Bonus links and transactions sketch

[jcushman]

This is a quick sketch to try to reduce opportunities for deadlocks in saving or moving links.

• Don't use a transaction in user.save() unless needed. I'm not sure if that's relevant, but why not.
• Add an update_bonus_links() helper that does bonus_links updating in the database instead of on the instance, and then marks the instance's bonus_links value as outdated. This means in principle you can do lots of these in parallel without needing a lock on the user -- with the exception that you could give someone an extra link or two if they came in at just the right time.
• Be a notch more selective in when to update bonus_links in Folder.save() -- I'm not sure how often this matters.
• For clarity, don't let bonus_links be null.

Sketch:

• update_bonus_links is mostly AI implemented, I didn't test, but I think it's about right.
• I didn't remove many saves or select_for_updates here -- not sure yet if this approach itself helps, or just lets us potentially remove some locks elsewhere, or misses the problem.

[jcushman]

Be a notch more selective in when to update bonus_links in Folder.save() -- I'm not sure how often this matters.

Oh, I suspect this would have made the difference here! The deadlock is on this line:

And the deadlocked call was "UPDATE "perma_linkuser" SET "bonus_links" = NULL WHERE "perma_linkuser"."id" = %s", which suggests bonus_links weren't in play. So moving the save() up into the if block above would have prevented it entirely.

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I think the other important question is, is this select_for_update still doing anything?

If that was just to protect the bonus_links count I think we can lose it, but maybe it's for something else too, in which case we could make it more specific.