libbacktrace: use __has_attribute for fallthrough

[official-gcc.git] / libgo / go / 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/goarch"
        "runtime/internal/atomic"
        "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*goarch.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 / goarch.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)
        ft  *functype      // type of fn (unlikely, but may be a heap pointer)
        ot  *ptrtype       // type of ptr to object (may be a heap pointer)
}

func queuefinalizer(p unsafe.Pointer, fn *funcval, ft *functype, 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.
                                // We allocate values of type finalizer in
                                // finblock values. Since these values are
                                // allocated by persistentalloc, they require
                                // special scanning during GC. finptrmask is a
                                // pointer mask to use while scanning.
                                // Since all the values in finalizer are
                                // pointers, just turn all bits on.
                                for i := range finptrmask {
                                        finptrmask[i] = 0xff
                                }
                        }
                }
                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.ft = ft
        f.ot = ot
        f.arg = p
        fingwake = true
        unlock(&finlock)
}

//go:nowritebarrier
func iterate_finq(callback func(*funcval, unsafe.Pointer, *functype, *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.ft, 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
)

func createfing() {
        // start the finalizer goroutine exactly once
        if fingCreate == 0 && atomic.Cas(&fingCreate, 0, 1) {
                expectSystemGoroutine()
                go runfinq()
        }
}

// This is the goroutine that runs all of the finalizers
func runfinq() {
        setSystemGoroutine()

        var (
                ef   eface
                ifac iface
        )

        gp := getg()
        gp.isFinalizerGoroutine = true
        for {
                lock(&finlock)
                fb := finq
                finq = nil
                if fb == nil {
                        fing = gp
                        fingwait = true
                        goparkunlock(&finlock, waitReasonFinalizerWait, traceEvGoBlock, 1)
                        continue
                }
                unlock(&finlock)
                for fb != nil {
                        for i := fb.cnt; i > 0; i-- {
                                f := &fb.fin[i-1]

                                if f.ft == nil {
                                        throw("missing type in runfinq")
                                }
                                fint := f.ft.in[0]
                                var param unsafe.Pointer
                                switch fint.kind & kindMask {
                                case kindPtr:
                                        // direct use of pointer
                                        param = unsafe.Pointer(&f.arg)
                                case kindInterface:
                                        ityp := (*interfacetype)(unsafe.Pointer(fint))
                                        if len(ityp.methods) == 0 {
                                                // set up with empty interface
                                                ef._type = &f.ot.typ
                                                ef.data = f.arg
                                                param = unsafe.Pointer(&ef)
                                        } else {
                                                // convert to interface with methods
                                                // this conversion is guaranteed to succeed - we checked in SetFinalizer
                                                ifac.tab = getitab(fint, &f.ot.typ, true)
                                                ifac.data = f.arg
                                                param = unsafe.Pointer(&ifac)
                                        }
                                default:
                                        throw("bad kind in runfinq")
                                }
                                // This is not a system goroutine while
                                // running the actual finalizer.
                                // This matters because we want this
                                // goroutine to appear in a stack dump
                                // if the finalizer crashes.
                                // The gc toolchain handles this using
                                // a global variable fingRunning,
                                // but we don't need that.
                                gp.isSystemGoroutine = false
                                reflectcall(f.ft, f.fn, false, false, &param, nil)
                                gp.isSystemGoroutine = true

                                // 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, 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 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 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 " + 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, false)

        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 gccgo we have no reliable way to detect them,
                // so we just return.
                return
        }

        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 len(ft.in) != 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.uncommontype == nil || etyp.uncommontype == 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 getitab(fint, etyp, true) == nil {
                        goto okarg
                }
        }
        throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string())
okarg:
        // make sure we have a finalizer goroutine
        createfing()

        systemstack(func() {
                data := f.data
                if !isDirectIface(ftyp) {
                        data = *(*unsafe.Pointer)(data)
                }
                if !addfinalizer(e.data, (*funcval)(data), ft, 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)
        }
}