FCU ignored after payload is processed

[Advaita Saha]

When a payload is sent to eth1, it processes and validates fine
but then fcu is called, and says it is updating to the old head and thus ignoring the request

Shouldn't behave like that, but it is because of the payload processing
In the new_payload, we are also persisting the header to the db

nimbus-eth1/nimbus/core/chain/forked_chain.nim

Lines 88 to 95 in ea74e03

 # We still need to write header to database 
 # because validateUncles still need it 
 let blockHash = header.blockHash() 
 if not c.db.persistHeader( 
       blockHash, 
       header, 
       c.com.startOfHistory): 
   return err("Could not persist header") 

Then when the fcu call takes place, it checks if the header is there in the db, and it is thus ignoring the fcu call

nimbus-eth1/nimbus/beacon/api_handler/api_forkchoice.nim

Lines 158 to 163 in ea74e03

 var canonHash: common.Hash256 
 if db.getBlockHash(header.number, canonHash) and canonHash == blockHash: 
   notice "Ignoring beacon update to old head", 
     blockHash=blockHash.short, 
     blockNumber=header.number 
   return validFCU(Opt.none(PayloadID), blockHash) 

A similar issue arising from the same persistence of header #2588

[Advaita Saha]

A probable fix to this might be to use the state, @jangko can comment further if it's viable or not

nimbus-eth1/nimbus/beacon/api_handler/api_forkchoice.nim

Line 159 in ea74e03

 if db.getBlockHash(header.number, canonHash) and canonHash == blockHash: 

This can be changed to

if db.getSavedStateBlockNumber() >= header.number :
  notice "Ignoring beacon update to old head", 
     blockHash=blockHash.short, 
     blockNumber=header.number 
   return validFCU(Opt.none(PayloadID), blockHash)