SCAN, SSCAN, HSCAN, and ZSCAN are not stable across failovers about valkey HOT 2 OPEN

There is a second issue that I want to make sure we don't drop, which is the cursor format is changing for SCAN for Redis 8 with the new per-slot dictionaries. The format is changing from <64 bits for DB cursor> to 00<14 bits for slot><48 bits for DB cursor>. To be specific, we aren't actually showing the bits, just the integer representation of those bits. We should be able to detect versioning issues, i.e. using the cursor from 7.0 node on an 8.0 node, so I would like to propose we update it to one of the following two options:

<2 version bits><14 bits for slot><48 bits for DB cursor>, we will bump from version 00 -> 01. This is likely the most backwards compatible. It also allows us in the future to do a third version if we want to re-organize the cursor bits. Going to more than 2 bits introduces the risk there are users are storing the cursor as a long long, and it will break.
-<14 bits for slot><48 bits for DB cursor>. The version will initially be 1, and we'll use the - to detect the new format. This gives us the most freedom to change the version more in the future.
@yossigo, I wasn't able to find our decision from the previous meeting. I know you were concerned with the format, which we agreed we should look into fixing, but I don't recall if you also wanted to try to make the SCAN command stable across failover. (I still want to try to make HSCAN, ZSCAN, and SSCAN stable) So that it might return duplicate items, but won't omit items because of the cursor shift.

from valkey.

This is a great topic to finish. So it's actually three things mentioned here:

1. Scan across failovers. Synching the hashtable seed. We can do that in the RDB or in another way when initiating replication or before. The seed may be sensitive security information though, but if anyone has permission to replicate, I think it's fine to let them fetch the seed too.
2. Cluster-wide scan. Per slot, either explicitly or implicitly. You were against exposing the slot to the user, while others liked it.
3. Cursor versioning. I don't see how you can use these bits though. Are these two LSB unused in an old cursor?

from valkey.