[gostls13.git] / src / runtime / mfinal.go

// 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 (
        "internal/abi"
        "internal/goarch"
        "internal/goexperiment"
        "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).
type finblock struct {
        _       sys.NotInHeap
        alllink *finblock
        next    *finblock
        cnt     uint32
        _       int32
        fin     [(_FinBlockSize - 2*goarch.PtrSize - 2*4) / unsafe.Sizeof(finalizer{})]finalizer
}

var fingStatus atomic.Uint32

// finalizer goroutine status.
const (
        fingUninitialized uint32 = iota
        fingCreated       uint32 = 1 << (iota - 1)
        fingRunningFinalizer
        fingWait
        fingWake
)

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 / goarch.PtrSize / 8]byte

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,
}

// lockRankMayQueueFinalizer records the lock ranking effects of a
// function that may call queuefinalizer.
func lockRankMayQueueFinalizer() {
        lockWithRankMayAcquire(&finlock, getLockRank(&finlock))
}

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.gcMiscSys))
                        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*goarch.PtrSize ||
                                        unsafe.Offsetof(finalizer{}.fn) != 0 ||
                                        unsafe.Offsetof(finalizer{}.arg) != goarch.PtrSize ||
                                        unsafe.Offsetof(finalizer{}.nret) != 2*goarch.PtrSize ||
                                        unsafe.Offsetof(finalizer{}.fint) != 3*goarch.PtrSize ||
                                        unsafe.Offsetof(finalizer{}.ot) != 4*goarch.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
        unlock(&finlock)
        fingStatus.Or(fingWake)
}

//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 {
        if ok := fingStatus.CompareAndSwap(fingCreated|fingWait|fingWake, fingCreated); ok {
                return fing
        }
        return nil
}

func createfing() {
        // start the finalizer goroutine exactly once
        if fingStatus.Load() == fingUninitialized && fingStatus.CompareAndSwap(fingUninitialized, fingCreated) {
                go runfinq()
        }
}

func finalizercommit(gp *g, lock unsafe.Pointer) bool {
        unlock((*mutex)(lock))
        // fingStatus should be modified after fing is put into a waiting state
        // to avoid waking fing in running state, even if it is about to be parked.
        fingStatus.Or(fingWait)
        return true
}

// This is the goroutine that runs all of the finalizers.
func runfinq() {
        var (
                frame    unsafe.Pointer
                framecap uintptr
                argRegs  int
        )

        gp := getg()
        lock(&finlock)
        fing = gp
        unlock(&finlock)

        for {
                lock(&finlock)
                fb := finq
                finq = nil
                if fb == nil {
                        gopark(finalizercommit, unsafe.Pointer(&finlock), waitReasonFinalizerWait, traceBlockSystemGoroutine, 1)
                        continue
                }
                argRegs = intArgRegs
                unlock(&finlock)
                if raceenabled {
                        racefingo()
                }
                for fb != nil {
                        for i := fb.cnt; i > 0; i-- {
                                f := &fb.fin[i-1]

                                var regs abi.RegArgs
                                // The args may be passed in registers or on stack. Even for
                                // the register case, we still need the spill slots.
                                // TODO: revisit if we remove spill slots.
                                //
                                // Unfortunately because we can have an arbitrary
                                // amount of returns and it would be complex to try and
                                // figure out how many of those can get passed in registers,
                                // just conservatively assume none of them do.
                                framesz := unsafe.Sizeof((any)(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")
                                }
                                r := frame
                                if argRegs > 0 {
                                        r = unsafe.Pointer(&regs.Ints)
                                } else {
                                        // 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)(r) = f.arg
                                case kindInterface:
                                        ityp := (*interfacetype)(unsafe.Pointer(f.fint))
                                        // set up with empty interface
                                        (*eface)(r)._type = &f.ot.Type
                                        (*eface)(r).data = f.arg
                                        if len(ityp.Methods) != 0 {
                                                // convert to interface with methods
                                                // this conversion is guaranteed to succeed - we checked in SetFinalizer
                                                (*iface)(r).tab = assertE2I(ityp, (*eface)(r)._type)
                                        }
                                default:
                                        throw("bad kind in runfinq")
                                }
                                fingStatus.Or(fingRunningFinalizer)
                                reflectcall(nil, unsafe.Pointer(f.fn), frame, uint32(framesz), uint32(framesz), uint32(framesz), &regs)
                                fingStatus.And(^fingRunningFinalizer)

