blocking-map.go

package main

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

type BlockingMap struct {
	Map     map[string]int
	Pending map[string][]*sync.Cond
	Lock    *sync.Mutex
}

func New() *BlockingMap {
	return &BlockingMap{Map: map[string]int{}, Pending: map[string][]*sync.Cond{}, Lock: new(sync.Mutex)}
}

func (m *BlockingMap) Put(key string, val int) {
	m.Lock.Lock()
	defer func() { m.Lock.Unlock() }()

	m.Map[key] = val
	for _, cv := range m.Pending[key] {
		cv.Signal()
	}
	m.Pending[key] = nil
}

func (m *BlockingMap) Get(key string, cv *sync.Cond) int {
	m.Lock.Lock()
	defer func() { m.Lock.Unlock() }()

	if val, ok := m.Map[key]; ok {
		return val
	}

	m.Pending[key] = append(m.Pending[key], cv)
	cv.L = m.Lock
	for {
		val, ok := m.Map[key]
		if ok {
			return val
		}
		cv.Wait()
	}
}

func main() {
	m := New()

	go func() {
		time.Sleep(1 * time.Second)
		m.Put("x", 1)
		time.Sleep(1 * time.Second)
		m.Put("y", 2)
		time.Sleep(1 * time.Second)
		m.Put("z", 3)

	}()

	cv := sync.NewCond(nil)

	i := m.Get("x", cv)
	fmt.Println(i)

	i = m.Get("y", cv)
	fmt.Println(i)

	i = m.Get("y", cv)
	fmt.Println(i)

	i = m.Get("z", cv)
	fmt.Println(i)
}