Concurrency Synchronization Techniques Provided in the sync Standard Package

Each sync.WaitGroup value maintains a counter internally. The initial value of the counter is zero.

The *WaitGroup type has three methods: Add(delta int), Done() and Wait().

For an addressable WaitGroup value wg,

• we can call the wg.Add(delta) method to change the counter value maintained by wg.
• the method call wg.Done() is totally equivalent to the method call wg.Add(-1).
• if a call wg.Add(delta) (or wg.Done()) modifies the counter maintained by wg to negative, panic will happen.
• when a goroutine calls wg.Wait(),
  • if the counter maintained by wg is already zero, then the call wg.Wait() can be viewed as a no-op.
  • otherwise (the counter is positive), the goroutine will enter blocking state. It will enter running state again (a.k.a., the call wg.Wait() returns) when another goroutine modifies the counter to zero, generally by calling wg.Done().

Please note that wg.Add(delta), wg.Done() and wg.Wait() are shorthands of (&wg).Add(delta), (&wg).Done() and (&wg).Wait(), respectively.

Generally, a WaitGroup value is used for the scenario that one goroutine waits until all of several other goroutines finish their respective jobs. An example:

package main

import (
	"log"
	"math/rand"
	"sync"
	"time"
)

func main() {
	rand.Seed(time.Now().UnixNano()) // needed before Go 1.20

	const N = 5
	var values [N]int32

	var wg sync.WaitGroup
	wg.Add(N)
	for i := 0; i < N; i++ {
		i := i
		go func() {
			values[i] = 50 + rand.Int31n(50)
			log.Println("Done:", i)
			wg.Done() // <=> wg.Add(-1)
		}()
	}

	wg.Wait()
	// All elements are guaranteed to be
	// initialized now.
	log.Println("values:", values)
}

In the above example, the main goroutine waits until all other N goroutines have populated their respective element value in values array. Here is one possible output result:

Done: 4
Done: 1
Done: 3
Done: 0
Done: 2
values: [71 89 50 62 60]

We can split the only Add method call in the above example into multiple ones.

...
	var wg sync.WaitGroup
	for i := 0; i < N; i++ {
		wg.Add(1) // will be invoked N times
		i := i
		go func() {
			values[i] = 50 + rand.Int31n(50)
			wg.Done()
		}()
	}
...

The Wait method can be called in multiple goroutines. When the counter becomes zero, all of them will be notified, in a broadcast way.

func main() {
	rand.Seed(time.Now().UnixNano()) // needed before Go 1.20

	const N = 5
	var values [N]int32

	var wgA, wgB sync.WaitGroup
	wgA.Add(N)
	wgB.Add(1)

	for i := 0; i < N; i++ {
		i := i
		go func() {
			wgB.Wait() // wait a notification
			log.Printf("values[%v]=%v \n", i, values[i])
			wgA.Done()
		}()
	}

	// The loop is guaranteed to finish before
	// any above wg.Wait calls returns.
	for i := 0; i < N; i++ {
		values[i] = 50 + rand.Int31n(50)
	}
	// Make a broadcast notification.
	wgB.Done()
	wgA.Wait()
}

A WaitGroup value can be reused after one call to its Wait method returns. But please note that each Add method call with a positive delta that occurs when the counter is zero must happen before any Wait call starts, otherwise, data races may happen.

A *sync.Once value has a Do(f func()) method, which takes a solo parameter with type func().

For an addressable Once value o, the method call o.Do(), which is a shorthand of (&o).Do(), can be concurrently executed multiple times, in multiple goroutines. The arguments of these o.Do() calls should (but are not required to) be the same function value.

Among these o.Do method calls, only exact one argument function will be invoked. The invoked argument function is guaranteed to exit before any o.Do method call returns. In other words, the code in the invoked argument function is guaranteed to be executed before any o.Do method call returns.

Generally, a Once value is used to ensure that a piece of code will be executed exactly once in concurrent programming.

