watchtowers: add thread safe min-heap

In this commit, a thread-safe min-heap is implemented. It will carry
sessionCloseItems which carry a sessionID and a block height at which
the session should be closed.

watchtower/wtclient/sess_close_min_heap.go

@@ -0,0 +1,95 @@
+package wtclient
+
+import (
+	"sync"
+
+	"github.com/lightningnetwork/lnd/queue"
+	"github.com/lightningnetwork/lnd/watchtower/wtdb"
+)
+
+// sessionCloseMinHeap is a thread-safe min-heap implementation that stores
+// sessionCloseItem items and prioritises the item with the lowest block height.
+type sessionCloseMinHeap struct {
+	queue queue.PriorityQueue
+	mu    sync.Mutex
+}
+
+// newSessionCloseMinHeap constructs a new sessionCloseMineHeap.
+func newSessionCloseMinHeap() *sessionCloseMinHeap {
+	return &sessionCloseMinHeap{}
+}
+
+// Len returns the length of the queue.
+func (h *sessionCloseMinHeap) Len() int {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+
+	return h.queue.Len()
+}
+
+// Empty returns true if the queue is empty.
+func (h *sessionCloseMinHeap) Empty() bool {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+
+	return h.queue.Empty()
+}
+
+// Push adds an item to the priority queue.
+func (h *sessionCloseMinHeap) Push(item *sessionCloseItem) {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+
+	h.queue.Push(item)
+}
+
+// Pop removes the top most item from the queue.
+func (h *sessionCloseMinHeap) Pop() *sessionCloseItem {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+
+	if h.queue.Empty() {
+		return nil
+	}
+
+	item := h.queue.Pop()
+
+	return item.(*sessionCloseItem) //nolint:forcetypeassert
+}
+
+// Top returns the top most item from the queue without removing it.
+func (h *sessionCloseMinHeap) Top() *sessionCloseItem {
+	h.mu.Lock()
+	defer h.mu.Unlock()
+
+	if h.queue.Empty() {
+		return nil
+	}
+
+	item := h.queue.Top()
+
+	return item.(*sessionCloseItem) //nolint:forcetypeassert
+}
+
+// sessionCloseItem represents a session that is ready to be deleted.
+type sessionCloseItem struct {
+	// sessionID is the ID of the session in question.
+	sessionID wtdb.SessionID
+
+	// deleteHeight is the block height after which we can delete the
+	// session.
+	deleteHeight uint32
+}
+
+// Less returns true if the current item's delete height is less than the
+// other sessionCloseItem's delete height. This results in lower block heights
+// being popped first from the heap.
+//
+// NOTE: this is part of the queue.PriorityQueueItem interface.
+func (s *sessionCloseItem) Less(other queue.PriorityQueueItem) bool {
+	o := other.(*sessionCloseItem).deleteHeight //nolint:forcetypeassert
+
+	return s.deleteHeight < o
+}
+
+var _ queue.PriorityQueueItem = (*sessionCloseItem)(nil)