# 8.10 终结器
TODO:
## 存活与终结
### SetFinalizer
```
type eface struct {
_type *_type
data unsafe.Pointer
}
func efaceOf(ep *interface{}) *eface {
return (*eface)(unsafe.Pointer(ep))
}
func SetFinalizer(obj interface{}, finalizer interface{}) {
...
e := efaceOf(&obj)
etyp := e._type
...
ot := (*ptrtype)(unsafe.Pointer(etyp))
...
// find the containing object
base, _, _ := findObject(uintptr(e.data), 0, 0)
if base == 0 {
if e.data == unsafe.Pointer(&zerobase) {
return
}
for datap := &firstmoduledata; datap != nil; datap = datap.next {
if datap.noptrdata <= uintptr(e.data) && uintptr(e.data) < datap.enoptrdata ||
datap.data <= uintptr(e.data) && uintptr(e.data) < datap.edata ||
datap.bss <= uintptr(e.data) && uintptr(e.data) < datap.ebss ||
datap.noptrbss <= uintptr(e.data) && uintptr(e.data) < datap.enoptrbss {
return
}
}
throw("runtime.SetFinalizer: pointer not in allocated block")
}
if uintptr(e.data) != base {
// As an implementation detail we allow to set finalizers for an inner byte
// of an object if it could come from tiny alloc (see mallocgc for details).
if ot.elem == nil || ot.elem.kind&kindNoPointers == 0 || ot.elem.size >= maxTinySize {
throw("runtime.SetFinalizer: pointer not at beginning of allocated block")
}
}
f := efaceOf(&finalizer)
ftyp := f._type
if ftyp == nil {
// switch to system stack and remove finalizer
systemstack(func() {
removefinalizer(e.data)
})
return
}
if ftyp.kind&kindMask != kindFunc {
throw("runtime.SetFinalizer: second argument is " + ftyp.string() + ", not a function")
}
ft := (*functype)(unsafe.Pointer(ftyp))
if ft.dotdotdot() {
throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string() + " because dotdotdot")
}
if ft.inCount != 1 {
throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string())
}
fint := ft.in()[0]
switch {
case fint == etyp:
// ok - same type
goto okarg
case fint.kind&kindMask == kindPtr:
if (fint.uncommon() == nil || etyp.uncommon() == nil) && (*ptrtype)(unsafe.Pointer(fint)).elem == ot.elem {
// ok - not same type, but both pointers,
// one or the other is unnamed, and same element type, so assignable.
goto okarg
}
case fint.kind&kindMask == kindInterface:
ityp := (*interfacetype)(unsafe.Pointer(fint))
if len(ityp.mhdr) == 0 {
// ok - satisfies empty interface
goto okarg
}
if _, ok := assertE2I2(ityp, *efaceOf(&obj)); ok {
goto okarg
}
}
throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string())
okarg:
// compute size needed for return parameters
nret := uintptr(0)
for _, t := range ft.out() {
nret = round(nret, uintptr(t.align)) + uintptr(t.size)
}
nret = round(nret, sys.PtrSize)
// make sure we have a finalizer goroutine
createfing()
systemstack(func() {
if !addfinalizer(e.data, (*funcval)(f.data), nret, fint, ot) {
throw("runtime.SetFinalizer: finalizer already set")
}
})
}
```
```
func removefinalizer(p unsafe.Pointer) {
s := (*specialfinalizer)(unsafe.Pointer(removespecial(p, _KindSpecialFinalizer)))
if s == nil {
return // there wasn't a finalizer to remove
}
lock(&mheap_.speciallock)
mheap_.specialfinalizeralloc.free(unsafe.Pointer(s))
unlock(&mheap_.speciallock)
}
func removespecial(p unsafe.Pointer, kind uint8) *special {
span := spanOfHeap(uintptr(p))
if span == nil {
throw("removespecial on invalid pointer")
}
// Ensure that the span is swept.
