深入理解Go-runtime.SetFinalizer原理剖析

finalizer是与对象关联的一个函数，通过runtime.SetFinalizer 来设置，它在对象被GC的时候，这个finalizer会被调用，以完成对象生命中最后一程。由于finalizer的存在，导致了对象在三色标记中，不可能被标为白色对象，也就是垃圾，所以，这个对象的生命也会得以延续一个GC周期。正如defer一样，我们也可以通过 Finalizer 完成一些类似于资源释放的操作

1. 结构概览

1.1. heap

type mspan struct {
    // 当前span上所有对象的special串成链表
    // special中有个offset，就是数据对象在span上的offset，通过offset，将数据对象和special关联起来
    specials    *special   // linked list of special records sorted by offset.
}

1.2. special

type special struct {
    next   *special // linked list in span
    // 数据对象在span上的offset
    offset uint16   // span offset of object
    kind   byte     // kind of special
}

1.3. specialfinalizer

type specialfinalizer struct {
    special special
    fn      *funcval // May be a heap pointer.
    // return的数据的大小
    nret    uintptr
    // 第一个参数的类型
    fint    *_type   // May be a heap pointer, but always live.
    // 与finalizer关联的数据对象的指针类型
    ot      *ptrtype // May be a heap pointer, but always live.
}

1.4. finalizer

type finalizer struct {
    fn   *funcval       // function to call (may be a heap pointer)
    arg  unsafe.Pointer // ptr to object (may be a heap pointer)
    nret uintptr        // bytes of return values from fn
    fint *_type         // type of first argument of fn
    ot   *ptrtype       // type of ptr to object (may be a heap pointer)
}

1.5. 全局变量

var finlock mutex  // protects the following variables
// 运行finalizer的g，只有一个g，不用的时候休眠，需要的时候再唤醒
var fing *g        // goroutine that runs finalizers
// finalizer的全局队列，这里是已经设置的finalizer串成的链表
var finq *finblock // list of finalizers that are to be executed
// 已经释放的finblock的链表，用finc缓存起来，以后需要使用的时候可以直接取走，避免再走一遍内存分配了
var finc *finblock // cache of free blocks
var finptrmask [_FinBlockSize / sys.PtrSize / 8]byte
var fingwait bool  // fing的标志位，通过 fingwait和fingwake，来确定是否需要唤醒fing
var fingwake bool
// 所有的blocks串成的链表
var allfin *finblock // list of all blocks

2. 源码分析

2.1. 创建finalizer

2.1.1. main

func main() {
    // i 就是后面说的 数据对象
    var i = 3
    // 这里的func 就是后面一直说的 finalizer
    runtime.SetFinalizer(&i, func(i *int) {
        fmt.Println(i, *i, "set finalizer")
    })
    time.Sleep(time.Second * 5)
}

2.1.2. SetFinalizer

根据 数据对象 ，生成一个special对象，并绑定到 数据对象 所在的span，串联到span.specials上，并且确保fing的存在

func SetFinalizer(obj interface{}, finalizer interface{}) {
    if debug.sbrk != 0 {
        // debug.sbrk never frees memory, so no finalizers run
        // (and we don't have the data structures to record them).
        return
    }
    e := efaceOf(&obj)
    etyp := e._type
    // ---- 省略数据校验的逻辑 ---
    ot := (*ptrtype)(unsafe.Pointer(etyp))

    // find the containing object
    // 在内存中找不到分配的地址时 base==0，setFinalizer 是在内存回收的时候调用，没有分配就不会回收
    base, _, _ := findObject(uintptr(e.data), 0, 0)

    f := efaceOf(&finalizer)
    ftyp := f._type
    // 如果 finalizer type == nil，尝试移除（没有的话，就不需要移除了）
    if ftyp == nil {
        // switch to system stack and remove finalizer
        systemstack(func() {
            removefinalizer(e.data)
        })
        return
    }
    // --- 对finalizer参数数量及类型进行校验 --
    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
    // 计算返回参数的大小并进行对齐
    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
    // 确保 finalizer 有一个 goroutine
    createfing()

    systemstack(func() {
        // 却换到g0，添加finalizer，并且不能重复设置
        if !addfinalizer(e.data, (*funcval)(f.data), nret, fint, ot) {
            throw("runtime.SetFinalizer: finalizer already set")
        }
    })
}

这里逻辑没什么复杂的，只是在参数、类型的判断等上面，比较的麻烦

2.1.3. removefinalizer

通过removespecial，找到数据对象p所对应的special对象，如果找到的话，释放mheap上对应的内存

func removefinalizer(p unsafe.Pointer) {
    // 根据数据p找到对应的special对象
    s := (*specialfinalizer)(unsafe.Pointer(removespecial(p, _KindSpecialFinalizer)))
    if s == nil {
        return // there wasn't a finalizer to remove
    }
    lock(&mheap_.speciallock)
    // 释放找到的special所对应的内存
    mheap_.specialfinalizeralloc.free(unsafe.Pointer(s))
    unlock(&mheap_.speciallock)
}

