fn: add new ConcurrentQueue[T] data structure

This is a version of the queue we have elsewhere, but we can get rid of
all the casting and interface usage in favor of a type param.

fn/conc_queue.go

@@ -0,0 +1,128 @@
+package fn
+
+import (
+	"sync"
+
+	"github.com/lightninglabs/neutrino/cache/lru"
+)
+
+// ConcurrentQueue is a typed concurrent-safe FIFO queue with unbounded
+// capacity. Clients interact with the queue by pushing items into the in
+// channel and popping items from the out channel. There is a goroutine that
+// manages moving items from the in channel to the out channel in the correct
+// order that must be started by calling Start().
+type ConcurrentQueue[T any] struct {
+	started sync.Once
+	stopped sync.Once
+
+	chanIn   chan T
+	chanOut  chan T
+	overflow *lru.List[T]
+
+	wg   sync.WaitGroup
+	quit chan struct{}
+}
+
+// NewConcurrentQueue constructs a ConcurrentQueue. The bufferSize parameter is
+// the capacity of the output channel. When the size of the queue is below this
+// threshold, pushes do n[?12;4$yot incur the overhead of the less efficient overflow
+// structure.
+func NewConcurrentQueue[T any](bufferSize int) *ConcurrentQueue[T] {
+	return &ConcurrentQueue[T]{
+		chanIn:   make(chan T),
+		chanOut:  make(chan T, bufferSize),
+		overflow: lru.NewList[T](),
+		quit:     make(chan struct{}),
+	}
+}
+
+// ChanIn returns a channel that can be used to push new items into the queue.
+func (cq *ConcurrentQueue[T]) ChanIn() chan<- T {
+	return cq.chanIn
+}
+
+// ChanOut returns a channel that can be used to pop items from the queue.
+func (cq *ConcurrentQueue[T]) ChanOut() <-chan T {
+	return cq.chanOut
+}
+
+// Start begins a goroutine that manages moving items from the in channel to the
+// out channel. The queue tries to move items directly to the out channel
+// minimize overhead, but if the out channel is full it pushes items to an
+// overflow queue. This must be called before using the queue.
+func (cq *ConcurrentQueue[T]) Start() {
+	cq.started.Do(cq.start)
+}
+
+func (cq *ConcurrentQueue[T]) start() {
+	cq.wg.Add(1)
+	go func() {
+		defer cq.wg.Done()
+
+	readLoop:
+		for {
+			nextElement := cq.overflow.Front()
+			if nextElement == nil {
+				// Overflow queue is empty so incoming items can
+				// be pushed directly to the output channel. If
+				// output channel is full though, push to
+				// overflow.
+				select {
+				case item, ok := <-cq.chanIn:
+					if !ok {
+						break readLoop
+					}
+					select {
+					case cq.chanOut <- item:
+						// Optimistically push directly
+						// to chanOut.
+					default:
+						cq.overflow.PushBack(item)
+					}
+				case <-cq.quit:
+					return
+				}
+			} else {
+				// Overflow queue is not empty, so any new items
+				// get pushed to the back to preserve order.
+				select {
+				case item, ok := <-cq.chanIn:
+					if !ok {
+						break readLoop
+					}
+					cq.overflow.PushBack(item)
+				case cq.chanOut <- nextElement.Value:
+					cq.overflow.Remove(nextElement)
+				case <-cq.quit:
+					return
+				}
+			}
+		}
+
+		// Incoming channel has been closed. Empty overflow queue into
+		// the outgoing channel.
+		nextElement := cq.overflow.Front()
+		for nextElement != nil {
+			select {
+			case cq.chanOut <- nextElement.Value:
+				cq.overflow.Remove(nextElement)
+			case <-cq.quit:
+				return
+			}
+			nextElement = cq.overflow.Front()
+		}
+
+		// Close outgoing channel.
+		close(cq.chanOut)
+	}()
+}
+
+// Stop ends the goroutine that moves items from the in channel to the out
+// channel. This does not clear the queue state, so the queue can be restarted
+// without dropping items.
+func (cq *ConcurrentQueue[T]) Stop() {
+	cq.stopped.Do(func() {
+		close(cq.quit)
+		cq.wg.Wait()
+	})
+}