p2p – When do checkpoint and header pre-sync rules start taking effect?

It’s related to this.

This mainly describes cases where there is already an active chain tip. I’m interested in when we don’t do IBD and when we do IBD.

As far as I know, the active chaintip is only set after fully synchronizing and finding the best chain (with the most accumulated tasks).

1. Do checkpoints and header presynchronization only take effect after setting at least one active chaintip? I came to my own conclusion, but I don’t know if it’s right.
   • It does not apply to the first IBD, and it does not apply to IBDs in which a non-first (not started at genesis) node has never set an active chaintip (full synchronization has not been reached). For a node that was synchronized once (some active chaintips are out of date but still there) and a node that restarts after 2 months (and thus starting IBD), checkpoints and header pre-syncs are applied since there is something to compare. .
2. As a result, is it possible for an attacker to create a low-difficulty chain from scratch and attack a node that does not have active chaintips sending it to the node? Nodes must store all of these headers because most operations do not know whether to create a chain or not, and since there are no active chaintips (nothing to compare), checkpoints and header pre-synchronization cannot be applied.