// Sweeping accesses the specials list w/o locks, so we have
// to synchronize with it. And it's just much safer.
mp := acquirem()
span.ensureSwept()
offset := uintptr(p) - span.base()
lock(&span.speciallock)
t := &span.specials
for {
s := *t
if s == nil {
break
}
// This function is used for finalizers only, so we don't check for
// "interior" specials (p must be exactly equal to s->offset).
if offset == uintptr(s.offset) && kind == s.kind {
*t = s.next
unlock(&span.speciallock)
releasem(mp)
return s
}
t = &s.next
}
unlock(&span.speciallock)
releasem(mp)
return nil
}
```
```
func createfing() {
// start the finalizer goroutine exactly once
if fingCreate == 0 && atomic.Cas(&fingCreate, 0, 1) {
go runfinq()
}
}
func runfinq() {
var (
frame unsafe.Pointer
framecap uintptr
)
for {
lock(&finlock)
fb := finq
finq = nil
if fb == nil {
gp := getg()
fing = gp
fingwait = true
goparkunlock(&finlock, waitReasonFinalizerWait, traceEvGoBlock, 1)
continue
}
unlock(&finlock)
...
for fb != nil {
for i := fb.cnt; i > 0; i-- {
f := &fb.fin[i-1]
framesz := unsafe.Sizeof((interface{})(nil)) + f.nret
if framecap < framesz {
// The frame does not contain pointers interesting for GC,
// all not yet finalized objects are stored in finq.
// If we do not mark it as FlagNoScan,
// the last finalized object is not collected.
frame = mallocgc(framesz, nil, true)
framecap = framesz
}
if f.fint == nil {
throw("missing type in runfinq")
}
// frame is effectively uninitialized
// memory. That means we have to clear
// it before writing to it to avoid
// confusing the write barrier.
*(*[2]uintptr)(frame) = [2]uintptr{}
switch f.fint.kind & kindMask {
case kindPtr:
// direct use of pointer
*(*unsafe.Pointer)(frame) = f.arg
case kindInterface:
ityp := (*interfacetype)(unsafe.Pointer(f.fint))
// set up with empty interface
(*eface)(frame)._type = &f.ot.typ
(*eface)(frame).data = f.arg
if len(ityp.mhdr) != 0 {
// convert to interface with methods
// this conversion is guaranteed to succeed - we checked in SetFinalizer
*(*iface)(frame) = assertE2I(ityp, *(*eface)(frame))
}
default:
throw("bad kind in runfinq")
}
fingRunning = true
reflectcall(nil, unsafe.Pointer(f.fn), frame, uint32(framesz), uint32(framesz))
fingRunning = false
// Drop finalizer queue heap references
// before hiding them from markroot.
// This also ensures these will be
// clear if we reuse the finalizer.
f.fn = nil
f.arg = nil
f.ot = nil
atomic.Store(&fb.cnt, i-1)
}
next := fb.next
lock(&finlock)
fb.next = finc
finc = fb
unlock(&finlock)
fb = next
}
}
}
```
```
func addfinalizer(p unsafe.Pointer, f *funcval, nret uintptr, fint *_type, ot *ptrtype) bool {
lock(&mheap_.speciallock)
s := (*specialfinalizer)(mheap_.specialfinalizeralloc.alloc())
unlock(&mheap_.speciallock)
s.special.kind = _KindSpecialFinalizer
s.fn = f
s.nret = nret
s.fint = fint
s.ot = ot
if addspecial(p, &s.special) {
// This is responsible for maintaining the same
// GC-related invariants as markrootSpans in any
// situation where it's possible that markrootSpans
// has already run but mark termination hasn't yet.
if gcphase != _GCoff {
base, _, _ := findObject(uintptr(p), 0, 0)
mp := acquirem()
gcw := &mp.p.ptr().gcw
// Mark everything reachable from the object
// so it's retained for the finalizer.
scanobject(base, gcw)
// Mark the finalizer itself, since the
// special isn't part of the GC'd heap.
scanblock(uintptr(unsafe.Pointer(&s.fn)), sys.PtrSize, &oneptrmask[0], gcw)
if gcBlackenPromptly {
gcw.dispose()
}
releasem(mp)
}
return true
}
// There was an old finalizer
lock(&mheap_.speciallock)
mheap_.specialfinalizeralloc.free(unsafe.Pointer(s))
unlock(&mheap_.speciallock)
return false
}
```
### KeepAlive
KeepAlive 会将某个参数标记为可达，从而能够保证某个对象在 调用 KeepAlive 之前都不会被垃圾回收所释放（因为被引用），进而这个对象设置的 Finalizer 也不会被运行，考虑下面的例子：
```
type File struct {d int}
d, err := syscall.Open("/file/path", syscall.O_RDONLY, 0)
// ...
