RegisterValidationInterface() takes a bare pointer, remembers it and other threads use it afterwards. This puts a burden on the caller to properly maintain the lifetime of the object which adds complexity and is a source of bugs (#25365, #26188 (comment)).

shared_ptr exists with the purpose of resolving that problem exactly. We already have RegisterSharedValidationInterface(). I think it should be possible to replace all usages of RegisterValidationInterface() with the "Shared" alternative. It is used in:

• BaseIndex::Start() passing this, is already suggested in #25365 (see last paragraph), #24230 "already does this".

• AppInitMain() passing g_zmq_notification_interface which is a global raw ptr, can be changed to global shared_ptr.

• AppInitMain() passing node.peerman.get() which is unique_ptr. Extracting the raw pointer from unique_ptr, saving it somewhere else and using it from other threads defeats the purpose of using unique_ptr because we must now manually manage the lifetime of the object. NodeContext::peerman should be changed from unique_ptr to shared_ptr.

This should make it possible to remove RegisterValidationInterface() and UnregisterValidationInterface() (which is already deprecated).

Chasing concept ACK.