core: reset to genesis when middle block is missing (ethereum#22135)

When a sethead/rewind finds that the targeted block is missing, it resets to genesis instead of crashing. Closes ethereum#22129

core/blockchain.go

@@ -524,8 +524,13 @@ func (bc *BlockChain) SetHeadBeyondRoot(head uint64, root common.Hash) (uint64,
 					if _, err := state.New(newHeadBlock.Root(), bc.stateCache, bc.snaps); err != nil {
 						log.Trace("Block state missing, rewinding further", "number", newHeadBlock.NumberU64(), "hash", newHeadBlock.Hash())
 						if pivot == nil || newHeadBlock.NumberU64() > *pivot {
-							newHeadBlock = bc.GetBlock(newHeadBlock.ParentHash(), newHeadBlock.NumberU64()-1)
-							continue
+							parent := bc.GetBlock(newHeadBlock.ParentHash(), newHeadBlock.NumberU64()-1)
+							if parent != nil {
+								newHeadBlock = parent
+								continue
+							}
+							log.Error("Missing block in the middle, aiming genesis", "number", newHeadBlock.NumberU64()-1, "hash", newHeadBlock.ParentHash())
+							newHeadBlock = bc.genesisBlock
 						} else {
 							log.Trace("Rewind passed pivot, aiming genesis", "number", newHeadBlock.NumberU64(), "hash", newHeadBlock.Hash(), "pivot", *pivot)
 							newHeadBlock = bc.genesisBlock