[noctune]

S3 recently got conditional writes and you can use do locking entirely in S3 - I don't think they are using this though. Must be too recent an addition.

[yencabulator]

I believe S3 can only do create-if-not-exist, which won't help for overwriting a pre-existing branch ref only-if-not-concurrently-updated.

https://docs.aws.amazon.com/AmazonS3/latest/userguide/condit...