limited size bg queue

semaphore.go

// from
//   https://github.com/sbhackerspace/sbhx-boardinator/blob/0b210dd088502ff2a55453897601ef2361679514/types/email.go#L65

func StartEmailQueue() {
	var semaphore = make(chan bool, 10) // Can only send 10 at once

	go func() {
		for {
			email := <-EmailQueue
			email.Status = QUEUED
			// Spawn goroutine immediately
			go func(e *Email) {
				// Will only block if 10 emails are already being sent.
				semaphore <- true
				defer func() {
					// Drain value from channel to make room
					// for another email sender
					<-semaphore
				}()

				err := e.Send()
				if err != nil {
					e.Failed(err)
					return
				}
				e.Status = SUCCESS
			}(email)
		}
	}()
}