// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Garbage collector: finalizers and block profiling.
package runtime
import "unsafe"
type finblock struct {
alllink *finblock
next *finblock
cnt int32
_ int32
fin [(_FinBlockSize - 2*ptrSize - 2*4) / unsafe.Sizeof(finalizer{})]finalizer
}
var finlock mutex // protects the following variables
var fing *g // goroutine that runs finalizers
var finq *finblock // list of finalizers that are to be executed
var finc *finblock // cache of free blocks
var finptrmask [_FinBlockSize / ptrSize / 8]byte
var fingwait bool
var fingwake bool
var allfin *finblock // list of all blocks
// NOTE: Layout known to queuefinalizer.
type finalizer struct {
fn *funcval // function to call
arg unsafe.Pointer // ptr to object
nret uintptr // bytes of return values from fn
fint *_type // type of first argument of fn
ot *ptrtype // type of ptr to object
}
var finalizer1 = [...]byte{
// Each Finalizer is 5 words, ptr ptr INT ptr ptr (INT = uintptr here)
// Each byte describes 8 words.
// Need 8 Finalizers described by 5 bytes before pattern repeats:
// ptr ptr INT ptr ptr
// ptr ptr INT ptr ptr
// ptr ptr INT ptr ptr
// ptr ptr INT ptr ptr
// ptr ptr INT ptr ptr
// ptr ptr INT ptr ptr
// ptr ptr INT ptr ptr
// ptr ptr INT ptr ptr
// aka
//
// ptr ptr INT ptr ptr ptr ptr INT
// ptr ptr ptr ptr INT ptr ptr ptr
// ptr INT ptr ptr ptr ptr INT ptr
// ptr ptr ptr INT ptr ptr ptr ptr
// INT ptr ptr ptr ptr INT ptr ptr
//
// Assumptions about Finalizer layout checked below.
1<<0 | 1<<1 | 0<<2 | 1<<3 | 1<<4 | 1<<5 | 1<<6 | 0<<7,
1<<0 | 1<<1 | 1<<2 | 1<<3 | 0<<4 | 1<<5 | 1<<6 | 1<<7,
1<<0 | 0<<1 | 1<<2 | 1<<3 | 1<<4 | 1<<5 | 0<<6 | 1<<7,
1<<0 | 1<<1 | 1<<2 | 0<<3 | 1<<4 | 1<<5 | 1<<6 | 1<<7,
0<<0 | 1<<1 | 1<<2 | 1<<3 | 1<<4 | 0<<5 | 1<<6 | 1<<7,
}
func queuefinalizer(p unsafe.Pointer, fn *funcval, nret uintptr, fint *_type, ot *ptrtype) {
lock(&finlock)
if finq == nil || finq.cnt == int32(len(finq.fin)) {
if finc == nil {
// Note: write barrier here, assigning to finc, but should be okay.
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*ptrSize ||
unsafe.Offsetof(finalizer{}.fn) != 0 ||
unsafe.Offsetof(finalizer{}.arg) != ptrSize ||
unsafe.Offsetof(finalizer{}.nret) != 2*ptrSize ||
unsafe.Offsetof(finalizer{}.fint) != 3*ptrSize ||
unsafe.Offsetof(finalizer{}.ot) != 4*ptrSize) {
throw("finalizer out of sync")
}
for i := range finptrmask {
finptrmask[i] = finalizer1[i%len(finalizer1)]
}
}
}
block := finc
finc = block.next
block.next = finq
finq = block
}
f := &finq.fin[finq.cnt]
finq.cnt++
f.fn = fn
f.nret = nret
f.fint = fint
f.ot = ot
f.arg = p
fingwake = true
unlock(&finlock)
}
//go:nowritebarrier
func iterate_finq(callback func(*funcval, unsafe.Pointer, uintptr, *_type, *ptrtype)) {
for fb := allfin; fb != nil; fb = fb.alllink {
for i := int32(0); i < fb.cnt; i++ {
f := &fb.fin[i]
callback(f.fn, f.arg, f.nret, f.fint, f.ot)
}
}
}
func wakefing() *g {
var res *g
lock(&finlock)
if fingwait && fingwake {
fingwait = false
fingwake = false
res = fing
}
unlock(&finlock)
return res
}
var (
fingCreate uint32
fingRunning bool
)
func createfing() {
// start the finalizer goroutine exactly once
if fingCreate == 0 && 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, "finalizer wait", traceEvGoBlock, 1)
continue
}
unlock(&finlock)
if raceenabled {
racefingo()
}
for fb != nil {
for i := fb.cnt; i > 0; i-- {
f := (*finalizer)(add(unsafe.Pointer(&fb.fin), uintptr(i-1)*unsafe.Sizeof(finalizer{})))
framesz := unsafe.Sizeof((interface{})(nil)) + uintptr(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, flagNoScan)
framecap = framesz
}
if f.fint == nil {
throw("missing type in runfinq")
}
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
assertE2I(ityp, *(*interface{})(frame), (*fInterface)(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 references to finalized object
f.fn = nil
f.arg = nil
f.ot = nil
fb.cnt = i - 1
}
next := fb.next
lock(&finlock)
fb.next = finc
finc = fb
unlock(&finlock)
fb = next
}
}
}
// SetFinalizer sets the finalizer associated with x to f.
