Incorrect GPG signature file during autoinstall when using custom GPG key

When using an APT mirror that creates its own GPG keys to sign the files (such as APTLY), and then attempting to use that in an autoinstall file for subiquity, although we provide the GPG key in the autoinstall yaml, it still uses the incorrect GPG key in sources.list.

In my APTLY environment example, I provide this snippet in my autoinstall user-data:

...
    apt:
      primary:
      - arches: [amd64, i386, default]
        uri: http://192.168.148.3/ubuntu
      security:
      - arches: [amd64, i386, default]
        uri: http://192.168.148.3/ubuntu
      geoip: false
      disable_suites: [backports, proposed]
      disable_components: [restricted, multiverse]
      conf: | # APT config
        Acquire::Check-Valid-Until "false";
        Acquire::AllowInsecureRepositories "true";
        Acquire::AllowDowngradeToInsecureRepositories "true";
        Acquire::AllowReleaseInfoChange "true";
      sources:
        #additional-repo.list:
        # source: "http://ubuntumirror.test.att.com/ubuntu/jammy ato_tools main"
        localrepokey:
          key: | # full key as block
            -----BEGIN PGP PUBLIC KEY BLOCK-----
 < . . . Content of the GPG key . . . :
            -----END PGP PUBLIC KEY BLOCK-----
...

However, subiquity still fails complaining about:

2024-08-13 16:24:51,746 DEBUG subiquity.server.controllers.mirror:180 The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 0C0A181F066B7A7C

And checking my installation environment, I see that subiquity adds my key at /tmp/tmpw3ph2pba/mount/etc/apt/trusted.gpg.d/localrepokey.asc, but looking at/tmp/tmpw3ph2pba/mount/etc/apt/sources.list.d/ubuntu.sources` I see it still has:

Signed-By: /usr/share/keyrings/ubuntu-archive-keyring.gpg

This is the incorrect GPG key.

If I run this:

cat ./trusted.gpg.d/localrepokey.asc | gpg --dearmor -o /tmp/tmpw3ph2pba/mount/etc/apt/keyrings/aptly.key

And then change the Signed-By line to point to this file (/tmp/tmpw3ph2pba/mount/etc/apt/keyrings/aptly.key), I can then run apt-get update in the chroot environment.

We would need a way to use the correct GPG key to the apt source when provided in the autoinstall user-data

Steps to reproduce this would involve setting up an aptly mirror using the following steps:

Add to sources.list:

deb http://repo.aptly.info/ nightly main

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys A0546A43624A8331
sudo apt update
sudo apt install aptly

gpg --no-default-keyring --keyring /usr/share/keyrings/ubuntu-archive-keyring.gpg --export | gpg --no-default-keyring --keyring trustedkeys.gpg --import

aptly mirror create -architectures=amd64 -with-udebs=true noble http://archive.ubuntu.com/ubuntu/ noble main restricted universe multiverse
aptly mirror create -architectures=amd64 -with-udebs=true noble-security http://archive.ubuntu.com/ubuntu/ noble-security main restricted universe multiverse
aptly mirror create -architectures=amd64 -with-udebs=true noble-updates http://archive.ubuntu.com/ubuntu/ noble-updates main restricted universe multiverse
aptly mirror create -architectures=amd64 -with-udebs=true noble-backports http://archive.ubuntu.com/ubuntu/ noble-backports main restricted universe multiverse

aptly mirror update noble && aptly mirror update noble-security && aptly mirror update noble-updates && aptly mirror update noble-backports

aptly snapshot create noble from mirror noble && aptly snapshot create noble-updates from mirror noble-updates && aptly snapshot create noble-security from mirror noble-security && aptly snapshot create noble-backports from mirror noble-backports

gpg --default-new-key-algo rsa4096 --gen-key --keyring pubring.gpg

aptly publish snapshot noble && aptly publish snapshot noble-updates && aptly publish snapshot noble-security && aptly publish snapshot noble-backports

sudo apt install apache2
sudo ln -s /home/ubuntu/.aptly/public/ /var/www/html/ubuntu
chmod 755 /home/ubuntu/

Export the gpg key and use it in the autoinstall yaml:

gpg --list-keys --keyring pubring.gpg
gpg --export --armor 066B7A7C > 066B7A7C.key

---

Workaround: provide sources content in deb822 format and include the text of the key in signed-by

      sources:
        ubuntu.sources:
          source: |
            Types: deb
            URIs: http://ip/ubuntu
            Suites: noble noble-updates noble-backports noble-security
            Components: main
            #Signed-By: /etc/apt/trusted.gpg.d/localrepokey.asc
            Signed-By: |
              -----BEGIN PGP PUBLIC KEY BLOCK-----
              .
              <key>
              -----END PGP PUBLIC KEY BLOCK-----