Golang 基础：底层并发原语 Mutex RWMutex Cond WaitGroup Once等使用和基本实现

// Package sync provides basic synchronization primitives such as mutual
// exclusion locks. Other than the Once and WaitGroup types, most are intended
// for use by low-level library routines. Higher-level synchronization is
// better done via channels and communication.
//
// Values containing the types defined in this package should not be copied.

    mu := sync.Mutex{}
    mu.Lock()       //加互斥锁
    mu.Unlock()

// A Mutex is a mutual exclusion lock.
// The zero value for a Mutex is an unlocked mutex.
//
// A Mutex must not be copied after first use.
type Mutex struct {
    state int32
    sema  uint32
}

var num int = 1
func testCopyMutex() {
    mu := sync.Mutex{}
    waitGroup := sync.WaitGroup{}

    waitGroup.Add(1)

    go func(copyMu sync.Mutex) {
        copyMu.Lock()
        num = 100
        fmt.Println("update num from sub-goroutine: ", num)
        time.Sleep(5 * time.Second)
        fmt.Println("read num from sub-goroutine: ", num)
        copyMu.Unlock()
        waitGroup.Done()
    }(mu)

    time.Sleep(time.Second)

    mu.Lock()
    num = 1
    fmt.Println("read num from main: ", num)
    mu.Unlock()

    waitGroup.Wait()
}

update num from sub-goroutine:  100
read num from main:  1
read num from sub-goroutine:  1

    go func(copyMu *sync.Mutex) {   //参数类型加 *
        copyMu.Lock()
        num = 100
        fmt.Println("update num from sub-goroutine: ", num)
        time.Sleep(5 * time.Second)
        fmt.Println("read num from sub-goroutine: ", num)
        copyMu.Unlock()
        waitGroup.Done()
    }(&mu)  //传递指针

// Lock locks m.
// If the lock is already in use, the calling goroutine
// blocks until the mutex is available.
func (m *Mutex) Lock() {
    // Fast path: grab unlocked mutex.
    if atomic.CompareAndSwapInt32(&m.state, 0, mutexLocked) {
        if race.Enabled {
            race.Acquire(unsafe.Pointer(m))
        }
        return
    }
    // Slow path (outlined so that the fast path can be inlined)
    m.lockSlow()
}

    rmu := sync.RWMutex{}
    rmu.RLock()     //读锁
    rmu.RUnlock()
    rmu.Lock()      //写锁
    rmu.Unlock()

    rmu.RLocker().Lock()        //通过 RLock 实现
    rmu.RLocker().Unlock()

// A RWMutex is a reader/writer mutual exclusion lock.
// The lock can be held by an arbitrary number of readers or a single writer.
// The zero value for a RWMutex is an unlocked mutex.
//
// A RWMutex must not be copied after first use.
//
// If a goroutine holds a RWMutex for reading and another goroutine might
// call Lock, no goroutine should expect to be able to acquire a read lock
// until the initial read lock is released. In particular, this prohibits
// recursive read locking. This is to ensure that the lock eventually becomes
// available; a blocked Lock call excludes new readers from acquiring the
// lock.
type RWMutex struct {
    w           Mutex  // held if there are pending writers
    writerSem   uint32 // semaphore for writers to wait for completing readers
    readerSem   uint32 // semaphore for readers to wait for completing writers
    readerCount int32  // number of pending readers
    readerWait  int32  // number of departing readers
}

// Lock locks rw for writing.
// If the lock is already locked for reading or writing,
// Lock blocks until the lock is available.
func (rw *RWMutex) Lock() {
    if race.Enabled {
        _ = rw.w.state
        race.Disable()
    }
    // First, resolve competition with other writers.
    rw.w.Lock()
    // Announce to readers there is a pending writer.
    r := atomic.AddInt32(&rw.readerCount, -rwmutexMaxReaders) + rwmutexMaxReaders
    // Wait for active readers.
    if r != 0 && atomic.AddInt32(&rw.readerWait, r) != 0 {
        runtime_SemacquireMutex(&rw.writerSem, false, 0)
    }
    if race.Enabled {
        race.Enable()
        race.Acquire(unsafe.Pointer(&rw.readerSem))
        race.Acquire(unsafe.Pointer(&rw.writerSem))
    }
}

    cond := sync.NewCond(&sync.Mutex{}) 参数为 sync.Locker 接口类型

    go func() {
        //    cond.L.Lock()
        //    for !condition() {
               cond.Wait()      //等待，一般放在循环里，查询一次，不满足就阻塞（释放锁），等被唤醒后，再检查下条件
        //    }
        //    ... make use of condition ...
        //    cond.L.Unlock()
    }()

    cond.L.Lock()   //获取构造传入的锁
    cond.Broadcast()    //通知所有等待的 goroutine，从 Wait 返回，重新获取锁
    cond.Signal()   //通知一个
    cond.L.Unlock()

