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User: Bypass repeatable-read when creating an actor_id

Authored by Anomie on Nov 29 2018, 2:03 PM.

Description

User: Bypass repeatable-read when creating an actor_id

When MySQL is using repeatable-read transaction isolation (which is the
default), the following sequence of events can occur:

  1. Request A: Begin a transaction.
  2. Request A: Try to select the actor ID for a user. Find no rows.
  3. Request B: Insert an actor ID for that user.
  4. Request A: Try to insert an actor ID for the user. Fails because one exists.
  5. Request A: Try to select the actor ID that must exist. Fail because of the snapshot created at step 2.

In MySQL we can avoid this issue at step #5 by using a locking select
(FOR UPDATE or LOCK IN SHARE MODE), so let's do that.

Bug: T210621
Change-Id: I6c1d255fdd14c6f49d2ea9790e7bd7d101e98ee4