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a8n: Return existing changeset on create conflict

Warren Gifford requested to merge a8n/idempotent-changeset-create into master

Created by: mrnugget

This fixes https://github.com/sourcegraph/src-cli/issues/52 by making the createChangesets query not fail when a changeset with the given external number and repository ID already exists.

It does so by modifying the existing query that INSERTs changesets to JOIN existing changesets and returning their values in case the we run into a conflict on the changesets_repo_external_id_unique constraint.

Since ON CONFLICT [...] DO NOTHING doesn't return a row, we need to RIGHT JOIN (instead of LEFT JOIN) the batch, so that we can access it in case the INSERT didn't return anything.

We use the batch to LEFT JOIN existing changesets rows and to be able to use their values in the SELECT. We use COALESCE so that we only "fallback" to existing values in case something doesn't exist.

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