watchtower/wtdb: add a generic Queue interface

watchtower/wtdb/queue.go

@@ -0,0 +1,22 @@
+package wtdb
+
+import "errors"
+
+// ErrEmptyQueue is returned from Pop if there are no items left in the Queue.
+var ErrEmptyQueue = errors.New("queue is empty")
+
+// Queue is an interface describing a FIFO queue for any generic type T.
+type Queue[T any] interface {
+	// Len returns the number of tasks in the queue.
+	Len() (uint64, error)
+
+	// Push pushes new T items to the tail of the queue.
+	Push(items ...T) error
+
+	// PopUpTo attempts to pop up to n items from the head of the queue. If
+	// no more items are in the queue then ErrEmptyQueue is returned.
+	PopUpTo(n int) ([]T, error)
+
+	// PushHead pushes new T items to the head of the queue.
+	PushHead(items ...T) error
+}