GitHub plugin locks with PG::UniqueViolation

The GitHub plugin locks regularly with following error:

`Reason: PG::UniqueViolation: ERROR: duplicate key value violates unique constraint "index_linked_accounts_on_uid_and_type" DETAIL: Key (uid, type)=(39604003, LinkedAccount::Github::Bot) already exists. : UPDATE "linked_accounts" SET "type" = 'LinkedAccount::Github::Bot', "updated_at" = '2020-05-04 05:19:03.094210', "cloudinary_id" = 'https://avatars2.githubusercontent.com/in/12637?v=4' WHERE "linked_accounts"."id" = 19320591 `

Unlocking it usually helps for some period, but the error eventually reappears (with exactly same avatar URL).


PS: Sometimes it also ends up with #1447.

Start by tracing the GitHub plugin's linked-account update and locking path around the reported PostgreSQL unique constraint, then compare the related behavior in issue #1447. Reproduce the recurring PG::UniqueViolation and determine what handling is needed so the plugin no longer remains locked by the duplicate (uid, type) record.

Written by the indexing model from the issue text.