An example:

package main

import (
	"log"
	"sync"
)

func main() {
	log.SetFlags(0)

	x := 0
	doSomething := func() {
		x++
		log.Println("Hello")
	}

	var wg sync.WaitGroup
	var once sync.Once
	for i := 0; i < 5; i++ {
		wg.Add(1)
		go func() {
			defer wg.Done()
			once.Do(doSomething)
			log.Println("world!")
		}()
	}

	wg.Wait()
	log.Println("x =", x) // x = 1
}

In the above example, Hello will be printed once, but world! will be printed five times. And Hello is guaranteed to be printed before all five world!.

Both of the *sync.Mutex and *sync.RWMutex types implement the sync.Locker interface. So they both have two methods, Lock and Unlock, to prevent multiple data users from using a piece of data concurrently.

Besides the Lock and Unlock methods, the *RWMutex type has two other methods, RLock and RUnlock, to avoid some data users (either writers or readers) and one data writer using a piece of data at the same time but allow some data readers to access the piece of data at the same time.

(Note, here the terminologies data reader and data writer should not be interpreted from literal. They are just used for explanation purpose. A data reader might modify data and a data writer might only read data.)

A Mutex value is often called a mutual exclusion lock. A zero Mutex value is an unlocked mutex. A Mutex value can only be locked when it is in unlocked status. In other words, once an addressable Mutex value m is locked successfully (a.k.a., a m.Lock() method call returns), a new attempt by a goroutine to lock the Mutex value will make the goroutine enter blocking state, until the Mutex value is unlocked (through a later m.Unlock() call).

Please note that m.Lock() and m.Unlock() are shorthands of (&m).Lock() and (&m).Unlock(), respectively.

An example of using sync.Mutex:

package main

import (
	"fmt"
	"runtime"
	"sync"
)

type Counter struct {
	m sync.Mutex
	n uint64
}

func (c *Counter) Value() uint64 {
	c.m.Lock()
	defer c.m.Unlock()
	return c.n
}

func (c *Counter) Increase(delta uint64) {
	c.m.Lock()
	c.n += delta
	c.m.Unlock()
}

func main() {
	var c Counter
	for i := 0; i < 100; i++ {
		go func() {
			for k := 0; k < 100; k++ {
				c.Increase(1)
			}
		}()
	}

	// The loop is just for demo purpose.
	for c.Value() < 10000 {
		runtime.Gosched()
	}
	fmt.Println(c.Value()) // 10000
}

In the above example, a Counter value uses a Mutex field to guarantee that the n field of the Counter value will be never used by multiple goroutines at the same time.

A RWMutex value is often called a reader+writer mutual exclusion lock. It has two locks, the write lock and the read lock. The locks of a zero RWMutex value are both unlocked. For an addressable RWMutex value rwm, data writers can lock the write lock of rwm through rwm.Lock() method calls, and data readers can lock the read lock of rwm through rwm.RLock() method calls. Method calls rwm.Unlock() and rwm.RUnlock() are used to unlock the write and read locks of rwm. The read lock of rwm maintains a lock count, which increases by one when rwm.Lock() is called successfully and decreases by one when rwm.Unlock() is called successfully. A zero lock count means the read lock is in unlocked status and a non-zero one (must be larger than one) means the read lock is locked.

Please note that rwm.Lock(), rwm.Unlock(), rwm.RLock() and rwm.RUnlock() are shorthands of (&m).Lock(), (&m).Unlock(), (&m).RLock() and (&m).RUnlock(), respectively.

For an addressable RWMutex value rwm, the following rules exist.

• The write lock of rwm may be locked only if neither of the read lock and write lock of rwm is in locked status. In other words, the write lock of rwm may only be held by at most one writer at any given time, and the read lock and write lock of rwm may not be held at the same time.
• When the write lock of rwm is in locked status, any newer attempts to lock the write lock or the read lock of rwm will be blocked until the initial write lock is unlocked.
• When the read lock of rwm is in locked status, any newer attempts to lock the write lock will be blocked until the read lock is unlocked. However, newer attempts to lock the read lock will succeed as long as the attempts are performed before any blocked attempts to lock the write lock (see the next rule for details). In other words, the read lock may be held by more than one readers at a time. The read lock will return to unlocked status when its lock count returns to zero.
• Assume the read lock of the value rwm is in locked status now, to avoid endless read locking, any newer attempts to lock the read lock after the a being blocked attempt to lock the write lock will be blocked.
• Assume the write lock of the value rwm is in locked status now, for the official standard Go compiler, to avoid endless write locking, the attempts to lock the read lock before releasing the write lock will succeed for sure once the write lock is unlocked, even if some of the attempts are made after some still being blocked attempts to lock the write lock.

