SetFinalizer设置一个finalizer关联到一个对象obj，当垃圾回收准备回收obj的时候，它会断开这个连接，并在单独的goroutine中执行finalizer(obj),这将让obj再次可用，但是不会再关联finalizer,
在SetFinalizer中:
- 先检查元素,如果类型为nil,或者不为指针，元素值为nil 都会报错
- 查找对象的heap地址
- 如果是函数类型为nil,则移除removefinalizer
- 检测参数类型,并给计算返回值size
-
createfing()确保finalizer goroutine正在运行 - 添加finalizer
func SetFinalizer(obj interface{}, finalizer interface{}) {
e := efaceOf(&obj)
etyp := e._type
if etyp == nil {
throw("runtime.SetFinalizer: first argument is nil")
}
if etyp.kind&kindMask != kindPtr {
throw("runtime.SetFinalizer: first argument is " + etyp.string() + ", not pointer")
}
ot := (*ptrtype)(unsafe.Pointer(etyp))
if ot.elem == nil {
throw("nil elem type!")
}
// find the containing object
base, _, _ := findObject(uintptr(e.data), 0, 0)
f := efaceOf(&finalizer)
ftyp := f._type
if ftyp == nil {
// switch to system stack and remove finalizer
systemstack(func() {
removefinalizer(e.data)
})
return
}
fint := ft.in()[0]
// 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")
}
})
}
从mheap.specialfinalizeralloc中申请一块内存,然后调用addspecial,如果成功则返回，失败则free掉刚才申请的内存
在addspecial ，根据p指针找到对应的span,轮训找到是否有相同offset和kind的数据存在，如果存在则finalizer函数已经存在，否则将其加入到span.specials中去(这里有根据offset排序)
// Adds a finalizer to the object p. Returns true if it succeeded.
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)
if addspecial(p, &s.special) {
return true
}
// There was an old finalizer
lock(&mheap_.speciallock)
mheap_.specialfinalizeralloc.free(unsafe.Pointer(s))
unlock(&mheap_.speciallock)
return false
}
func addspecial(p unsafe.Pointer, s *special) bool {
span := spanOfHeap(uintptr(p))
lock(&span.speciallock)
// Find splice point, check for existing record.
t := &span.specials
for {
x := *t
if x == nil {
break
}
if offset == uintptr(x.offset) && kind == x.kind {
unlock(&span.speciallock)
releasem(mp)
return false // already exists
}
if offset < uintptr(x.offset) || (offset == uintptr(x.offset) && kind < x.kind) {
break
}
t = &x.next
}
// Splice in record, fill in offset.
s.offset = uint16(offset)
s.next = *t
*t = s
unlock(&span.speciallock)
releasem(mp)
return true
}
清理
sweep函数中，如果span有special record,先检查是否有finalizer, 如果没有的话则调用freespecial,将其加入到finq
var finq *finblock // list of finalizers that are to be executed
func (s *mspan) sweep(preserve bool) bool {
// The allocBits indicate which unmarked objects don't need to be
// processed since they were free at the end of the last GC cycle
// and were not allocated since then.
// If the allocBits index is >= s.freeindex and the bit
// is not marked then the object remains unallocated
// since the last GC.
// This situation is analogous to being on a freelist.
// Unlink & free special records for any objects we're about to free.
// Two complications here:
// 1. An object can have both finalizer and profile special records.
// In such case we need to queue finalizer for execution,
// mark the object as live and preserve the profile special.
// 2. A tiny object can have several finalizers setup for different offsets.
// If such object is not marked, we need to queue all finalizers at once.
// Both 1 and 2 are possible at the same time.
specialp := &s.specials
special := *specialp
for special != nil {
// A finalizer can be set for an inner byte of an object, find object beginning.
objIndex := uintptr(special.offset) / size
p := s.base() + objIndex*size
mbits := s.markBitsForIndex(objIndex)
if !mbits.isMarked() {
// This object is not marked and has at least one special record.
// Pass 1: see if it has at least one finalizer.
hasFin := false
endOffset := p - s.base() + size
for tmp := special; tmp != nil && uintptr(tmp.offset) < endOffset; tmp = tmp.next {
if tmp.kind == _KindSpecialFinalizer {
// Stop freeing of object if it has a finalizer.
mbits.setMarkedNonAtomic()
hasFin = true
break
}
}
// Pass 2: queue all finalizers _or_ handle profile record.
for special != nil && uintptr(special.offset) < endOffset {
// Find the exact byte for which the special was setup
// (as opposed to object beginning).
p := s.base() + uintptr(special.offset)
if special.kind == _KindSpecialFinalizer || !hasFin {
// Splice out special record.
y := special
special = special.next
*specialp = special
freespecial(y, unsafe.Pointer(p), size)
} else {
// This is profile record, but the object has finalizers (so kept alive).
// Keep special record.
specialp = &special.next
special = *specialp
}
}
} else {
// object is still live: keep special record
specialp = &special.next
special = *specialp
}
}
}
执行
在createfing中新建了一个goroutine处理这个队列, 获取finq队列中的数据
如果队列为空，则休眠, findrunable会检查唤醒,
否则循环，调用reflectcall(nil, unsafe.Pointer(f.fn), frame, uint32(framesz), uint32(framesz))执行fin函数
```func createfing() {
// start the finalizer goroutine exactly once
if fingCreate == 0 && atomic.Cas(&fingCreate, 0, 1) {
go runfinq()
}
}
// This is the goroutine that runs all of the finalizers
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)
if raceenabled {
racefingo()
}
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
}
}
}