, so Build-Depends-Indep are not guaranteed to be satisfiable on other architectures.

[bugproxy (bugproxy)]

------- Comment From <email address hidden> 2020-03-04 07:23 EDT-------
Thanks for the clarification.

[Dimitri John Ledkov (xnox)]

Or you can for example dput qemu*.dsc into a launchpad ppa, which will perform builds for all architectures in the right way. And will generate a matching set of arch:all packages on amd64 and all the relevant architecture specific packages for arch:any packages.