这个Golang程序中的同步问题

php小编苹果今天为大家介绍一个有趣的话题——"这个Golang程序中的同步问题"。在编写并发程序时，我们经常会遇到同步问题，即多个线程之间的竞争和协调。Golang作为一门并发编程语言，提供了丰富的同步机制和工具，但也存在一些常见的同步问题需要我们注意和解决。本文将详细探讨这些问题，并给出相应的解决方案，帮助大家更好地理解和应对Golang中的同步挑战。让我们一起来探索吧！

问题内容

我正在尝试创建一个充当代理服务器并可以动态切换到新端点的程序。但我遇到一个问题，在调用 switchovertonewendpoint() 后，仍然有一些代理对象连接到原始端点 8.8.8.8 ，应该将其关闭。

package main

import (
    "net"
    "sync"
    "sync/atomic"
    "time"
)

type proxy struct {
    id       int32
    from, to *net.tcpconn
}

var switchover int32 = 0

func setswitchover() {
    atomic.storeint32((*int32)(&switchover), 1)
}

func switchoverenabled() bool {
    return atomic.loadint32((*int32)(&switchover)) == 1
}

var proxies map[int32]*proxy = make(map[int32]*proxy, 0)
var proxyseq int32 = 0
var mu sync.rwmutex

func addproxy(from *net.tcpconn) {
    mu.lock()
    proxyseq += 1
    proxy := &proxy{id: proxyseq, from: from}
    proxies[proxyseq] = proxy
    mu.unlock()

    var toaddr string
    if switchoverenabled() {
        toaddr = "1.1.1.1"
    } else {
        toaddr = "8.8.8.8"
    }
    tcpaddr, _ := net.resolvetcpaddr("tcp4", toaddr)
    toconn, err := net.dialtcp("tcp", nil, tcpaddr)
    if err != nil {
        panic(err)
    }
    proxy.to = toconn
}

func switchovertonewendpoint() {
    mu.rlock()
    closedproxies := proxies
    mu.runlock()

    setswitchover()
    for _, proxy := range closedproxies {
        proxy.from.close()
        proxy.to.close()
        mu.lock()
        delete(proxies, proxy.id)
        mu.unlock()
    }
}

func main() {
    tcpaddr, _ := net.resolvetcpaddr("tcp4", "0.0.0.0:5432")
    ln, _ := net.listentcp("tcp", tcpaddr)
    go func() {
        time.sleep(time.second * 30)
        switchovertonewendpoint()
    }()
    for {
        clientconn, err := ln.accepttcp()
        if err != nil {
            panic(err)
        }
        go addproxy(clientconn)
    }
}

想了一会儿，我猜问题出在

mu.rlock()
    closedproxies := proxies
    mu.runlock()

但我不确定这是否是根本原因，以及是否可以通过将其替换为以下内容来修复它：

closedProxies := make([]*Proxy, 0)
    mu.RLock()
    for _, proxy := range proxies {
        closedProxies = append(closedProxies, proxy)
    }
    mu.RUnlock()

由于该案例很难重现，所以有专业人士可以提供想法或提示吗？欢迎任何评论。提前致谢。

解决方法

问题

改变是必要的。在最初的实现中， latedproxies 持有相同的映射。请参阅此演示：

package main

import "fmt"

func main() {
    proxies := make(map[int]int, 0)
    for i := 0; i < 10; i++ {
        proxies[i] = i
    }

    closeproxies := proxies

    proxies[10] = 10
    proxies[11] = 11

    for k := range closeproxies {
        delete(proxies, k)
    }

    fmt.printf("items left: %d\n", len(proxies))
    // output:
    //   items left: 0
}

但这不是根本原因。可以在复制 closeproxies 之后但在调用 setswitchover 之前添加新代理。在这种情况下，新代理连接到旧地址，但不在 closeproxies 中。我认为这是根本原因。

