Add support for managing appstream catalogs as part of the index

[joebonrichie]

Summary

Why

We want up-to-date appstream catalog information as some applications contain changelog information in their appstream metadata. This changelog information is then shown in front-ends such as gnome-software. As is it currently, appstream-catalog is downloaded as part of a separate package, this then means that gnome-software will show out of date changelog information for packages. e.g. an update to gnome-clocks 49.0 exists and the changelog for this release is included an update for appstream-catalog. However, as the latest appstream-catalog is not yet installed gnome-software will either show out of date changelog information or simply fall back to the generic summary message used for the update. By updating the appstream-catalog as part of the eopkg index we can then ensure we always have the most up-to-date changelog information for available updates.

Example

Before up-to-date appstream catalog is installed:

After up-to-date appstream catalog is installed:

How

A supplemental index file can be provided (appstream.xml) that is picked up similarly in nature to other supplemental files e.g. components.xml, groups.xml.

An example of an appstream.xml file is:

<?xml version="1.0" encoding="UTF-8"?>
<PISI>
    <AppstreamCatalogs>
        <AppstreamCatalog>
            <Origin>solus-unstable-main</Origin>
            <URI>https://appstream.getsol.us/data/unstable/main/Components-x86_64.xml.gz</URI>
            <Icons size="64x64">
                <URI>https://appstream.getsol.us/data/unstable/main/icons-64x64.tar.gz</URI>
            </Icons>
            <Icons size="64x64@2">
                <URI>https://appstream.getsol.us/data/unstable/main/[email protected]</URI>
            </Icons>
            <Icons size="128x128">
                <URI>https://appstream.getsol.us/data/unstable/main/icons-128x128.tar.gz</URI>
            </Icons>
        </AppstreamCatalog>
    </AppstreamCatalogs>
</PISI>

Multiple appstream catalogs can be specified.

When the repository is indexed the </AppstreamCatalog> tag(s) is included in the eopkg-index.xml file.

The origin should match the origin in the Components-x86_64.xml.gz file.

When an </AppstreamCatalog> is included in the eopkg index, eopkg will then download the appstream catalog and it's icons to /var/lib/swcatalog/xml/ if the catalog is not installed. If the catalog is installed then eopkg will check against the Last-Modified header to see if newer tarballs exist on the remote then download and install them if necessary.

For the example appstream.xml file above, eopkg will install to the following paths:
/var/lib/swcatalog/xml/solus-unstable-main.xml.gz
/var/lib/swcatalog/xml/solus-unstable-main.xml.gz.eopkg
/var/lib/swcatalog/icons/solus-unstable-main/128x128/
/var/lib/swcatalog/icons/solus-unstable-main/64x64/
/var/lib/swcatalog/icons/solus-unstable-main/64x64@2/

Eopkg will place a tag on the catalog so it knows that appstream catalog is managed by itself (in this case /var/lib/swcatalog/xml/solus-unstable-main.xml.gz.eopkg) . If the index no longer contains an appstream catalog matching the tag, eopkg will then nuke those paths as an obsolete catalog when the index is updated.

Test Plan

• Add appstream.xml to local repo
• Index local repo
• Verify </AppstreamCatalog> exists in the eopkg.index.xml file
• Update repo
• Verify that the appstream catalog & icons get installed to their correct paths
• Update appstream catalog remotely
• Update repo
• Verify that the appstream catalog updated tarballs get installed
• Remove appstream.xml from local repo
• Index local repo
• Verify </AppstreamCatalog> no longer exists in eopkg-index.xml file
• Update repo
• Verify that the obsolete catalog gets uninstalled

TODO: been a while since i've looked at this code but i think i wasn't particularly happy with how obsolete catalogs were managed