So if you are fond of adding PPAs to your Ubuntu system (who isn’t?), sooner or later you will reach this limit and suddenly the next
sudo apt-get update you do will throw multiple “…public key is unavailable: NO_PUBKEY …” errors, which will be cryptic as hell.
You will first blame your internet connection, and then be paranoid that perhaps your VPN host is mucking with your security, and then after trying multiple
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys XXXXXXXXXXXXXXXXX with processed:1 unchanged:1 results, you will realize that you need to ask Google, which will eventually tell you that there’s this bug.
And that, yeah, you have to go check
/etc/apt/trusted.gpg.d and remove duplicates and entries that aren’t in use anymore, because apparently, it happens that even after a ppa-purge the gpg keyring remains in there, filling up space, waiting for this resource limit to be reached, finally putting a rather innocent apt-get update into tantrums.
So, do a
sudo apt-key list right now, and keep those gpg files at
/etc/apt/trusted.gpg.d to less than 40 entries, perhaps while pondering about the life of Werner Koch, and the decisions that led him to institute such a resource limit.