还有一个问题。在设置 to 字段之前，将向 proxies 添加新代理。程序可能希望在设置 to 字段之前关闭此代理，从而导致恐慌。

可靠的设计

这个想法是将所有端点放入一个切片中，并让每个端点管理自己的代理列表。所以我们只需要跟踪当前端点的索引。当我们想要切换到另一个端点时，我们只需要更改索引，并告诉过时的端点清除其代理。剩下的唯一复杂的事情是确保过时的端点可以清除其所有代理。请参阅下面的实现：

manager.go

这就是这个想法的实现。

package main

import (
    "sync"
)

// conn is abstraction of a connection to make manager easy to test.
type conn interface {
    close() error
}

// dialer is abstraction of a dialer to make manager easy to test.
type dialer interface {
    dial(addr string) (conn, error)
}

type manager struct {
    // mucurrent protects the "current" member.
    mucurrent sync.rwmutex
    current   int // when current is -1, the manager is shuted down.
    endpoints []*endpoint

    // mu protects the whole switch action.
    mu sync.mutex
}

func newmanager(dialer dialer, addresses ...string) *manager {
    if len(addresses) < 2 {
        panic("a manger should handle at least 2 addresses")
    }

    endpoints := make([]*endpoint, len(addresses))
    for i, addr := range addresses {
        endpoints[i] = &endpoint{
            address: addr,
            dialer:  dialer,
        }
    }
    return &manager{
        endpoints: endpoints,
    }
}

func (m *manager) addproxy(from conn) {
    // 1. addproxy will wait when the write lock of m.mucurrent is taken.
    // once the write lock is released, addproxy will connect to the new endpoint.
    // switch only holds the write lock for a short time, and switch is called
    // not so frequently, so addproxy won't wait too much.
    // 2. switch will wait if there is any addproxy holding the read lock of
    // m.mucurrent. that means switch waits longer. the advantage is that when
    // e.clear is called in switch, all addproxy requests to the old endpoint
    // are done. so it's safe to call e.clear then.
    m.mucurrent.rlock()
    defer m.mucurrent.runlock()

    current := m.current

    // do not accept any new connection when m has been shutdown.
    if current == -1 {
        from.close()
        return
    }

    m.endpoints[current].addproxy(from)
}

func (m *manager) switch() {
    // in a real world, switch is called not so frequently.
    // so it's ok to add a lock here.
    // and it's necessary to make sure the old endpoint is cleared and ready
    // for use in the future.
    m.mu.lock()
    defer m.mu.unlock()

    // take the write lock of m.mucurrent.
    // it waits for all the addproxy requests holding the read lock to finish.
    m.mucurrent.lock()
    old := m.current

    // do nothing when m has been shutdown.
    if old == -1 {
        m.mucurrent.unlock()
        return
    }
    next := old + 1
    if next >= len(m.endpoints) {
        next = 0
    }
    m.current = next
    m.mucurrent.unlock()

    // when it reaches here, all addproxy requests to the old endpoint are done.
    // and it's safe to call e.clear now.
    m.endpoints[old].clear()
}

func (m *manager) shutdown() {
    m.mu.lock()
    defer m.mu.unlock()

    m.mucurrent.lock()
    current := m.current
    m.current = -1
    m.mucurrent.unlock()

    m.endpoints[current].clear()
}

type proxy struct {
    from, to conn
}

type endpoint struct {
    address string
    dialer  dialer

    mu      sync.mutex
    proxies []*proxy
}

func (e *endpoint) clear() {
    for _, p := range e.proxies {
        p.from.close()
        p.to.close()
    }

