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Add lexicographic based tags pagination and counting

João Pereira requested to merge db-api-tags-list into database

Context

This MR adds a way to lexicographically find and count tags within a repository based on their name. This is needed for the GET /v2/<name>/tags/list route.

Related to #120 (closed).

Rationale

Although defined by the Docker Distribution spec, currently the registry doesn't implement pagination for tags. This is due to technical limitations (see #13 (comment 271769891)).

However, if it's ever implemented, it'll have the same constraints as the catalog pagination, as it would rely on the same filesystem walk method:

  • The pagination is done based on the tags name;
  • Navigating to the next page is done by providing a last query parameter, which should be set to the name of the last tag returned on the current response/page;
  • Even if there are no tags with a name equal to last, all tags with a name lexicographically before last are ignored;
  • Tags are sorted lexicographically, with a possible limit (n query parameter) being applied after they're sorted.

We have to respect the constraints above. For this reason, the new methods are likely going to be used exclusively by the tag list API handler.

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