這個Golang程式中的同步問題

php小編蘋果今天為大家介紹一個有趣的主題—"這個Golang程式中的同步問題"。在編寫並發程式時，我們經常會遇到同步問題，即多個執行緒之間的競爭和協調。 Golang作為一門同時進行程式語言，提供了豐富的同步機制和工具，但也存在一些常見的同步問題需要我們注意和解決。本文將詳細探討這些問題，並給予對應的解決方案，幫助大家更能理解並應對Golang中的同步挑戰。讓我們一起來探索吧！

問題內容

我正在嘗試建立一個充當代理伺服器並且可以動態切換到新端點的程式。但我遇到一個問題，在呼叫 switchovertonewendpoint() 後，仍然有一些代理物件連接到原始端點 8.8.8.8 ，應該將其關閉。

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)
    }
}

想了一會兒，我猜問題出在

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

但我不確定這是否是根本原因，以及是否可以透過將其替換為以下內容來修復它：

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

由於這個案例很難重現，所以有專業人士可以提供想法或提示嗎？歡迎任何評論。提前致謝。

解決方法

問題

改變是必要的。在最初的實作中， latedproxies 持有相同的映射。請參閱此演示：

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
}

但這不是根本原因。可以在複製 closeproxies 之後但在呼叫 setswitchover 之前新增代理。在這種情況下，新代理連接到舊位址，但不在 closeproxies 中。我認為這是根本原因。

還有一個問題。在設定 to 欄位之前，將向 proxies 新增代理程式。程式可能希望在設定 to 欄位之前關閉此代理，從而導致恐慌。

可靠的設計

這個想法是將所有端點放入一個切片中，並讓每個端點管理自己的代理清單。所以我們只需要追蹤當前端點的索引。當我們想要切換到另一個端點時，我們只需要更改索引，並告訴過時的端點清除其代理程式。剩下的唯一複雜的事情是確保過時的端點可以清除其所有代理。請參閱下面的實作：

manager.go

這就是這個想法的實現。

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

這個示範展示如何使用先前實作的manager類型：

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

使用以下指令執行測試：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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