Potentially "overlocking" in cursor?

[SpencerMalone]

We get a lot of issues caused by...
Error 1213 (40001): Deadlock found when trying to get lock; try restarting transaction when mass migrating data, and I was trying to understand why the select locks are so aggressive.

I'm guessing that the locking in...
https://github.com/Shopify/ghostferry/blob/master/cursor.go#L174-L176
is an implementation of around https://github.com/Shopify/ghostferry/blob/master/tlaplus/ghostferry.tla#L332-L354 in the design spec, and I was wondering if there was any reason we couldn't use a FOR SHARE / LOCK IN SHARE MODE in the cursor to reduce the write lock churn when migrating lots of pieces of data at once? This issue is half bug / half feature request / half question, but the IRC seemed pretty empty when I poked at it, so I figured I'd broach the subject here.

[grodowski]

Hey @SpencerMalone! Since #386 the cursor locking has been changed to a shared lock, you might want to give this a try.