Created attachment 131027 [details]
A python test case showing the issue

Steps to reproduce:
1. run gst-install-plugins-helper ¹
2. wait for it to return

Alternative steps to reproduce:
1. run the attached python script
2. wait for it to finish

What happens:
gst-install-plugins-helper returns immediately.

What should happen:
According to the documentation², the gst_install_plugins_sync function should not return before the plugin was either installed or the installation was aborted or failed. The gst_install_plugins_async function callback should act the same. The only way this can work is if gst-install-plugins-helper waits until the frontend (in my case gnome-software) does its job and then reports the  correct return value³.

I am running these software versions on Fedora 25:
gstreamer1-plugins-base-1.10.4-1.fc25.x86_64
PackageKit-1.1.5-1.fc25.x86_64
gnome-software-3.22.7-1.fc25.x86_64

Additional info:
Relevant GStreamer code: https://cgit.freedesktop.org/gstreamer/gst-plugins-base/tree/gst-libs/gst/pbutils/install-plugins.c
Some relevant PackageKit code: https://cgit.freedesktop.org/packagekit/tree/contrib/gstreamer-plugin/pk-gstreamer-install.c

¹ for example with these arguments:
`--interaction=show-confirm-search --desktop-id=exaile.desktop "gstreamer|1.0|exaile.py|MPEG-1 Layer 3 (MP3)-Decoder|decoder-audio/mpeg, mpegversion=(int)1, layer=(int)3"`
² https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gst-plugins-base-libs/html/gst-plugins-base-libs-gstpbutilsinstallplugins.html#gst-install-plugins-sync
³ https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gst-plugins-base-libs/html/gst-plugins-base-libs-gstpbutilsinstallplugins.html#GstInstallPluginsReturn