The last two rules are to ensure both readers and writers have chances to acquire locks.

Please note, locks are not bound to goroutines. A lock may be locked in one goroutine and unlocked in another one later. In other words, a lock doesn't know which goroutine successfully locked or unlocked it.

The type of the m field of the Counter type in the last example can be changed to sync.RWMutex, as the following code shows, to get a better performance when the Value method is called very frequently but the Increase method is called not frequently.

...
type Counter struct {
	//m sync.Mutex
	m sync.RWMutex
	n uint64
}

func (c *Counter) Value() uint64 {
	//c.m.Lock()
	//defer c.m.Unlock()
	c.m.RLock()
	defer c.m.RUnlock()
	return c.n
}
...

Another use scenario of sync.RWMutex values is to slice a write job into several small ones. Please read the next section for an example.

By the last two rules mentioned above, the following program is very possible to output abdc.

package main

import (
	"fmt"
	"time"
	"sync"
)

func main() {
	var m sync.RWMutex
	go func() {
		m.RLock()
		fmt.Print("a")
		time.Sleep(time.Second)
		m.RUnlock()
	}()
	go func() {
		time.Sleep(time.Second * 1 / 4)
		m.Lock()
		fmt.Print("b")
		time.Sleep(time.Second)
		m.Unlock()
	}()
	go func() {
		time.Sleep(time.Second * 2 / 4)
		m.Lock()
		fmt.Print("c")
		m.Unlock()
	}()
	go func () {
		time.Sleep(time.Second * 3 / 4)
		m.RLock()
		fmt.Print("d")
		m.RUnlock()
	}()
	time.Sleep(time.Second * 3)
	fmt.Println()
}

Please note, the above example is only for explanation purpose. It uses time.Sleep calls to do concurrency synchronizations, which is a bad practice for production code.

sync.Mutex and sync.RWMutex values can also be used to make notifications, though there are many other better ways to do the same job. Here is an example which makes a notification by using a sync.Mutex value.

package main

import (
	"fmt"
	"sync"
	"time"
)

func main() {
	var m sync.Mutex
	m.Lock()
	go func() {
		time.Sleep(time.Second)
		fmt.Println("Hi")
		m.Unlock() // make a notification
	}()
	m.Lock() // wait to be notified
	fmt.Println("Bye")
}

In the above example, the text Hi is guaranteed to be printed before the text Bye. About the memory order guarantees made by sync.Mutex and sync.RWMutex values, please read memory order guarantees in Go.

The sync.Cond type provides an efficient way to do notifications among goroutines.

Each sync.Cond value holds a sync.Locker field with name L. The field value is often a value of type *sync.Mutex or *sync.RWMutex.

The *sync.Cond type has three methods, Wait(), Signal() and Broadcast().

Each sync.Cond value also maintains a FIFO (first in first out) waiting goroutine queue. For an addressable sync.Cond value c,

• c.Wait() must be called when c.L is locked, otherwise, a c.Wait() will cause panic. A c.Wait() call will
  1. first push the current caller goroutine into the waiting goroutine queue maintained by c,
  2. then call c.L.Unlock() to unlock/unhold the lock c.L.

  3. then make the current caller goroutine enter blocking state.

     (The caller goroutine will be unblocked by another goroutine through calling c.Signal() or c.Broadcast() later.)

     Once the caller goroutine is unblocked and enters running state again, c.L.Lock() will be called (in the resumed c.Wait() call) to try to lock and hold the lock c.L again, The c.Wait() call will exit after the c.L.Lock() call returns.
• a c.Signal() call will unblock the first goroutine in (and remove it from) the waiting goroutine queue maintained by c, if the queue is not empty.
• a c.Broadcast() call will unblock all the goroutines in (and remove them from) the waiting goroutine queue maintained by c, if the queue is not empty.

