// Copyright 2023 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.

package runtime

import (
	"runtime/internal/atomic"
	"unsafe"
)

// Pinner represents a set of pinned Go objects. An object can be pinned with
// the Pin method and all pinned objects of a Pinner can be unpinned with the
// Unpin method.
type Pinner struct {
	*pinner
}

// Pin a Go object. The object will not be moved or freed by the garbage
// collector until the Unpin method has been called. The pointer to a pinned
// object can be directly stored in C memory or can be contained in Go memory
// passed to C functions. If the pinned object iftself contains pointers to Go
// objects, these objects must be pinned separately if they are going to be
// accessed from C code. The argument must be a pointer of any type or an
// unsafe.Pointer. It 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. If one of these conditions is not met, Pin will panic.
func (p *Pinner) Pin(pointer any) {
	if p.pinner == nil {
		p.pinner = new(pinner)
		SetFinalizer(p.pinner, func(i *pinner) {
			if i.refs != nil {
				i.unpin() // only required to make the test idempotent
				pinnerLeakPanic()
			}
		})
	}
	ptr := pinnerGetPtr(&pointer)

	setPinned(ptr, true)
	p.refs = append(p.refs, ptr)
}

// Unpin all pinned objects of the Pinner.
func (p *Pinner) Unpin() {
	p.pinner.unpin()
}

type pinner struct {
	refs []unsafe.Pointer
}

func (p *pinner) unpin() {
	if p == nil || p.refs == nil {
		return
	}
	for i := range p.refs {
		setPinned(p.refs[i], false)
		p.refs[i] = nil
	}
	p.refs = nil
}

func pinnerGetPtr(i *any) unsafe.Pointer {
	e := efaceOf(i)
	etyp := e._type
	if etyp == nil {
		panic(errorString("runtime.Pinner: argument is nil"))
	}
	if kind := etyp.Kind_ & kindMask; kind != kindPtr && kind != kindUnsafePointer {
		panic(errorString("runtime.Pinner: argument is not a pointer: " + toRType(etyp).string()))
	}
	if inUserArenaChunk(uintptr(e.data)) {
		// Arena-allocated objects are not eligible for pinning.
		panic(errorString("runtime.Pinner: object was allocated into an arena"))
	}
	return e.data
}

// isPinned checks if a Go pointer is pinned.
// nosplit, because it's called from nosplit code in cgocheck.
//
//go:nosplit
func isPinned(ptr unsafe.Pointer) bool {
	span := spanOfHeap(uintptr(ptr))
	if span == nil {
		// this code is only called for Go pointer, so this must be a
		// linker-allocated global object.
		return true
	}
	pinnerBits := span.getPinnerBits()
	// these pinnerBits might get unlinked by a concurrently running sweep, but
	// that's OK because gcBits don't get cleared until the following GC cycle
	// (nextMarkBitArenaEpoch)
	if pinnerBits == nil {
		return false
	}
	objIndex := span.objIndex(uintptr(ptr))
	pinState := pinnerBits.ofObject(objIndex)
	KeepAlive(ptr) // make sure ptr is alive until we are done so the span can't be freed
	return pinState.isPinned()
}

// setPinned marks or unmarks a Go pointer as pinned.
func setPinned(ptr unsafe.Pointer, pin bool) {
	span := spanOfHeap(uintptr(ptr))
	if span == nil {
		if isGoPointerWithoutSpan(ptr) {
			// this is a linker-allocated or zero size object, nothing to do.
			return
		}
		panic(errorString("runtime.Pinner.Pin: argument is not a Go pointer"))
	}

	// ensure that the span is swept, b/c sweeping accesses the specials list
	// w/o locks.
	mp := acquirem()
	span.ensureSwept()
	KeepAlive(ptr) // make sure ptr is still alive after span is swept

	objIndex := span.objIndex(uintptr(ptr))

	lock(&span.speciallock) // guard against concurrent calls of setPinned on same span

	pinnerBits := span.getPinnerBits()
	if pinnerBits == nil {
		pinnerBits = span.newPinnerBits()
		span.setPinnerBits(pinnerBits)
	}
	pinState := pinnerBits.ofObject(objIndex)
	if pin {
		if pinState.isPinned() {
			// multiple pins on same object, set multipin bit
			pinState.setMultiPinned(true)
			// and increase the pin counter
			// TODO(mknyszek): investigate if systemstack is necessary here
			systemstack(func() {
				offset := objIndex * span.elemsize
				span.incPinCounter(offset)
			})
		} else {
			// set pin bit
			pinState.setPinned(true)
		}
	} else {
		// unpin
		if pinState.isPinned() {
			if pinState.isMultiPinned() {
				var exists bool
				// TODO(mknyszek): investigate if systemstack is necessary here
				systemstack(func() {
					offset := objIndex * span.elemsize
					exists = span.decPinCounter(offset)
				})
				if !exists {
					// counter is 0, clear multipin bit
					pinState.setMultiPinned(false)
				}
			} else {
				// no multipins recorded. unpin object.
				pinState.setPinned(false)
			}
		} else {
			// unpinning unpinned object, bail out
			throw("runtime.Pinner: object already unpinned")
		}
	}
	unlock(&span.speciallock)
	releasem(mp)
	return
}

