apt-get update hangs indefinitely, appstreamcli using 100% CPU

[Jason Gerard DeRose]

Just ran `sudo apt-get update`, which is hanging indefinitely after the downloads complete, the last CLI output from which is:

Fetched 733 kB in 1s (399 kB/s)

Looking at `top` suggests `appstreamcli` is the culprit as it's pegged at near 100% CPU usage:

 100 0.1 18:29.02 appstreamcli

If I kill the hung `sudo apt-get update` process with Control+C, the `appstreamcli` process goes away.

Retrying `sudo apt-get update` has the same result.

On a whim I also tried `sudo apt update`, but it seems to have the same problem. It hangs indefinitely and running it results in an `appstreamcli` process consuming 100% CPU.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: appstream 0.9.4-1
ProcVersionSignature: Ubuntu 4.4.0-22.40-generic 4.4.8
Uname: Linux 4.4.0-22-generic x86_64
NonfreeKernelModules: nvidia_uvm nvidia_modeset nvidia
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
CurrentDesktop: Unity
Date: Thu May 19 20:53:40 2016
SourcePackage: appstream
UpgradeStatus: No upgrade log present (probably fresh install)

[Jason Gerard DeRose (jderose)]

Also, the graphical Ubuntu Software Updater is likewise hanging indefinitely.

And I've confirmed this on 3 different systems so far.

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.