这里的函数，虽然叫removefinalizer， 但是这里暂时跟finalizer结构体没有关系，都是在跟special结构体打交道，后面的addfinalizer也是一样的

2.1.4. removespecial

遍历数据所在的span的specials，如果找到了指定数据p的special的话，就从specials中移除，并返回

func removespecial(p unsafe.Pointer, kind uint8) *special {
    // 找到数据p所在的span
    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()
    // 保证span被清扫过了
    span.ensureSwept()
    // 获取数据p的偏移量，根据偏移量去寻找p对应的special
    offset := uintptr(p) - span.base()

    lock(&span.speciallock)
    t := &span.specials
    // 遍历span.specials这个链表
    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 {
            // 找到了，修改指针，将当前找到的special移除
            *t = s.next
            unlock(&span.speciallock)
            releasem(mp)
            return s
        }
        t = &s.next
    }
    unlock(&span.speciallock)
    releasem(mp)
    // 没有找到，就返回nil
    return nil
}

2.1.5. addfinalizer

正好跟removefinalizer相反，这个就是根据数据对象p，创建对应的special，然后添加到span.specials链表上面

func addfinalizer(p unsafe.Pointer, f *funcval, nret uintptr, fint *_type, ot *ptrtype) bool {
    lock(&mheap_.speciallock)
    // 分配出来一块内存供finalizer使用
    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) {

        return true
    }

    // There was an old finalizer
    // 没有添加成功，是因为p已经有了一个special对象了
    lock(&mheap_.speciallock)
    mheap_.specialfinalizeralloc.free(unsafe.Pointer(s))
    unlock(&mheap_.speciallock)
    return false
}

2.1.6. addspecial

这里是添加special的主逻辑

func addspecial(p unsafe.Pointer, s *special) bool {
    span := spanOfHeap(uintptr(p))
    if span == nil {
        throw("addspecial on invalid pointer")
    }
    // 同 removerspecial一样，确保这个span已经清扫过了
    mp := acquirem()
    span.ensureSwept()

    offset := uintptr(p) - span.base()
    kind := s.kind

    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 {
            // 已经存在了，不能在增加了，一个数据对象，只能绑定一个finalizer
            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.
    // 添加到 specials 队列尾
    s.offset = uint16(offset)
    s.next = *t
    *t = s
    unlock(&span.speciallock)
    releasem(mp)

    return true
}

2.1.7. createfing

这个函数是保证，创建了finalizer之后，有一个goroutine去运行，这里只运行一次，这个goroutine会由全局变量 fing 记录

func createfing() {
    // start the finalizer goroutine exactly once
    // 进创建一个goroutine，进行时刻监控运行
    if fingCreate == 0 && atomic.Cas(&fingCreate, 0, 1) {
        // 开启一个goroutine运行
        go runfinq()
    }
}

2.2. 执行finalizer

在上面的 createfing 的会尝试创建一个goroutine去执行，接下来就分析一下执行流程吧

func runfinq() {
    var (
        frame    unsafe.Pointer
        framecap uintptr
    )

    for {
        lock(&finlock)
        // 获取finq 全局队列，并清空全局队列
        fb := finq
        finq = nil
        if fb == nil {
            // 如果全局队列为空，休眠当前g，等待被唤醒
            gp := getg()
            fing = gp
            // 设置fing的状态标志位
            fingwait = true
            goparkunlock(&finlock, waitReasonFinalizerWait, traceEvGoBlock, 1)
            continue
        }
        unlock(&finlock)
        // 循环执行runq链表里的fin数组
        for fb != nil {
            for i := fb.cnt; i > 0; i-- {
                f := &fb.fin[i-1]
                // 获取存储当前finalizer的返回数据的大小，如果比之前大，则分配
                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.
                // 清空frame内存存储
                *(*[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")
                }
                // 调用finalizer函数
                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.
                // 清空finalizer的属性
                f.fn = nil
                f.arg = nil
                f.ot = nil
                atomic.Store(&fb.cnt, i-1)
            }
            // 将已经完成的finalizer放入finc以作缓存，避免再次分配内存
            next := fb.next
            lock(&finlock)
            fb.next = finc
            finc = fb
            unlock(&finlock)
            fb = next
        }
    }
}

