The use case is building snaps containing both 32- and 64-bit packages (i.e. to support older software in the graphics-core22 stack [1]). I've not found a way to build a amd64 + i386 (or arm64 + armhf) snap, because either repository refresh fails (case A), or snapcraft holds on to an archive cache from before `dpkg --add-architecture` (case B). ## Case A Given this YAML: (using the ARM example as this stops working when building a snap for an architecture that doesn't exist in the added repository - this way you can reproduce on amd64) ```yaml name: arch-test base: core22 version: '0.1' summary: Architecture test description: |   Multi-architecture test. grade: devel confinement: devmode package-repositories:   - type: apt     url: http://ports.ubuntu.com/ubuntu-ports/     suites: [jammy]     components: [main, universe]     architectures: [armhf]     key-id: F6ECB3762474EDA9D21B7022871920D1991BC93C     key-server: keyserver.ubuntu.com parts:   part:     plugin: nil     stage-packages:     - libc6:armhf ``` ```plain $ snapcraft Launching instance... Add repository architecture: armhf Failed to refresh package list: failed to run apt update. Failed to execute pack in instance. Full execution log: '/home/michal/.cache/snapcraft/log/snapcraft-20230304-212001.588931.log' ``` Because: ```plain 2023-03-04 21:20:35.447 :: 2023-03-04 20:20:35.054 :: E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/jammy/main/binary-armhf/Packages 404 Not Found [IP: 185.125.190.36 80] 2023-03-04 21:20:35.447 :: 2023-03-04 20:20:35.054 :: E: Failed to fetch http://security.ubuntu.com/ubuntu/dists/jammy-security/main/binary-armhf/Packages 404 Not Found [IP: 185.125.190.39 80] 2023-03-04 21:20:35.447 :: 2023-03-04 20:20:35.054 :: E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/jammy-updates/main/binary-armhf/Packages 404 Not Found [IP: 185.125.190.39 80] 2023-03-04 21:20:35.447 :: 2023-03-04 20:20:35.054 :: E: Failed to fetch http://archive.ubuntu.com/ubuntu/dists/jammy-backports/main/binary-armhf/Packages 404 Not Found [IP: 185.125.190.39 80] 2023-03-04 21:20:35.447 :: 2023-03-04 20:20:35.054 :: E: Some index files failed to download. They have been ignored, or old ones used instead. ``` The apt repository definitions in the build base are not architecture-specific, so adding an architecture outside of that repository results in above errors. ## Case B Another approach: ```yaml # ... (no package-repositories) parts:   aaa:     plugin: nil     stage-packages:     - libc6   arch-add:     plugin: nil     override-pull: |       if [ "${CRAFT_TARGET_ARCH}" == "amd64" ]; then         dpkg --add-architecture i386         apt-get update       fi   pack-i386:     after: [arch-add]     plugin: nil     stage-packages:     - libc6:i386 ``` This will fail to find `libc6:i386`, because Snapcraft seems to hold to an apt cache from before processing parts: ```plain Launching instance... Executed: pull arch-add Stage package not found in part 'pack': libc6:i386. Failed to execute pack in instance. Full execution log: '/home/michal/.cache/snapcraft/log/snapcraft-20230304-213302.303889.log' ``` Re-running the build will complete, because Snapcraft will have reloaded the now-multiarch cache. [1] https://github.com/MirServer/glvnd-core22/