当我们在写go程序时可能会看到类似的提示：
call of xxx copies lock value: sync.WaitGroup contains sync.noCopy
在sync包的文档开始也进行了类似说明：
Values containing the types defined in this package should not be copied.
或者在读详细文档时，经常看到出现频率很高的一句：
must not be copied after first use
比如
A Mutex must not be copied after first use.
type Mutex struct {
// contains filtered or unexported fields
}
The zero Map is empty and ready for use. A Map must not be copied after first use.
type Map struct {
// contains filtered or unexported fields
}
A Cond must not be copied after first use.
type Cond struct {
// L is held while observing or changing the condition
L Locker
// contains filtered or unexported fields
}
A Builder is used to efficiently build a string using Write methods. It minimizes memory copying. The zero value is ready to use. Do not copy a non-zero Builder.
type Builder struct {
// contains filtered or unexported fields
}
爱问为什么的同学可能会问，为什么不让copy？
因为你copy一个Mutex的值没有任何意义，甚至会带来一些安全隐患。看下下面的代码：
type Temp struct {
lock sync.Mutex
}
func (t *Temp) Lock() {
t.lock.Lock()
}
func (t Temp) Unlock() {
t.lock.Unlock()
}
func main() {
t := Temp{lock: sync.Mutex{}}
t.Lock()
t.Unlock()
t.Lock()
}
运行这段代码会出现死锁，原因就是Unlock方法是值作为接收者，unlock的Mutex是副本Mutex。
所以有的时候，我们可能不想让某个类型被拷贝，只想通过指针来使用，例如你的结构体有pointer字段，你不想让这个结构被拷贝，原因是拷贝后的指针字段指向同一个内容，这样会存在不安全的场景。
那如何防止拷贝某个类型呢？有下面两种方式：
- 运行时检查
- 使用go vet
运行时检查
比如strings.Builder
// A Builder is used to efficiently build a string using Write methods.
// It minimizes memory copying. The zero value is ready to use.
// Do not copy a non-zero Builder.
type Builder struct {
addr *Builder // of receiver, to detect copies by value
buf []byte
}
......
// noescape hides a pointer from escape analysis. noescape is
// the identity function but escape analysis doesn't think the
// output depends on the input. noescape is inlined and currently
// compiles down to zero instructions.
// USE CAREFULLY!
// This was copied from the runtime; see issues 23382 and 7921.
//go:nosplit
func noescape(p unsafe.Pointer) unsafe.Pointer {
x := uintptr(p)
return unsafe.Pointer(x ^ 0)
}
func (b *Builder) copyCheck() {
if b.addr == nil {
// This hack works around a failing of Go's escape analysis
// that was causing b to escape and be heap allocated.
// See issue 23382.
// TODO: once issue 7921 is fixed, this should be reverted to
// just "b.addr = b".
b.addr = (*Builder)(noescape(unsafe.Pointer(b)))
} else if b.addr != b {
panic("strings: illegal use of non-zero Builder copied by value")
}
}
// WriteString appends the contents of s to b's buffer.
// It returns the length of s and a nil error.
func (b *Builder) WriteString(s string) (int, error) {
b.copyCheck()
b.buf = append(b.buf, s...)
return len(s), nil
}
// test.go
var b strings.Builder
for i := 3; i >= 1; i-- {
fmt.Fprintf(&b, "%d...", i)
}
b.WriteString("ignition")
fmt.Println(b.String())
a := b
a.WriteString("hello")
fmt.Println(a.String())
strings.Builder的作用是最小化内存拷贝，这里边涉及到逃逸分析，关于逃逸分析可参考这篇文章，通过内部的addr字段防止拷贝。如果拷贝之后调用WriteString，进入到copyCheck就会进入else分支，引发panic。sync.Cond也在运行时进行了检查：
type Cond struct {
noCopy noCopy
L Locker
notify notifyList
checker copyChecker
}
// Signal wakes one goroutine waiting on c, if there is any.
//
// It is allowed but not required for the caller to hold c.L
// during the call.
func (c *Cond) Signal() {
c.checker.check()
runtime_notifyListNotifyOne(&c.notify)
}
// Broadcast wakes all goroutines waiting on c.