看完上面的流程的时候，突然发现有点懵逼

1. 全局队列finq中是什么时候被插入数据 finalizer的？
2. g如果休眠了，那怎么被唤醒呢？

先针对第一个问题分析：

插入队列的操作，要追溯到我们之前分析的GC 深入理解Go-垃圾回收机制 了，在sweep 中有下面一段函数

2.2.1. sweep

func (s *mspan) sweep(preserve bool) bool {
    ....
    specialp := &s.specials
    special := *specialp
    for special != nil {
        ....
        if special.kind == _KindSpecialFinalizer || !hasFin {
            // Splice out special record.
            y := special
            special = special.next
            *specialp = special
            // 加入全局finq队列的入口就在这里了
            freespecial(y, unsafe.Pointer(p), size)
        }
        ....
    }
    ....
}

2.2.2. freespecial

在gc的时候，不仅要把special对应的内存释放掉，而且把specials整理创建对应dinalizer对象，并插入到 finq队列里面

func freespecial(s *special, p unsafe.Pointer, size uintptr) {
    switch s.kind {
    case _KindSpecialFinalizer:
        // 把这个finalizer加入到全局队列
        sf := (*specialfinalizer)(unsafe.Pointer(s))
        queuefinalizer(p, sf.fn, sf.nret, sf.fint, sf.ot)
        lock(&mheap_.speciallock)
        mheap_.specialfinalizeralloc.free(unsafe.Pointer(sf))
        unlock(&mheap_.speciallock)
    // 下面两种情况不在分析范围内，省略
    case _KindSpecialProfile:
        sp := (*specialprofile)(unsafe.Pointer(s))
        mProf_Free(sp.b, size)
        lock(&mheap_.speciallock)
        mheap_.specialprofilealloc.free(unsafe.Pointer(sp))
        unlock(&mheap_.speciallock)
    default:
        throw("bad special kind")
        panic("not reached")
    }
}

2.2.3. queuefinalizer

func queuefinalizer(p unsafe.Pointer, fn *funcval, nret uintptr, fint *_type, ot *ptrtype) {
    lock(&finlock)
    // 如果finq为空或finq的内部数组已经满了，则从finc或重新分配 来获取block并插入到finq的链表头
    if finq == nil || finq.cnt == uint32(len(finq.fin)) {
        if finc == nil {
            finc = (*finblock)(persistentalloc(_FinBlockSize, 0, &memstats.gc_sys))
            finc.alllink = allfin
            allfin = finc
            if finptrmask[0] == 0 {
                // Build pointer mask for Finalizer array in block.
                // Check assumptions made in finalizer1 array above.
                if (unsafe.Sizeof(finalizer{}) != 5*sys.PtrSize ||
                    unsafe.Offsetof(finalizer{}.fn) != 0 ||
                    unsafe.Offsetof(finalizer{}.arg) != sys.PtrSize ||
                    unsafe.Offsetof(finalizer{}.nret) != 2*sys.PtrSize ||
                    unsafe.Offsetof(finalizer{}.fint) != 3*sys.PtrSize ||
                    unsafe.Offsetof(finalizer{}.ot) != 4*sys.PtrSize) {
                    throw("finalizer out of sync")
                }
                for i := range finptrmask {
                    finptrmask[i] = finalizer1[i%len(finalizer1)]
                }
            }
        }
        // 从finc中移除并获取链表头
        block := finc
        finc = block.next
        // 将从finc获取到的链表挂载到finq的队列头，finq指向新的block
        block.next = finq
        finq = block
    }
    // 根据finq.cnt获取索引对应的block
    f := &finq.fin[finq.cnt]
    atomic.Xadd(&finq.cnt, +1) // Sync with markroots
    // 设置相关属性
    f.fn = fn
    f.nret = nret
    f.fint = fint
    f.ot = ot
    f.arg = p
    // 设置唤醒标志
    fingwake = true
    unlock(&finlock)
}

至此，也就明白了，runq全局队列是怎么被填充的了

那么，第二个问题，当fing被休眠后，怎么被唤醒呢？

这里就需要追溯到，深入理解Go-goroutine的实现及Scheduler分析 这篇文章了

2.2.4. findrunnable

在 findrunnable 中有一段代码如下：

func findrunnable() (gp *g, inheritTime bool) {
    // 通过状态位判断是否需要唤醒 fing， 通过wakefing来判断并返回fing
    if fingwait && fingwake {
        if gp := wakefing(); gp != nil {
            // 唤醒g，并从休眠出继续执行
            ready(gp, 0, true)
        }
    }
}

2.2.5. wakefing

这里不仅会对状态位 fingwait fingwake做二次判断，而且，如果状态位符合唤醒要求的话，需要重置两个状态位

func wakefing() *g {
    var res *g
    lock(&finlock)
    if fingwait && fingwake {
        fingwait = false
        fingwake = false
        res = fing
    }
    unlock(&finlock)
    return res
}

3. 参考文档

• 《Go语言学习笔记》--雨痕