p := &File{d}
runtime.SetFinalizer(p, func(p *File) {
syscall.Close(p.d)
})
var buf [10]byte
n, err := syscall.Read(p.d, buf[:])
// 确保在 Read 返回之前， p 都不会被 finalize 掉
runtime.KeepAlive(p)
// 此后不再使用 p
```
KeepAlive 的源码非常简单：
```
func KeepAlive(x interface{}) {
if cgoAlwaysFalse {
println(x)
}
}
```
保留一个引用只需要产生一个参数传递，而这里针对 cgo 做了特殊处理，即 产生了一个`println`调用来保证编译器不会将其优化掉。
- 第一部分 :基础篇
- 第1章 Go语言的前世今生
- 1.2 Go语言综述
- 1.3 顺序进程通讯
- 1.4 Plan9汇编语言
- 第2章 程序生命周期
- 2.1 从go命令谈起
- 2.2 Go程序编译流程
- 2.3 Go 程序启动引导
- 2.4 主Goroutine的生与死
- 第3 章 语言核心
- 3.1 数组.切片与字符串
- 3.2 散列表
- 3.3 函数调用
- 3.4 延迟语句
- 3.5 恐慌与恢复内建函数
- 3.6 通信原语
- 3.7 接口
- 3.8 运行时类型系统
- 3.9 类型别名
- 3.10 进一步阅读的参考文献
- 第4章 错误
- 4.1 问题的演化
- 4.2 错误值检查
- 4.3 错误格式与上下文
- 4.4 错误语义
- 4.5 错误处理的未来
- 4.6 进一步阅读的参考文献
- 第5章 同步模式
- 5.1 共享内存式同步模式
- 5.2 互斥锁
- 5.3 原子操作
- 5.4 条件变量
- 5.5 同步组
- 5.6 缓存池
- 5.7 并发安全散列表
- 5.8 上下文
- 5.9 内存一致模型
- 5.10 进一步阅读的文献参考
- 第二部分 运行时篇
- 第6章 并发调度
- 6.1 随机调度的基本概念
- 6.2 工作窃取式调度
- 6.3 MPG模型与并发调度单
- 6.4 调度循环
- 6.5 线程管理
- 6.6 信号处理机制
- 6.7 执行栈管理
- 6.8 协作与抢占
- 6.9 系统监控
- 6.10 网络轮询器
- 6.11 计时器
- 6.12 非均匀访存下的调度模型
- 6.13 进一步阅读的参考文献
- 第7章 内存分配
- 7.1 设计原则
- 7.2 组件
- 7.3 初始化
- 7.4 大对象分配
- 7.5 小对象分配
- 7.6 微对象分配
- 7.7 页分配器
- 7.8 内存统计
- 第8章 垃圾回收
- 8.1 垃圾回收的基本想法
- 8.2 写屏幕技术
- 8.3 调步模型与强弱触发边界
- 8.4 扫描标记与标记辅助
- 8.5 免清扫式位图技术
- 8.6 前进保障与终止检测
- 8.7 安全点分析
- 8.8 分代假设与代际回收
- 8.9 请求假设与实务制导回收
- 8.10 终结器
- 8.11 过去,现在与未来
- 8.12 垃圾回收统一理论
- 8.13 进一步阅读的参考文献
- 第三部分 工具链篇
- 第9章 代码分析
- 9.1 死锁检测
- 9.2 竞争检测
- 9.3 性能追踪
- 9.4 代码测试
- 9.5 基准测试
- 9.6 运行时统计量
- 9.7 语言服务协议
- 第10章 依赖管理
- 10.1 依赖管理的难点
- 10.2 语义化版本管理
- 10.3 最小版本选择算法
- 10.4 Vgo 与dep之争
- 第12章 泛型
- 12.1 泛型设计的演进
- 12.2 基于合约的泛型
- 12.3 类型检查技术
- 12.4 泛型的未来
- 12.5 进一步阅读的的参考文献
- 第13章 编译技术
- 13.1 词法与文法
- 13.2 中间表示
- 13.3 优化器
- 13.4 指针检查器
- 13.5 逃逸分析
- 13.6 自举
- 13.7 链接器
- 13.8 汇编器
- 13.9 调用规约
- 13.10 cgo与系统调用
- 结束语: Go去向何方?