Semaphore server

servaphore.go

package main

import (
	"log"
	"net/http"
	"time"
)

type empty struct{}
type semaphore chan empty

// acquire n resources
func (s semaphore) P(n int) {
	e := empty{}
	for i := 0; i < n; i++ {
		s <- e
	}
}

// release n resources
func (s semaphore) V(n int) {
	for i := 0; i < n; i++ {
		<-s
	}
}

type store map[string]semaphore

func (s store) P(tag string, n int) chan empty {
	log.Printf("Trying to lock")
	if _, ok := s[tag]; !ok {
		log.Printf("Created new tag %s", tag)
		s[tag] = make(semaphore, 1)
	}

	ch := make(chan empty, 1)
	log.Printf("TRYING TO LOCK %s", tag)
	s[tag].P(n)
	ch <- empty{}
	log.Printf("LOCKED %s", tag)
	return ch
}

func (s store) V(tag string, n int) {
	if _, ok := s[tag]; !ok {
		s[tag] = make(semaphore, 1)
	}

	s[tag].V(n)
}

var semaphoreStore store = make(store)

func handler(w http.ResponseWriter, r *http.Request) {
	action := r.URL.Query().Get("action")
	tag := r.URL.Query().Get("tag")

	log.Printf("Found tag %s and action %s", tag, action)

	timeout := time.NewTimer(10 * time.Second)

	switch action {
	case "lock":
		select {
		case <-semaphoreStore.P(tag, 1):
			log.Printf("GOT A LOCK")
			w.WriteHeader(200)
			return
		case <-timeout.C:
			log.Printf("TIMED OUT WAITING FOR LOCK")
			w.WriteHeader(403)
			return
		}

	case "unlock":
		semaphoreStore.V(tag, 1)
		log.Printf("UNLOCKED")
		w.WriteHeader(200)
		return
	}

	log.Printf("No case was found")
	w.WriteHeader(500)
	return
}

func main() {
	log.Printf("Starting semaphore server")

	http.HandleFunc("/", handler)
	log.Fatal(http.ListenAndServe(":8080", nil))
}

servaphore_test.go

package main

// :(