Fantom-foundation/go-lachesis

package gossip

import "sync"

// execQueue implements a queue that executes function calls in a single thread,
// in the same order as they have been queued.
type execQueue struct {
    mu        sync.Mutex
    cond      *sync.Cond
    funcs     []func()
    closeWait chan struct{}
}

// newExecQueue creates a new execution queue.
func newExecQueue(capacity int) *execQueue {
    q := &execQueue{funcs: make([]func(), 0, capacity)}
    q.cond = sync.NewCond(&q.mu)
    go q.loop()
    return q
}

func (q *execQueue) loop() {
    for f := q.waitNext(false); f != nil; f = q.waitNext(true) {
        f()
    }
    close(q.closeWait)
}

func (q *execQueue) waitNext(drop bool) (f func()) {
    q.mu.Lock()
    if drop && len(q.funcs) > 0 {
        // Remove the function that just executed. We do this here instead of when
        // dequeuing so len(q.funcs) includes the function that is running.
        q.funcs = append(q.funcs[:0], q.funcs[1:]...)
    }
    for !q.isClosed() {
        if len(q.funcs) > 0 {
            f = q.funcs[0]
            break
        }
        q.cond.Wait()
    }
    q.mu.Unlock()
    return f
}

func (q *execQueue) isClosed() bool {
    return q.closeWait != nil
}

// canQueue returns true if more function calls can be added to the execution queue.
func (q *execQueue) canQueue() bool {
    q.mu.Lock()
    ok := !q.isClosed() && len(q.funcs) < cap(q.funcs)
    q.mu.Unlock()
    return ok
}

// queue adds a function call to the execution queue. Returns true if successful.
func (q *execQueue) queue(f func()) bool {
    q.mu.Lock()
    ok := !q.isClosed() && len(q.funcs) < cap(q.funcs)
    if ok {
        q.funcs = append(q.funcs, f)
        q.cond.Signal()
    }
    q.mu.Unlock()
    return ok
}

// clear drops all queued functions
func (q *execQueue) clear() {
    q.mu.Lock()
    q.funcs = q.funcs[:0]
    q.mu.Unlock()
}

// quit stops the exec queue.
// quit waits for the current execution to finish before returning.
func (q *execQueue) quit() {
    q.mu.Lock()
    if !q.isClosed() {
        q.closeWait = make(chan struct{})
        q.cond.Signal()
    }
    q.mu.Unlock()
    <-q.closeWait
}