Please note that c.Wait(), c.Signal() and c.Broadcast() are shorthands of (&c).Wait(), (&c).Signal() and (&c).Broadcast(), respectively.

c.Signal() and c.Broadcast() are often used to notify the status of a condition is changed, Generally, c.Wait() should be called in a loop of checking whether or not a condition has got satisfied.

In an idiomatic sync.Cond use case, generally, one goroutine waits for changes of a certain condition, and some other goroutines change the condition and send notifications. Here is an example:

package main

import (
	"fmt"
	"math/rand"
	"sync"
	"time"
)

func main() {
	rand.Seed(time.Now().UnixNano()) // needed before Go 1.20

	const N = 10
	var values [N]string

	cond := sync.NewCond(&sync.Mutex{})

	for i := 0; i < N; i++ {
		d := time.Second * time.Duration(rand.Intn(10)) / 10
		go func(i int) {
			time.Sleep(d) // simulate a workload

			// Changes must be made when
			// cond.L is locked.
			cond.L.Lock()
			values[i] = string('a' + i)

			// Notify when cond.L lock is locked.
			cond.Broadcast()
			cond.L.Unlock()

			// "cond.Broadcast()" can also be put
			// here, when cond.L lock is unlocked.
			//cond.Broadcast()
		}(i)
	}

	// This function must be called when
	// cond.L is locked.
	checkCondition := func() bool {
		fmt.Println(values)
		for i := 0; i < N; i++ {
			if values[i] == "" {
				return false
			}
		}
		return true
	}

	cond.L.Lock()
	defer cond.L.Unlock()
	for !checkCondition() {
		// Must be called when cond.L is locked.
		cond.Wait()
	}
}

One possible output:

[         ]
[     f    ]
[  c   f    ]
[  c   f  h  ]
[ b c   f  h  ]
[a b c   f  h  j]
[a b c   f g h i j]
[a b c  e f g h i j]
[a b c d e f g h i j]

For there is only one goroutine (the main goroutine) waiting to be unblocked in this example, the cond.Broadcast() call can be replaced with cond.Signal(). As the comments suggest, cond.Broadcast() and cond.Signal() are not required to be called when cond.L is locked.

To avoid data races, each of the ten parts of the user defined condition should only be modified when cond.L is locked. The checkCondition function and the cond.Wait method should be also called when cond.L is locked.

In fact, for the above specified example, the cond.L field can also be a *sync.RWMutex value, and each of the ten parts of the user defined condition can be modified when the read lock of cond.L is held, just as the following code shows:

...
	cond := sync.NewCond(&sync.RWMutex{})
	cond.L.Lock()

	for i := 0; i < N; i++ {
		d := time.Second * time.Duration(rand.Intn(10)) / 10
		go func(i int) {
			time.Sleep(d)
			cond.L.(*sync.RWMutex).RLock()
			values[i] = string('a' + i)
			cond.L.(*sync.RWMutex).RUnlock()
			cond.Signal()
		}(i)
	}
...

In the above example, the sync.RWMutex value is used unusually. Its read lock is held by some goroutines which modify array elements, and its write lock is used by the main goroutine to read array elements.

The user defined condition monitored by a Cond value can be a void. For such cases, the Cond value is used for notifications purely. For example, the following program will print abc or bac.

package main

import (
	"fmt"
	"sync"
)

func main() {
	wg := sync.WaitGroup{}
	wg.Add(1)
	cond := sync.NewCond(&sync.Mutex{})
	cond.L.Lock()
	go func() {
		cond.L.Lock()
		go func() {
			cond.L.Lock()
			cond.Broadcast()
			cond.L.Unlock()
		}()
		cond.Wait()
		fmt.Print("a")
		cond.L.Unlock()
		wg.Done()
	}()
	cond.Wait()
	fmt.Print("b")
	cond.L.Unlock()
	wg.Wait()
	fmt.Println("c")
}

If it needs, multiple sync.Cond values can share the same sync.Locker. However, such cases are rare in practice.

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