Go并发编程小测验：你能答对几道题？

以下是12个Go并发编程的小测验,看看你能答对几道题？ (持续增加中)

1 Mutex

package main
import (
	"fmt"
	"sync"
)
var mu sync.Mutex
var chain string
func main() {
	chain = "main"
	A()
	fmt.Println(chain)
}
func A() {
	mu.Lock()
	defer mu.Unlock()
	chain = chain + " --> A"
	B()
}
func B() {
	chain = chain + " --> B"
	C()
}
func C() {
	mu.Lock()
	defer mu.Unlock()
	chain = chain + " --> C"
}

A: 不能编译
B: 输出 main --> A --> B --> C
C: 输出 main
D: panic

2 RWMutex

package main
import (
	"fmt"
	"sync"
	"time"
)
var mu sync.RWMutex
var count int
func main() {
	go A()
	time.Sleep(2 * time.Second)
	mu.Lock()
	defer mu.Unlock()
	count++
	fmt.Println(count)
}
func A() {
	mu.RLock()
	defer mu.RUnlock()
	B()
}
func B() {
	time.Sleep(5 * time.Second)
	C()
}
func C() {
	mu.RLock()
	defer mu.RUnlock()
}

A: 不能编译
B: 输出 1
C: 程序hang住
D: panic

3 Waitgroup

package main
import (
	"sync"
	"time"
)
func main() {
	var wg sync.WaitGroup
	wg.Add(1)
	go func() {
		time.Sleep(time.Millisecond)
		wg.Done()
		wg.Add(1)
	}()
	wg.Wait()
}

A: 不能编译
B: 无输出，正常退出
C: 程序hang住
D: panic

4 双检查实现单例

package doublecheck
import (
	"sync"
)
type Once struct {
	m    sync.Mutex
	done uint32
}
func (o *Once) Do(f func()) {
	if o.done == 1 {
		return
	}
	o.m.Lock()
	defer o.m.Unlock()
	if o.done == 0 {
		o.done = 1
		f()
	}
}

A: 不能编译
B: 可以编译，正确实现了单例
C: 可以编译，不正确的单例实现
D: 可以编译，但是程序运行会panic

5 Mutex

package main
import (
	"fmt"
	"sync"
)
type MyMutex struct {
	count int
	sync.Mutex
}
func main() {
	var mu MyMutex
	mu.Lock()
	var mu2 = mu
	mu.count++
	mu.Unlock()
	mu2.Lock()
	mu2.count++
	mu2.Unlock()
	fmt.Println(mu.count, mu2.count)
}

A: 不能编译
B: 输出 1, 1
C: 输出 1, 2
D: panic

6 Pool

package main
import (
	"bytes"
	"fmt"
	"runtime"
	"sync"
	"time"
)
var pool = sync.Pool{New: func() interface{} { return new(bytes.Buffer) }}
func main() {
	go func() {
		for {
			processRequest(1 << 28) // 256MiB
		}
	}()
	for i := 0; i < 1000; i++ {
		go func() {
			for {
				processRequest(1 << 10) // 1KiB
			}
		}()
	}
	var stats runtime.MemStats
	for i := 0; ; i++ {
		runtime.ReadMemStats(&stats)
		fmt.Printf("Cycle %d: %dB\n", i, stats.Alloc)
		time.Sleep(time.Second)
		runtime.GC()
	}
}
func processRequest(size int) {
	b := pool.Get().(*bytes.Buffer)
	time.Sleep(500 * time.Millisecond)
	b.Grow(size)
	pool.Put(b)
	time.Sleep(1 * time.Millisecond)
}

A: 不能编译
B: 可以编译，运行时正常，内存稳定
C: 可以编译，运行时内存可能暴涨
D: 可以编译，运行时内存先暴涨，但是过一会会回收掉

7 channel

package main
import (
	"fmt"
	"runtime"
	"time"
)
func main() {
	var ch chan int
	go func() {
		ch = make(chan int, 1)
		ch <- 1
	}()
	go func(ch chan int) {
		time.Sleep(time.Second)
		<-ch
	}(ch)
	c := time.Tick(1 * time.Second)
	for range c {
		fmt.Printf("#goroutines: %d\n", runtime.NumGoroutine())
	}
}

A: 不能编译
B: 一段时间后总是输出 #goroutines: 1
C: 一段时间后总是输出 #goroutines: 2
D: panic

8 channel

package main
import "fmt"
func main() {
	var ch chan int
	var count int
	go func() {
		ch <- 1
	}()
	go func() {
		count++
		close(ch)
	}()
	<-ch
	fmt.Println(count)
}

A: 不能编译
B: 输出 1
C: 输出 0
D: panic

9 Map

package main
import (
	"fmt"
	"sync"
)
func main() {
	var m sync.Map
	m.LoadOrStore("a", 1)
	m.Delete("a")
	fmt.Println(m.Len())
}

A: 不能编译
B: 输出 1
C: 输出 0
D: panic

10 happens before

package main
var c = make(chan int)
var a int
func f() {
	a = 1
	<-c
}
func main() {
	go f()
	c <- 0
	print(a)
}

A: 不能编译
B: 输出 1
C: 输出 0
D: panic

11 自定义Map

package main
import "sync"
type Map struct {
	m map[int]int
	sync.Mutex
}
func (m *Map) Get(key int) (int, bool) {
	m.Lock()
	defer m.Unlock()
	i, ok := m.m[key]
	return i, ok
}
func (m *Map) Put(key, value int) {
	m.Lock()
	defer m.Unlock()
	m.m[key] = value
}
func (m *Map) Len() int {
	return len(m.m)
}
func main() {
	var wg sync.WaitGroup
	wg.Add(2)
	m := Map{m: make(map[int]int)}
	go func() {
		for i := 0; i < 10000000; i++ {
			m.Put(i, i)
		}
		wg.Done()
	}()
	go func() {
		for i := 0; i < 10000000; i++ {
			m.Len()
		}
		wg.Done()
	}()
	wg.Wait()
}

go run quiz.go运行。

A: 不能编译
B: 可运行，无并发问题
C: 可运行，有并发问题
D: panic

12 slice

package main
import (
	"fmt"
	"sync"
)
func main() {
	var wg sync.WaitGroup
	wg.Add(2)
	var ints = make([]int, 0, 1000)
	go func() {
		for i := 0; i < 1000; i++ {
			ints = append(ints, i)
		}
		wg.Done()
	}()
	go func() {
		for i := 0; i < 1000; i++ {
			ints = append(ints, i)
		}
		wg.Done()
	}()
	wg.Wait()
	fmt.Println(len(ints))
}

A: 不能编译
B: 输出2000
C: 输出可能不是2000
D: panic

13 goroutine

package main
import (
	"fmt"
	"sync"
	"time"
)
type T struct {
	V int
}
func (t *T) Incr(wg *sync.WaitGroup) {
	t.V++
	wg.Done()
}
func (t *T) Print() {
	time.Sleep(1e9)
	fmt.Print(t.V)
}
func main() {
	var wg sync.WaitGroup
	wg.Add(10)
	var ts = make([]T, 10)
	for i := 0; i < 10; i++ {
		ts[i] = T{i}
	}
	for _, t := range ts {
		go t.Incr(&wg)
	}
	wg.Wait()
	for _, t := range ts {
		go t.Print()
	}
	time.Sleep(5 * time.Second)
}

A: 输出12345678910
B: 输出0123456789
C: 输出9999999999
D: 输出10101010101010101010