Transient error in apt-add-repository can lead to missing apt keys

Hit this with the landscape-client charm, which uses charm helpers for setting up apt repositories and installing packages.

[from unit-landscape-client-31.log]
017-01-06 04:19:06 INFO install gpg: requesting key 4652B4E6 from hkp server keyserver.ubuntu.com
2017-01-06 04:19:06 INFO install gpgkeys: key 35F77D63B5CEC106C577ED856E85A86E4652B4E6 can't be retrieved
2017-01-06 04:19:06 INFO install gpg: no valid OpenPGP data found.
2017-01-06 04:19:06 INFO install gpg: Total number processed: 0
2017-01-06 04:19:06 INFO install gpg: keyserver communications error: keyserver helper general error
2017-01-06 04:19:06 INFO install gpg: keyserver communications error: unknown pubkey algorithm
2017-01-06 04:19:06 INFO install gpg: keyserver receive failed: unknown pubkey algorithm
2017-01-06 04:19:07 INFO install Hit:1 http://archive.ubuntu.com/ubuntu xenial InRelease
2017-01-06 04:19:07 INFO install Get:2 http://security.ubuntu.com/ubuntu xenial-security InRelease [102 kB]
2017-01-06 04:19:07 INFO install Ign:3 http://ubuntu-cloud.archive.canonical.com/ubuntu xenial-updates/newton InRelease
2017-01-06 04:19:07 INFO install Get:4 http://ppa.launchpad.net/landscape/trunk/ubuntu xenial InRelease [17.6 kB]
2017-01-06 04:19:07 INFO install Get:5 http://archive.ubuntu.com/ubuntu xenial-updates InRelease [102 kB]
2017-01-06 04:19:08 INFO install Get:6 http://ubuntu-cloud.archive.canonical.com/ubuntu xenial-updates/newton Release [7881 B]
2017-01-06 04:19:08 INFO install Get:7 http://archive.ubuntu.com/ubuntu xenial-backports InRelease [102 kB]
2017-01-06 04:19:08 INFO install Get:8 http://ubuntu-cloud.archive.canonical.com/ubuntu xenial-updates/newton Release.gpg [543 B]
2017-01-06 04:19:08 INFO install Ign:4 http://ppa.launchpad.net/landscape/trunk/ubuntu xenial InRelease
2017-01-06 04:19:08 INFO install Get:9 http://ppa.launchpad.net/landscape/trunk/ubuntu xenial/main amd64 Packages [1244 B]
2017-01-06 04:19:08 INFO install Get:10 http://ppa.launchpad.net/landscape/trunk/ubuntu xenial/main Translation-en [484 B]
2017-01-06 04:19:10 INFO install Fetched 334 kB in 1s (308 kB/s)
2017-01-06 04:19:11 INFO install Reading package lists...
2017-01-06 04:19:11 INFO install W: GPG error: http://ppa.launchpad.net/landscape/trunk/ubuntu xenial InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 6E85A86E4652B4E6
2017-01-06 04:19:11 INFO install W: The repository 'http://ppa.launchpad.net/landscape/trunk/ubuntu xenial InRelease' is not signed.
2017-01-06 04:19:11 INFO juju-log Installing ['landscape-client'] with options: ['--option=Dpkg::Options::=--force-c
onfold']
2017-01-06 04:19:11 INFO install Reading package lists...
2017-01-06 04:19:12 INFO install Building dependency tree...
2017-01-06 04:19:12 INFO install Reading state information...
2017-01-06 04:19:12 INFO install The following additional packages will be installed:
2017-01-06 04:19:12 INFO install bc landscape-common python-attr python-configobj python-gdbm python-pam
2017-01-06 04:19:12 INFO install python-pyasn1-modules python-pycurl python-serial python-service-identity
2017-01-06 04:19:12 INFO install python-twisted-bin python-twisted-core python-twisted-web
2017-01-06 04:19:12 INFO install Suggested packages:
2017-01-06 04:19:12 INFO install python-configobj-doc python-gdbm-dbg python-pam-dbg libcurl4-gnutls-dev
2017-01-06 04:19:12 INFO install python-pycurl-dbg python-pycurl-doc python-wxgtk3.0 | python-wxgtk
2017-01-06 04:19:12 INFO install python-twisted-bin-dbg python-tk python-gtk2 python-glade2 python-qt3
2017-01-06 04:19:12 INFO install python-wxgtk3.0
2017-01-06 04:19:12 INFO install The following NEW packages will be installed:
2017-01-06 04:19:12 INFO install bc landscape-client landscape-common python-attr python-configobj
2017-01-06 04:19:12 INFO install python-gdbm python-pam python-pyasn1-modules python-pycurl python-serial
2017-01-06 04:19:12 INFO install python-service-identity python-twisted-bin python-twisted-core
2017-01-06 04:19:12 INFO install python-twisted-web
2017-01-06 04:19:12 INFO install 0 upgraded, 14 newly installed, 0 to remove and 9 not upgraded.
2017-01-06 04:19:12 INFO install Need to get 2525 kB of archives.
2017-01-06 04:19:12 INFO install After this operation, 15.6 MB of additional disk space will be used.
2017-01-06 04:19:12 INFO install WARNING: The following packages cannot be authenticated!
2017-01-06 04:19:12 INFO install landscape-common landscape-client
2017-01-06 04:19:12 INFO install E: There were unauthenticated packages and -y was used without --allow-unauthenticated

It looks like the original attempt to fetch the key "4652B4E6" failed and was not retried. This was probably part of an apt-add-repository. Since the key wasn't added, the next 'apt-get update' failed to verify the packages in the newly added archive. Then the landscape-common and landscape-client packages failed to installed because the archive signature wasn't available.

Key retrieval can be a transient issue and is often recovered by an "apt-get update". I suspect the original apt-add-repository returned some error code and it was ignored. Retrying the command should resolve the problem in this case.