fix: preserve live collection during alias-based reindex

[tharropoulos]

Change Summary

• prevent reindex from deleting the live alias target before the new collection is ready
• swap the alias to the newly built collection first, then delete the old backing collection
• add a regression test that verifies reindex does not remove the live alias target before the alias swap
• make temporary reindex collection names unique to avoid same-second collection name collisions during repeated runs
• fix reindex causes downtime when alias is already set up, delete_collection called before new collection is ready #23

PR Checklist

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[haleyngonadi]

hi @tharropoulos, it's me again :)

Do you think this will also resolve the issue of joins requiring re-indexing when the alias of one of their references changes? For example, when I run Book.reindex, and I have it referenced in another model like so: { "name" => "book_id", "type" => "string", "reference": "#{::Book.index_name}.id" }, I immediately get an error afterwards: "No reference field found." but the reference existed until I had to reindex.

[tharropoulos]

@haleyngonadi It should fix that case as well, as the alias was dangling before this fix. I actually just wrote a test to verify that joins continue to work when reindexing the referenced collection.

That said, I'd still be a little careful about generalizing too far from one passing case. With JOINs, when aliases are used in reference fields, related collections may need to be reindexed together because of how internal reference IDs are handled. Here're some docs around Aliases with JOINs

[dpashutskii]

Thank you for fixing it. @tharropoulos, will you ship a new gem release?