[#1324] local_object_storage: Implement tree service backend

In this commit we implement algorithm for CRDT trees from
https://martin.klepmann.com/papers/move-op.pdf

Each tree is identified by the ID of a container it belongs to
and the tree name itself. Essentially, it is a sequence of operations
which should be applied in chronological order to get a usual tree
representation.

There are 2 backends for now: bbolt database and in-memory.
In-memory backend is here for debugging and will eventually act
as a memory-cache for the on-disk database.

Signed-off-by: Evgenii Stratonikov <evgeniy@nspcc.ru>

pkg/local_object_storage/pilorama/inmemory.go

@@ -0,0 +1,235 @@
+package pilorama
+
+// nodeInfo couples parent and metadata.
+type nodeInfo struct {
+	Parent    Node
+	Meta      Meta
+	Timestamp Timestamp
+}
+
+// state represents state being replicated.
+type state struct {
+	operations []LogMove
+	tree
+}
+
+// newState constructs new empty tree.
+func newState() *state {
+	return &state{
+		tree: *newTree(),
+	}
+}
+
+// undo un-does op and changes s in-place.
+func (s *state) undo(op *LogMove) {
+	children := s.tree.childMap[op.Parent]
+	for i := range children {
+		if children[i] == op.Child {
+			if len(children) > 1 {
+				s.tree.childMap[op.Parent] = append(children[:i], children[i+1:]...)
+			} else {
+				delete(s.tree.childMap, op.Parent)
+			}
+			break
+		}
+	}
+
+	if op.HasOld {
+		s.tree.infoMap[op.Child] = op.Old
+		oldChildren := s.tree.childMap[op.Old.Parent]
+		for i := range oldChildren {
+			if oldChildren[i] == op.Child {
+				return
+			}
+		}
+		s.tree.childMap[op.Old.Parent] = append(oldChildren, op.Child)
+	} else {
+		delete(s.tree.infoMap, op.Child)
+	}
+}
+
+// Apply puts op in log at a proper position, re-applies all subsequent operations
+// from log and changes s in-place.
+func (s *state) Apply(op *Move) error {
+	var index int
+	for index = len(s.operations); index > 0; index-- {
+		if s.operations[index-1].Time <= op.Time {
+			break
+		}
+	}
+
+	if index == len(s.operations) {
+		s.operations = append(s.operations, s.do(op))
+		return nil
+	}
+
+	s.operations = append(s.operations[:index+1], s.operations[index:]...)
+	for i := len(s.operations) - 1; i > index; i-- {
+		s.undo(&s.operations[i])
+	}
+
+	s.operations[index] = s.do(op)
+
+	for i := index + 1; i < len(s.operations); i++ {
+		s.operations[i] = s.do(&s.operations[i].Move)
+	}
+	return nil
+}
+
+// do performs a single move operation on a tree.
+func (s *state) do(op *Move) LogMove {
+	lm := LogMove{
+		Move: Move{
+			Parent: op.Parent,
+			Meta:   op.Meta,
+			Child:  op.Child,
+		},
+	}
+
+	_, parentInTree := s.tree.infoMap[op.Parent]
+	shouldPut := !s.tree.isAncestor(op.Child, op.Parent) &&
+		!(op.Parent != 0 && op.Parent != TrashID && !parentInTree)
+	shouldRemove := op.Parent == TrashID
+
+	p, ok := s.tree.infoMap[op.Child]
+	if ok {
+		lm.HasOld = true
+		lm.Old = p
+	}
+
+	if !shouldPut {
+		return lm
+	}
+
+	if shouldRemove {
+		if ok {
+			s.removeChild(op.Child, p.Parent)
+		}
+		delete(s.tree.infoMap, op.Child)
+		return lm
+	}
+
+	if !ok {
+		p.Timestamp = op.Time
+	} else {
+		s.removeChild(op.Child, p.Parent)
+	}
+
+	p.Meta = op.Meta
+	p.Parent = op.Parent
+	s.tree.infoMap[op.Child] = p
+	s.tree.childMap[op.Parent] = append(s.tree.childMap[op.Parent], op.Child)
+
+	return lm
+}
+
+func (s *state) removeChild(child, parent Node) {
+	oldChildren := s.tree.childMap[parent]
+	for i := range oldChildren {
+		if oldChildren[i] == child {
+			s.tree.childMap[parent] = append(oldChildren[:i], oldChildren[i+1:]...)
+			break
+		}
+	}
+}
+
+func (s *state) timestamp() Timestamp {
+	if len(s.operations) == 0 {
+		return 0
+	}
+	return s.operations[len(s.operations)-1].Time + 1
+}
+
+func (s *state) findSpareID() Node {
+	id := uint64(1)
+	for _, ok := s.infoMap[id]; ok; _, ok = s.infoMap[id] {
+		id++
+	}
+	return id
+}
+
+// tree is a mapping from the child nodes to their parent and metadata.
+type tree struct {
+	infoMap  map[Node]nodeInfo
+	childMap map[Node][]Node
+}
+
+func newTree() *tree {
+	return &tree{
+		childMap: make(map[Node][]Node),
+		infoMap:  make(map[Node]nodeInfo),
+	}
+}
+
+// isAncestor returns true if parent is an ancestor of a child.
+// For convenience, also return true if parent == child.
+func (t tree) isAncestor(parent, child Node) bool {
+	for c := child; c != parent; {
+		p, ok := t.infoMap[c]
+		if !ok {
+			return false
+		}
+		c = p.Parent
+	}
+	return true
+}
+
+// getPathPrefix descends by path constructed from values of attr until
+// there is no node corresponding to a path element. Returns the amount of nodes
+// processed and ID of the last node.
+func (t tree) getPathPrefix(attr string, path []string) (int, Node) {
+	var curNode Node
+
+loop:
+	for i := range path {
+		children := t.childMap[curNode]
+		for j := range children {
+			f := t.infoMap[children[j]].Meta.GetAttr(attr)
+			if string(f) == path[i] {
+				curNode = children[j]
+				continue loop
+			}
+		}
+		return i, curNode
+	}
+
+	return len(path), curNode
+}
+
+// get returns list of nodes which have the specified path from root
+// descending by values of attr from meta.
+func (t tree) get(attr string, path []string, latest bool) []Node {
+	if len(path) == 0 {
+		return nil
+	}
+
+	i, curNode := t.getPathPrefix(attr, path[:len(path)-1])
+	if i < len(path)-1 {
+		return nil
+	}
+
+	var nodes []Node
+	var lastTs Timestamp
+
+	children := t.childMap[curNode]
+	for i := range children {
+		info := t.infoMap[children[i]]
+		fileName := string(info.Meta.GetAttr(attr))
+		if fileName == path[len(path)-1] {
+			if latest {
+				if info.Timestamp >= lastTs {
+					nodes = append(nodes[:0], children[i])
+				}
+			} else {
+				nodes = append(nodes, children[i])
+			}
+		}
+	}
+
+	return nodes
+}
+
+// getMeta returns meta information of node n.
+func (t tree) getMeta(n Node) Meta {
+	return t.infoMap[n].Meta
+}