mark the new chain as canonical when calling db.setHead

the new chain marked as canonical is from the new head down to
forking point. most of the time, the forking point is the parent
itself.

nimbus/db/db_chain.nim

@@ -313,24 +313,43 @@ proc headerExists*(self: BaseChainDB; blockHash: Hash256): bool =
   ## Returns True if the header with the given block hash is in our DB.
   self.db.contains(genericHashKey(blockHash).toOpenArray)
 
+proc markCanonicalChain(self: BaseChainDB, header: BlockHeader, headerHash: Hash256) =
+  ## mark this chain as canonical by adding block number to hash lookup
+  ## down to forking point
+  var
+    currHash = headerHash
+    currHeader = header
+
+  while currHash != Hash256():
+    let key = blockNumberToHashKey(currHeader.blockNumber)
+    let data = self.db.get(key.toOpenArray)
+    if data.len == 0:
+      # not marked, mark it
+      self.db.put(key.toOpenArray, rlp.encode(currHash))
+    elif rlp.decode(data, Hash256) != currHash:
+      # replace prev chain
+      self.db.put(key.toOpenArray, rlp.encode(currHash))
+    else:
+      # forking point, done
+      break
+
+    currHash = currHeader.parentHash
+    currHeader = self.getBlockHeader(currHeader.parentHash)
+
 proc setHead*(self: BaseChainDB, blockHash: Hash256): bool =
-  # TODO: apply addBlockNumberToHashLookup to forkPoint
-  if self.headerExists(blockHash):
-    var header: BlockHeader
-    if not self.getBlockHeader(blockHash, header):
-      return false
-    self.addBlockNumberToHashLookup(header)
-    self.db.put(canonicalHeadHashKey().toOpenArray, rlp.encode(blockHash))
-    return true
-  else:
+  var header: BlockHeader
+  if not self.getBlockHeader(blockHash, header):
     return false
 
+  self.markCanonicalChain(header, blockHash)
+  self.db.put(canonicalHeadHashKey().toOpenArray, rlp.encode(blockHash))
+  return true
+
 proc setHead*(self: BaseChainDB, header: BlockHeader, writeHeader = false) =
-  # TODO: apply addBlockNumberToHashLookup to forkPoint
   var headerHash = rlpHash(header)
   if writeHeader:
     self.db.put(genericHashKey(headerHash).toOpenArray, rlp.encode(header))
-  self.addBlockNumberToHashLookup(header)
+  self.markCanonicalChain(header, headerHash)
   self.db.put(canonicalHeadHashKey().toOpenArray, rlp.encode(headerHash))
 
 proc persistReceipts*(self: BaseChainDB, receipts: openArray[Receipt]): Hash256 =