Building a struct which mode switches

mode_switcher.go

// test application for how to switch modes
package main

import (
	"fmt"
	"sync"
	"time"
)

type ReadMode interface {
}

type WriteMode interface {
	ReadMode
}

type ModeSwitchable struct {
	cond *sync.Cond
	readers int
	writers int
}

func (ms *ModeSwitchable) WithReadonly(fn func (ds ReadMode)) {
	ms.cond.L.Lock()
	for ms.writers > 0 {
		ms.cond.Wait()
	}
	ms.readers++
	ms.cond.L.Unlock()
	ms.cond.Broadcast()

	fn(ReadMode(ms))
	ms.readers--
	ms.cond.Broadcast()
}

func (ms *ModeSwitchable) WithAny(fn func (ds ReadMode)) {
	fn(ReadMode(ms))
}

func (ms *ModeSwitchable) WhenWriteable(fn func (ds WriteMode)) {
	ms.cond.L.Lock()
	for ms.readers > 0 {
		ms.cond.Wait()
	}
	ms.writers++
	ms.cond.L.Unlock()
	ms.cond.Broadcast()

	fn(WriteMode(ms))
	ms.writers--
	ms.cond.Broadcast()
}

func main() {
	fmt.Println("Started")
	ms := ModeSwitchable{}
	ms.cond = sync.NewCond(new(sync.Mutex))
	ms.readers = 0
	ms.writers = 0

	go ms.WhenWriteable(func(ds WriteMode) {
		fmt.Println("Long running writable started...", time.Now())
		time.Sleep(time.Second*10)
		fmt.Println("Long running writable finished", time.Now())
	})

	wg := new(sync.WaitGroup)

	wg.Add(1)
	go ms.WithReadonly(func(ds ReadMode) {
		fmt.Println("Readonly during writeable", time.Now())
		wg.Done()
	})

	wg.Add(1)
	go ms.WithReadonly(func(ds ReadMode) {
		fmt.Println("Readonly during writeable", time.Now())
		wg.Done()
	})

	wg.Add(1)
	go ms.WithReadonly(func(ds ReadMode) {
		fmt.Println("Readonly during writeable", time.Now())
		wg.Done()
	})

	time.Sleep(time.Second)
	go ms.WhenWriteable(func(ds WriteMode) {
		fmt.Println("Quick writeable 1 started", time.Now())
		time.Sleep(time.Second * 3)
		fmt.Println("Quick writeable 1 finished", time.Now())
	})

	go ms.WhenWriteable(func(ds WriteMode) {
		fmt.Println("Quick writeable 2 started", time.Now())
		time.Sleep(time.Second * 3)
		fmt.Println("Quick writeable 2 finished", time.Now())
	})

	wg.Wait()

	// Test trying to write during readonly
	go ms.WithReadonly(func(ds ReadMode) {
		fmt.Println("Long running readonly started...", time.Now())
		time.Sleep(time.Second*10)
		fmt.Println("Long running readonly finished", time.Now())
	})

	time.Sleep(time.Second)
	wg.Add(1)
	go ms.WhenWriteable(func(ds WriteMode) {
		fmt.Println("Quick writeable 3 started", time.Now())
		time.Sleep(time.Second * 3)
		fmt.Println("Quick writeable 3 finished", time.Now())
		wg.Done()
	})
	wg.Add(1)
	go ms.WhenWriteable(func(ds WriteMode) {
		fmt.Println("Quick writeable 4 started", time.Now())
		time.Sleep(time.Second * 3)
		fmt.Println("Quick writeable 4 finished", time.Now())
		wg.Done()
	})

	wg.Wait()
}