// Cond implements a condition variable, a rendezvous point
// for goroutines waiting for or announcing the occurrence
// of an event.
//
// Each Cond has an associated Locker L (often a *Mutex or *RWMutex),
// which must be held when changing the condition and
// when calling the Wait method.
//
// A Cond must not be copied after first use.
type Cond struct {
    noCopy noCopy

    // L is held while observing or changing the condition
    L Locker

    notify  notifyList
    checker copyChecker
}

// NewCond returns a new Cond with Locker l.
func NewCond(l Locker) *Cond {
    return &Cond{L: l}
}

func (c *Cond) Wait() {
    c.checker.check()
    t := runtime_notifyListAdd(&c.notify)
    c.L.Unlock()
    runtime_notifyListWait(&c.notify, t)
    c.L.Lock()
}

func (c *Cond) Broadcast() {
    c.checker.check()
    runtime_notifyListNotifyAll(&c.notify)
}

    waitGroup := sync.WaitGroup{}
    waitGroup.Add(1)        //需要等待数为 1
    go func() {
        waitGroup.Done()    //减去需要等待数
    }()

    waitGroup.Wait()    //等待数为 0 才继续执行，循环检查 

type WaitGroup struct {
    noCopy noCopy

    // 64-bit value: high 32 bits are counter, low 32 bits are waiter count.
    // 64-bit atomic operations require 64-bit alignment, but 32-bit
    // compilers do not ensure it. So we allocate 12 bytes and then use
    // the aligned 8 bytes in them as state, and the other 4 as storage
    // for the sema.
    state1 [3]uint32
}

// Wait blocks until the WaitGroup counter is zero.
func (wg *WaitGroup) Wait() {
    statep, semap := wg.state()
    if race.Enabled {
        _ = *statep // trigger nil deref early
        race.Disable()
    }
    for {
        state := atomic.LoadUint64(statep)
        v := int32(state >> 32)
        w := uint32(state)
        if v == 0 {
            // Counter is 0, no need to wait.
            if race.Enabled {
                race.Enable()
                race.Acquire(unsafe.Pointer(wg))
            }
            return
        }
        // Increment waiters count.
        //...
    }
}

    once := sync.Once{}
    once.Do(func() {
        fmt.Println("do the work that only need exec once")
    })

type Once struct {
    // done indicates whether the action has been performed.
    // It is first in the struct because it is used in the hot path.
    // The hot path is inlined at every call site.
    // Placing done first allows more compact instructions on some architectures (amd64/386),
    // and fewer instructions (to calculate offset) on other architectures.
    done uint32
    m    Mutex
}

func (o *Once) Do(f func()) {
    // Note: Here is an incorrect implementation of Do:
    //
    //  if atomic.CompareAndSwapUint32(&o.done, 0, 1) {
    //      f()
    //  }
    //
    // Do guarantees that when it returns, f has finished.
    // This implementation would not implement that guarantee:
    // given two simultaneous calls, the winner of the cas would
    // call f, and the second would return immediately, without
    // waiting for the first's call to f to complete.
    // This is why the slow path falls back to a mutex, and why
    // the atomic.StoreUint32 must be delayed until after f returns.

    if atomic.LoadUint32(&o.done) == 0 {
        // Outlined slow-path to allow inlining of the fast-path.
        o.doSlow(f)
    }
}
func (o *Once) doSlow(f func()) {
    o.m.Lock()
    defer o.m.Unlock()
    if o.done == 0 {
        defer atomic.StoreUint32(&o.done, 1)
        f()
    }
}

        state := atomic.LoadUint64(statep)
        v := int32(state >> 32)
        w := uint32(state)