//
// It is allowed but not required for the caller to hold c.L
// during the call.
func (c *Cond) Broadcast() {
c.checker.check()
runtime_notifyListNotifyAll(&c.notify)
}
type copyChecker uintptr
func (c *copyChecker) check() {
if uintptr(*c) != uintptr(unsafe.Pointer(c)) &&
!atomic.CompareAndSwapUintptr((*uintptr)(c), 0, uintptr(unsafe.Pointer(c))) &&
uintptr(*c) != uintptr(unsafe.Pointer(c)) {
panic("sync.Cond is copied")
}
}
这里举个简单的例子：
type cond struct {
checker copyChecker
}
type copyChecker uintptr
func (c *copyChecker) check() {
fmt.Printf("Before: c: %v, *c: %v, uintptr(*c): %v, uintptr(unsafe.Pointer(c)): %v\n", c, *c, uintptr(*c), uintptr(unsafe.Pointer(c)))
fmt.Println(atomic.CompareAndSwapUintptr((*uintptr)(c), 0, uintptr(unsafe.Pointer(c))))
fmt.Printf("After: c: %v, *c: %v, uintptr(*c): %v, uintptr(unsafe.Pointer(c)): %v\n", c, *c, uintptr(*c), uintptr(unsafe.Pointer(c)))
}
// main.go
var a cond
a.checker.check()
b := a
b.checker.check()
会输出如下信息：
Before: c: 0xc0000b0008, *c: 0, uintptr(*c): 0, uintptr(unsafe.Pointer(c)): 824634441736
true
After: c: 0xc0000b0008, *c: 824634441736, uintptr(*c): 824634441736, uintptr(unsafe.Pointer(c)): 824634441736
Before: c: 0xc0000b0018, *c: 824634441736, uintptr(*c): 824634441736, uintptr(unsafe.Pointer(c)): 824634441752
false
After: c: 0xc0000b0018, *c: 824634441736, uintptr(*c): 824634441736, uintptr(unsafe.Pointer(c)): 824634441752
在运行时检查都是使用了指针来进行测试是否发生了拷贝。
Go vet工具
vet是兽医的意思，而go的吉祥物是一支地鼠，而go vet工具报告可能出现的错误，所以这个命名还是蛮有意思的，猜测设计者应该是这个用意。
假如我们copy了sync.Cond，使用vet工具，在编译器就可以给开发者以提示，比如下面代码：
func main() {
cc := sync.Cond{}
copycc := cc
fmt.Println(copycc)
}
使用go vet 工具来检查
go vet -copylocks -json
{
"goLandTest/escapeAna": {
"copylocks": [
{
"posn": "/Users/hongyi/xxx/Go/src/goLandTest/escapeAna/main.go:69:12",
"message": "assignment copies lock value to copycc: sync.Cond contains sync.noCopy"
},
{
"posn": "/Users/hongyi/xxx/Go/src/goLandTest/escapeAna/main.go:70:14",
"message": "call of fmt.Println copies lock value: sync.Cond contains sync.noCopy"
}
]
}
}
常见的IDE如vs code就集成了这个插件，当保存代码的时候会使用vet工具来检查代码。关于vet的更多用法请参考官方文档
那如何让go vet来检查copy的呢？答案是在结构体内部嵌入noCopy，noCopy是一个结构体，在cond.go中可以找到
// noCopy may be embedded into structs which must not be copied
// after the first use.
//
// See https://golang.org/issues/8005#issuecomment-190753527
// for details.
type noCopy struct{}
// Lock is a no-op used by -copylocks checker from `go vet`.
func (*noCopy) Lock() {}
func (*noCopy) Unlock() {}
type Cond struct {
noCopy noCopy
....
}
type WaitGroup struct {
noCopy noCopy
state1 [3]uint32
}
如果你想让你自己定义的类型不能被copy，你可以在你的包中简单的定义一个noCopy结构，并在你的类型里嵌入这个结构，然后go vet工具就会为你做剩下的检查工作。
type noCopy struct{}
func (*noCopy) Lock() {}
func (*noCopy) Unlock() {}
type YourType struct {
noCopy noCopy
...
}
参考
what-does-nocopy-after-first-use-mean-in-golang-and-how
Golang escape analysis
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