// When the garbage collector finds an unreachable block
// with an associated finalizer, it clears the association and runs
// f(x) in a separate goroutine. This makes x reachable again, but
// now without an associated finalizer. Assuming that SetFinalizer
// is not called again, the next time the garbage collector sees
// that x is unreachable, it will free x.
//
// SetFinalizer(x, nil) clears any finalizer associated with x.
//
// The argument x must be a pointer to an object allocated by
// calling new or by taking the address of a composite literal.
// The argument f must be a function that takes a single argument
// to which x's type can be assigned, and can have arbitrary ignored return
// values. If either of these is not true, SetFinalizer aborts the
// program.
//
// Finalizers are run in dependency order: if A points at B, both have
// finalizers, and they are otherwise unreachable, only the finalizer
// for A runs; once A is freed, the finalizer for B can run.
// If a cyclic structure includes a block with a finalizer, that
// cycle is not guaranteed to be garbage collected and the finalizer
// is not guaranteed to run, because there is no ordering that
// respects the dependencies.
//
// The finalizer for x is scheduled to run at some arbitrary time after
// x becomes unreachable.
// There is no guarantee that finalizers will run before a program exits,
// so typically they are useful only for releasing non-memory resources
// associated with an object during a long-running program.
// For example, an os.File object could use a finalizer to close the
// associated operating system file descriptor when a program discards
// an os.File without calling Close, but it would be a mistake
// to depend on a finalizer to flush an in-memory I/O buffer such as a
// bufio.Writer, because the buffer would not be flushed at program exit.
//
// It is not guaranteed that a finalizer will run if the size of *x is
// zero bytes.
//
// It is not guaranteed that a finalizer will run for objects allocated
// in initializers for package-level variables. Such objects may be
// linker-allocated, not heap-allocated.
//
// A single goroutine runs all finalizers for a program, sequentially.
// If a finalizer must run for a long time, it should do so by starting
// a new goroutine.
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 := (*eface)(unsafe.Pointer(&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(e.data)
if base == nil {
// 0-length objects are okay.
if e.data == unsafe.Pointer(&zerobase) {
return
}
// Global initializers might be linker-allocated.
// var Foo = &Object{}
// func main() {
// runtime.SetFinalizer(Foo, nil)
// }
// The relevant segments are: noptrdata, data, bss, noptrbss.
// We cannot assume they are in any order or even contiguous,
// due to external linking.
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 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 := (*eface)(unsafe.Pointer(&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))
ins := *(*[]*_type)(unsafe.Pointer(&ft.in))
if ft.dotdotdot || len(ins) != 1 {
throw("runtime.SetFinalizer: cannot pass " + *etyp._string + " to finalizer " + *ftyp._string)
}
fint := ins[0]
switch {
case fint == etyp:
// ok - same type
goto okarg
case fint.kind&kindMask == kindPtr:
if (fint.x == nil || fint.x.name == nil || etyp.x == nil || etyp.x.name == 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 assertE2I2(ityp, obj, nil) {
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 *(*[]*_type)(unsafe.Pointer(&ft.out)) {
nret = round(nret, uintptr(t.align)) + uintptr(t.size)
}
nret = round(nret, 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")
}
})
}
// Look up pointer v in heap. Return the span containing the object,
// the start of the object, and the size of the object. If the object
// does not exist, return nil, nil, 0.
func findObject(v unsafe.Pointer) (s *mspan, x unsafe.Pointer, n uintptr) {
c := gomcache()
c.local_nlookup++
if ptrSize == 4 && c.local_nlookup >= 1<<30 {
// purge cache stats to prevent overflow
lock(&mheap_.lock)
purgecachedstats(c)
unlock(&mheap_.lock)
}
// find span
arena_start := uintptr(unsafe.Pointer(mheap_.arena_start))
arena_used := uintptr(unsafe.Pointer(mheap_.arena_used))
if uintptr(v) < arena_start || uintptr(v) >= arena_used {
return
}
p := uintptr(v) >> pageShift
q := p - arena_start>>pageShift
s = *(**mspan)(add(unsafe.Pointer(mheap_.spans), q*ptrSize))
if s == nil {
return
}
x = unsafe.Pointer(uintptr(s.start) << pageShift)
if uintptr(v) < uintptr(x) || uintptr(v) >= uintptr(unsafe.Pointer(s.limit)) || s.state != mSpanInUse {
s = nil
x = nil
return
}
n = uintptr(s.elemsize)
if s.sizeclass != 0 {
x = add(x, (uintptr(v)-uintptr(x))/n*n)
}
return
}