    // assign a new slice to e.proxies, and the gc will collect the old one.
    e.proxies = []*proxy{}
}

func (e *endpoint) addproxy(from conn) {
    toconn, err := e.dialer.dial(e.address)
    if err != nil {
        // close the from connection so that the client will reconnect?
        from.close()
        return
    }

    e.mu.lock()
    defer e.mu.unlock()
    e.proxies = append(e.proxies, &proxy{from: from, to: toconn})
}

main.go

这个演示展示了如何使用之前实现的manager类型：

package main

import (
    "net"
    "time"
)

type realdialer struct{}

func (d realdialer) dial(addr string) (conn, error) {
    tcpaddr, err := net.resolvetcpaddr("tcp4", addr)
    if err != nil {
        return nil, err
    }
    return net.dialtcp("tcp", nil, tcpaddr)
}

func main() {
    manager := newmanager(realdialer{}, "1.1.1.1", "8.8.8.8")

    tcpaddr, _ := net.resolvetcpaddr("tcp4", "0.0.0.0:5432")
    ln, _ := net.listentcp("tcp", tcpaddr)

    go func() {
        for range time.tick(30 * time.second) {
            manager.switch()
        }
    }()
    for {
        clientconn, err := ln.accepttcp()
        if err != nil {
            panic(err)
        }
        go manager.addproxy(clientconn)
    }
}

manager_test.go

使用以下命令运行测试：go test ./... -race -count 10

package main

import (
    "errors"
    "math/rand"
    "sync"
    "sync/atomic"
    "testing"
    "time"

    "github.com/google/uuid"
)

func TestManager(t *testing.T) {
    addresses := []string{"1.1.1.1", "8.8.8.8"}
    dialer := newDialer(addresses...)
    manager := NewManager(dialer, addresses...)

    ch := make(chan int, 1)

    var wg sync.WaitGroup
    wg.Add(1)
    go func() {
        for range ch {
            manager.Switch()
        }
        wg.Done()
    }()

    count := 1000
    total := count * 10

    wg.Add(total)

    fromConn := &fakeFromConn{}
    for i := 0; i < total; i++ {
        if i%count == count-1 {
            ch <- 0
        }
        go func() {
            manager.AddProxy(fromConn)
            wg.Done()
        }()
    }
    close(ch)

    wg.Wait()

    manager.Shutdown()

    for _, s := range dialer.servers {
        left := len(s.conns)
        if left != 0 {
            t.Errorf("server %s, unexpected connections left: %d", s.addr, left)
        }
    }

    closedCount := fromConn.closedCount.Load()

    if closedCount != int32(total) {
        t.Errorf("want closed count: %d, got: %d", total, closedCount)
    }
}

type fakeFromConn struct {
    closedCount atomic.Int32
}

func (c *fakeFromConn) Close() error {
    c.closedCount.Add(1)

    return nil
}

type fakeToConn struct {
    id     uuid.UUID
    server *fakeServer
}

func (c *fakeToConn) Close() error {
    if c.id == uuid.Nil {
        return nil
    }

    c.server.removeConn(c.id)

    return nil
}

type fakeServer struct {
    addr  string
    mu    sync.Mutex
    conns map[uuid.UUID]bool
}

func (s *fakeServer) addConn() (uuid.UUID, error) {
    s.mu.Lock()
    defer s.mu.Unlock()

    id, err := uuid.NewRandom()
    if err == nil {
        s.conns[id] = true
    }
    return id, err
}

func (s *fakeServer) removeConn(id uuid.UUID) {
    s.mu.Lock()
    defer s.mu.Unlock()

    delete(s.conns, id)
}

type fakeDialer struct {
    servers map[string]*fakeServer
}

func newDialer(addresses ...string) *fakeDialer {
    servers := make(map[string]*fakeServer)
    for _, addr := range addresses {
        servers[addr] = &fakeServer{
            addr:  addr,
            conns: make(map[uuid.UUID]bool),
        }
    }
    return &fakeDialer{
        servers: servers,
    }
}

func (d *fakeDialer) Dial(addr string) (Conn, error) {
    n := rand.Intn(100)
    if n == 0 {
        return nil, errors.New("fake network error")
    }
    // Simulate network latency.
    time.Sleep(time.Duration(n) * time.Millisecond)

    s := d.servers[addr]
    id, err := s.addConn()
    if err != nil {
        return nil, err
    }
    conn := &fakeToConn{
        id:     id,
        server: s,
    }
    return conn, nil
}

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