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Add snapshot models

Nicolas Dandrimont requested to merge generated-differential-D268-source into generated-differential-D268-target

Snapshots are the new, improved occurrences; They're the topmost object in the Software Heritage Merkle Tree, and represent a full picture of an origin at a given time.

Snapshots contain a list of named pointers to objects in the Software Heritage archive, as well as an intrinsic identifier. The full specification is supported: pointers to all types of objects, dangling pointers, as well as alias branches.

They're implemented with a somewhat classic fully normalised model; Foreign keys use a sha1_git, which makes more sense regarding pointing at non-existent objects, at the expense of some economies of size.

Backwards compatibility both ways with occurrences is ensured: when adding a snapshot linked to an origin visit, the corresponding occurrences are created in occurrence_history; when querying the snapshot for an origin visit where we haven't generated the snapshot yet, a virtual snapshot with id None is returned. This lets us migrate to the new tables gently.

Close #567 (closed).

Test Plan

Integration tests are included.

Migrated from D268 (view on Phabricator)

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