core: mirror full node reorg logic in light client too (#20931)

* core: fix the condition of reorg

* core: fix nitpick to only retrieve head once

* core: don't reorg if received chain is longer at same diff

Co-authored-by: Péter Szilágyi <peterke@gmail.com>

core/headerchain.go

@@ -149,7 +149,8 @@ func (hc *HeaderChain) WriteHeader(header *types.Header) (status WriteStatus, er
 	if ptd == nil {
 		return NonStatTy, consensus.ErrUnknownAncestor
 	}
-	localTd := hc.GetTd(hc.currentHeaderHash, hc.CurrentHeader().Number.Uint64())
+	head := hc.CurrentHeader().Number.Uint64()
+	localTd := hc.GetTd(hc.currentHeaderHash, head)
 	externTd := new(big.Int).Add(header.Difficulty, ptd)
 
 	// Irrelevant of the canonical status, write the td and header to the database
@@ -165,7 +166,15 @@ func (hc *HeaderChain) WriteHeader(header *types.Header) (status WriteStatus, er
 	// If the total difficulty is higher than our known, add it to the canonical chain
 	// Second clause in the if statement reduces the vulnerability to selfish mining.
 	// Please refer to http://www.cs.cornell.edu/~ie53/publications/btcProcFC.pdf
-	if externTd.Cmp(localTd) > 0 || (externTd.Cmp(localTd) == 0 && mrand.Float64() < 0.5) {
+	reorg := externTd.Cmp(localTd) > 0
+	if !reorg && externTd.Cmp(localTd) == 0 {
+		if header.Number.Uint64() < head {
+			reorg = true
+		} else if header.Number.Uint64() == head {
+			reorg = mrand.Float64() < 0.5
+		}
+	}
+	if reorg {
 		// If the header can be added into canonical chain, adjust the
 		// header chain markers(canonical indexes and head header flag).
 		//