                                // 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 isGoPointerWithoutSpan(p unsafe.Pointer) bool {
        // 0-length objects are okay.
        if p == unsafe.Pointer(&zerobase) {
                return true
        }

        // 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(p) && uintptr(p) < datap.enoptrdata ||
                        datap.data <= uintptr(p) && uintptr(p) < datap.edata ||
                        datap.bss <= uintptr(p) && uintptr(p) < datap.ebss ||
                        datap.noptrbss <= uintptr(p) && uintptr(p) < datap.enoptrbss {
                        return true
                }
        }
        return false
}

// 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, because it may share same address with other zero-size
// objects in memory. See https://go.dev/ref/spec#Size_and_alignment_guarantees.
//
// 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.
//
// Note that because finalizers may execute arbitrarily far into the future
// after an object is no longer referenced, the runtime is allowed to perform
// a space-saving optimization that batches objects together in a single
// allocation slot. The finalizer for an unreferenced object in such an
// allocation may never run if it always exists in the same batch as a
// referenced object. Typically, this batching only happens for tiny
// (on the order of 16 bytes or less) and pointer-free objects.
//
// 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, such as os.File, 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 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.
//
// In the terminology of the Go memory model, a call
// SetFinalizer(x, f) “synchronizes before” the finalization call f(x).
// However, there is no guarantee that KeepAlive(x) or any other use of x
// “synchronizes before” f(x), so in general a finalizer should use a mutex
// or other synchronization mechanism if it needs to access mutable state in x.
// For example, consider a finalizer that inspects a mutable field in x
// that is modified from time to time in the main program before x
// becomes unreachable and the finalizer is invoked.
// The modifications in the main program and the inspection in the finalizer
// need to use appropriate synchronization, such as mutexes or atomic updates,
// to avoid read-write races.
func SetFinalizer(obj any, finalizer any) {
        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 " + toRType(etyp).string() + ", not pointer")
        }
        ot := (*ptrtype)(unsafe.Pointer(etyp))
        if ot.Elem == nil {
                throw("nil elem type!")
        }

        if inUserArenaChunk(uintptr(e.data)) {
                // Arena-allocated objects are not eligible for finalizers.
                throw("runtime.SetFinalizer: first argument was allocated into an arena")
        }

        // find the containing object
        base, span, _ := findObject(uintptr(e.data), 0, 0)

        if base == 0 {
                if isGoPointerWithoutSpan(e.data) {
                        return
                }
                throw("runtime.SetFinalizer: pointer not in allocated block")
        }

        // Move base forward if we've got an allocation header.
        if goexperiment.AllocHeaders && !span.spanclass.noscan() && !heapBitsInSpan(span.elemsize) && span.spanclass.sizeclass() != 0 {
                base += mallocHeaderSize
        }

        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.PtrBytes != 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 " + toRType(ftyp).string() + ", not a function")
        }
        ft := (*functype)(unsafe.Pointer(ftyp))
        if ft.IsVariadic() {
                throw("runtime.SetFinalizer: cannot pass " + toRType(etyp).string() + " to finalizer " + toRType(ftyp).string() + " because dotdotdot")
        }
        if ft.InCount != 1 {
                throw("runtime.SetFinalizer: cannot pass " + toRType(etyp).string() + " to finalizer " + toRType(ftyp).string())
        }
        fint := ft.InSlice()[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.Methods) == 0 {
                        // ok - satisfies empty interface
                        goto okarg
                }
                if itab := assertE2I2(ityp, efaceOf(&obj)._type); itab != nil {
                        goto okarg
                }
        }
        throw("runtime.SetFinalizer: cannot pass " + toRType(etyp).string() + " to finalizer " + toRType(ftyp).string())
okarg:
        // compute size needed for return parameters
        nret := uintptr(0)
        for _, t := range ft.OutSlice() {
                nret = alignUp(nret, uintptr(t.Align_)) + t.Size_
        }
        nret = alignUp(nret, goarch.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.
//
// Note: KeepAlive should only be used to prevent finalizers from
// running prematurely. In particular, when used with [unsafe.Pointer],
// the rules for valid uses of unsafe.Pointer still apply.
func KeepAlive(x any) {
        // 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)
        }
}