// 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 (
"runtime/internal/atomic"
"runtime/internal/sys"
"unsafe"
)
// finblock is an array of finalizers to be executed. finblocks are
// arranged in a linked list for the finalizer queue.
//
// finblock is allocated from non-GC'd memory, so any heap pointers
// must be specially handled. GC currently assumes that the finalizer
// queue does not grow during marking (but it can shrink).
//
//go:notinheap
type finblock struct {
alllink *finblock
next *finblock
cnt uint32
_ int32
fin [(_FinBlockSize - 2*sys.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 / sys.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 (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)
}
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) {
if gcphase != _GCoff {
// Currently we assume that the finalizer queue won't
// grow during marking so we don't have to rescan it
// during mark termination. If we ever need to lift
// this assumption, we can do it by adding the
// necessary barriers to queuefinalizer (which it may
// have automatically).
throw("queuefinalizer during GC")
}
lock(&finlock)
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)]
}
}
}
block := finc
finc = block.next
block.next = finq
finq = block
}
f := &finq.fin[finq.cnt]
atomic.Xadd(&finq.cnt, +1) // Sync with markroots
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 := uint32(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 && 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
}
}
}
// SetFinalizer sets the finalizer associated with obj to the provided
// finalizer function. When the garbage collector finds an unreachable block
// with an associated finalizer, it clears the association and runs
// finalizer(obj) in a separate goroutine. This makes obj reachable again,
// but now without an associated finalizer. Assuming that SetFinalizer
// is not called again, the next time the garbage collector sees
// that obj is unreachable, it will free obj.
//
// SetFinalizer(obj, nil) clears any finalizer associated with obj.
//
// The argument obj must be a pointer to an object allocated by calling
// new, by taking the address of a composite literal, or by taking the
// address of a local variable.
// The argument finalizer must be a function that takes a single argument
// to which obj's type can be assigned, and can have arbitrary ignored return
// values. If either of these is not true, SetFinalizer may abort 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 is scheduled to run at some arbitrary time after the
// program can no longer reach the object to which obj points.
// 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 *obj 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 finalizer may run as soon as an object becomes unreachable.
// In order to use finalizers correctly, the program must ensure that
// the object is reachable until it is no longer required.
// Objects stored in global variables, or that can be found by tracing
// pointers from a global variable, are reachable. For other objects,
// pass the object to a call of the KeepAlive function to mark the
// last point in the function where the object must be reachable.
//
// For example, if p points to a struct that contains a file descriptor d,
// and p has a finalizer that closes that file descriptor, and if the last
// use of p in a function is a call to syscall.Write(p.d, buf, size), then
// p may be unreachable as soon as the program enters syscall.Write. The
// finalizer may run at that moment, closing p.d, causing syscall.Write
// to fail because it is writing to a closed file descriptor (or, worse,
// to an entirely different file descriptor opened by a different goroutine).
// To avoid this problem, call runtime.KeepAlive(p) after the call to
// syscall.Write.
//
// 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 := 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)
if base == 0 {
// 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 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.ptrdata != 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")
}
})
}
// Mark KeepAlive as noinline so that it is easily detectable as an intrinsic.
//go:noinline
// KeepAlive marks its argument as currently reachable.
// This ensures that the object is not freed, and its finalizer is not run,
// before the point in the program where KeepAlive is called.
//
// A very simplified example showing where KeepAlive is required:
// type File struct { d int }
// d, err := syscall.Open("/file/path", syscall.O_RDONLY, 0)
// // ... do something if err != nil ...
// p := &File{d}
// runtime.SetFinalizer(p, func(p *File) { syscall.Close(p.d) })
// var buf [10]byte
// n, err := syscall.Read(p.d, buf[:])
// // Ensure p is not finalized until Read returns.
// runtime.KeepAlive(p)
// // No more uses of p after this point.
//
// Without the KeepAlive call, the finalizer could run at the start of
// syscall.Read, closing the file descriptor before syscall.Read makes
// the actual system call.
func KeepAlive(x interface{}) {
// Introduce a use of x that the compiler can't eliminate.
// This makes sure x is alive on entry. We need x to be alive
// on entry for "defer runtime.KeepAlive(x)"; see issue 21402.
if cgoAlwaysFalse {
println(x)
}
}
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