blockchain: Add InactiveTips() to blockindex

InactiveTips() returns all the tips of the branches of the blockchain
tree that are not in the best chain. This function is useful for
supporting the getchaintips rpc call.

blockchain/blockindex.go

@@ -377,6 +377,44 @@ func (bi *blockIndex) UnsetStatusFlags(node *blockNode, flags blockStatus) {
 	bi.Unlock()
 }
 
+// InactiveTips returns all the block nodes that aren't in the best chain.
+//
+// This function is safe for concurrent access.
+func (bi *blockIndex) InactiveTips(bestChain *chainView) []*blockNode {
+	bi.RLock()
+	defer bi.RUnlock()
+
+	// Look through the entire blockindex and look for nodes that aren't in
+	// the best chain. We're gonna keep track of all the orphans and the parents
+	// of the orphans.
+	orphans := make(map[chainhash.Hash]*blockNode)
+	orphanParent := make(map[chainhash.Hash]*blockNode)
+	for hash, node := range bi.index {
+		found := bestChain.Contains(node)
+		if !found {
+			orphans[hash] = node
+			orphanParent[node.parent.hash] = node.parent
+		}
+	}
+
+	// If an orphan isn't pointed to by another orphan, it is a chain tip.
+	//
+	// We can check this by looking for the orphan in the orphan parent map.
+	// If the orphan exists in the orphan parent map, it means that another
+	// orphan is pointing to it.
+	tips := make([]*blockNode, 0, len(orphans))
+	for hash, orphan := range orphans {
+		_, found := orphanParent[hash]
+		if !found {
+			tips = append(tips, orphan)
+		}
+
+		delete(orphanParent, hash)
+	}
+
+	return tips
+}
+
 // flushToDB writes all dirty block nodes to the database. If all writes
 // succeed, this clears the dirty set.
 func (bi *blockIndex) flushToDB() error {