core: define and test chain rewind corner cases (#21409)

* core: define and test chain reparation cornercases

* core: write up a variety of set-head tests

* core, eth: unify chain rollbacks, handle all the cases

* core: make linter smile

* core: remove commented out legacy code

* core, eth/downloader: fix review comments

* core: revert a removed recovery mechanism

eth/downloader/downloader_test.go

@@ -341,25 +341,52 @@ func (dl *downloadTester) InsertReceiptChain(blocks types.Blocks, receipts []typ
 	return len(blocks), nil
 }
 
-// Rollback removes some recently added elements from the chain.
-func (dl *downloadTester) Rollback(hashes []common.Hash) {
+// SetHead rewinds the local chain to a new head.
+func (dl *downloadTester) SetHead(head uint64) error {
 	dl.lock.Lock()
 	defer dl.lock.Unlock()
 
-	for i := len(hashes) - 1; i >= 0; i-- {
-		if dl.ownHashes[len(dl.ownHashes)-1] == hashes[i] {
-			dl.ownHashes = dl.ownHashes[:len(dl.ownHashes)-1]
+	// Find the hash of the head to reset to
+	var hash common.Hash
+	for h, header := range dl.ownHeaders {
+		if header.Number.Uint64() == head {
+			hash = h
 		}
-		delete(dl.ownChainTd, hashes[i])
-		delete(dl.ownHeaders, hashes[i])
-		delete(dl.ownReceipts, hashes[i])
-		delete(dl.ownBlocks, hashes[i])
-
-		delete(dl.ancientChainTd, hashes[i])
-		delete(dl.ancientHeaders, hashes[i])
-		delete(dl.ancientReceipts, hashes[i])
-		delete(dl.ancientBlocks, hashes[i])
 	}
+	for h, header := range dl.ancientHeaders {
+		if header.Number.Uint64() == head {
+			hash = h
+		}
+	}
+	if hash == (common.Hash{}) {
+		return fmt.Errorf("unknown head to set: %d", head)
+	}
+	// Find the offset in the header chain
+	var offset int
+	for o, h := range dl.ownHashes {
+		if h == hash {
+			offset = o
+			break
+		}
+	}
+	// Remove all the hashes and associated data afterwards
+	for i := offset + 1; i < len(dl.ownHashes); i++ {
+		delete(dl.ownChainTd, dl.ownHashes[i])
+		delete(dl.ownHeaders, dl.ownHashes[i])
+		delete(dl.ownReceipts, dl.ownHashes[i])
+		delete(dl.ownBlocks, dl.ownHashes[i])
+
+		delete(dl.ancientChainTd, dl.ownHashes[i])
+		delete(dl.ancientHeaders, dl.ownHashes[i])
+		delete(dl.ancientReceipts, dl.ownHashes[i])
+		delete(dl.ancientBlocks, dl.ownHashes[i])
+	}
+	dl.ownHashes = dl.ownHashes[:offset+1]
+	return nil
+}
+
+// Rollback removes some recently added elements from the chain.
+func (dl *downloadTester) Rollback(hashes []common.Hash) {
 }
 
 // newPeer registers a new block download source into the downloader.