// Copyright 2014 Google Inc. All rights reserved.

// Use of this source code is governed by the Apache 2.0

// license that can be found in the LICENSE file.

package internal

// This file implements a network dialer that limits the number of concurrent connections.

// It is only used for API calls.

import (

	"log"

	"net"

	"runtime"

	"sync"

	"time"

)

var limitSem = make(chan int, 100) // TODO(dsymonds): Use environment variable.

func limitRelease() {

	// non-blocking

	select {

	case <-limitSem:

	default:

		// This should not normally happen.

		log.Print("appengine: unbalanced limitSem release!")

	}

}

func limitDial(network, addr string) (net.Conn, error) {

	limitSem <- 1

	// Dial with a timeout in case the API host is MIA.

	// The connection should normally be very fast.

	if err != nil {

		limitRelease()

		return nil, err

	}

	lc := &limitConn{Conn: conn}

	runtime.SetFinalizer(lc, (*limitConn).Close) // shouldn't usually be required

	return lc, nil

}

type limitConn struct {

	close sync.Once

	net.Conn

}

func (lc *limitConn) Close() error {

	defer lc.close.Do(func() {

		limitRelease()

		runtime.SetFinalizer(lc, nil)

	})

	return lc.Conn.Close()

}