type pinState struct {
	bytep   *uint8
	byteVal uint8
	mask    uint8
}

// nosplit, because it's called by isPinned, which is nosplit
//
//go:nosplit
func (v *pinState) isPinned() bool {
	return (v.byteVal & v.mask) != 0
}

func (v *pinState) isMultiPinned() bool {
	return (v.byteVal & (v.mask << 1)) != 0
}

func (v *pinState) setPinned(val bool) {
	v.set(val, false)
}

func (v *pinState) setMultiPinned(val bool) {
	v.set(val, true)
}

// set sets the pin bit of the pinState to val. If multipin is true, it
// sets/unsets the multipin bit instead.
func (v *pinState) set(val bool, multipin bool) {
	mask := v.mask
	if multipin {
		mask <<= 1
	}
	if val {
		atomic.Or8(v.bytep, mask)
	} else {
		atomic.And8(v.bytep, ^mask)
	}
}

// pinnerBits is the same type as gcBits but has different methods.
type pinnerBits gcBits

// ofObject returns the pinState of the n'th object.
// nosplit, because it's called by isPinned, which is nosplit
//
//go:nosplit
func (p *pinnerBits) ofObject(n uintptr) pinState {
	bytep, mask := (*gcBits)(p).bitp(n * 2)
	byteVal := atomic.Load8(bytep)
	return pinState{bytep, byteVal, mask}
}

func (s *mspan) pinnerBitSize() uintptr {
	return divRoundUp(s.nelems*2, 8)
}

// newPinnerBits returns a pointer to 8 byte aligned bytes to be used for this
// span's pinner bits. newPinneBits is used to mark objects that are pinned.
// They are copied when the span is swept.
func (s *mspan) newPinnerBits() *pinnerBits {
	return (*pinnerBits)(newMarkBits(s.nelems * 2))
}

// nosplit, because it's called by isPinned, which is nosplit
//
//go:nosplit
func (s *mspan) getPinnerBits() *pinnerBits {
	return (*pinnerBits)(atomic.Loadp(unsafe.Pointer(&s.pinnerBits)))
}

func (s *mspan) setPinnerBits(p *pinnerBits) {
	atomicstorep(unsafe.Pointer(&s.pinnerBits), unsafe.Pointer(p))
}

// refreshPinnerBits replaces pinnerBits with a fresh copy in the arenas for the
// next GC cycle. If it does not contain any pinned objects, pinnerBits of the
// span is set to nil.
func (s *mspan) refreshPinnerBits() {
	p := s.getPinnerBits()
	if p == nil {
		return
	}

	hasPins := false
	bytes := alignUp(s.pinnerBitSize(), 8)

	// Iterate over each 8-byte chunk and check for pins. Note that
	// newPinnerBits guarantees that pinnerBits will be 8-byte aligned, so we
	// don't have to worry about edge cases, irrelevant bits will simply be
	// zero.
	for _, x := range unsafe.Slice((*uint64)(unsafe.Pointer(&p.x)), bytes/8) {
		if x != 0 {
			hasPins = true
			break
		}
	}

	if hasPins {
		newPinnerBits := s.newPinnerBits()
		memmove(unsafe.Pointer(&newPinnerBits.x), unsafe.Pointer(&p.x), bytes)
		s.setPinnerBits(newPinnerBits)
	} else {
		s.setPinnerBits(nil)
	}
}

// incPinCounter is only called for multiple pins of the same object and records
// the _additional_ pins.
func (span *mspan) incPinCounter(offset uintptr) {
	var rec *specialPinCounter
	ref, exists := span.specialFindSplicePoint(offset, _KindSpecialPinCounter)
	if !exists {
		lock(&mheap_.speciallock)
		rec = (*specialPinCounter)(mheap_.specialPinCounterAlloc.alloc())
		unlock(&mheap_.speciallock)
		// splice in record, fill in offset.
		rec.special.offset = uint16(offset)
		rec.special.kind = _KindSpecialPinCounter
		rec.special.next = *ref
		*ref = (*special)(unsafe.Pointer(rec))
		spanHasSpecials(span)
	} else {
		rec = (*specialPinCounter)(unsafe.Pointer(*ref))
	}
	rec.counter++
}

// decPinCounter decreases the counter. If the counter reaches 0, the counter
// special is deleted and false is returned. Otherwise true is returned.
func (span *mspan) decPinCounter(offset uintptr) bool {
	ref, exists := span.specialFindSplicePoint(offset, _KindSpecialPinCounter)
	if !exists {
		throw("runtime.Pinner: decreased non-existing pin counter")
	}
	counter := (*specialPinCounter)(unsafe.Pointer(*ref))
	counter.counter--
	if counter.counter == 0 {
		*ref = counter.special.next
		if span.specials == nil {
			spanHasNoSpecials(span)
		}
		lock(&mheap_.speciallock)
		mheap_.specialPinCounterAlloc.free(unsafe.Pointer(counter))
		unlock(&mheap_.speciallock)
		return false
	}
	return true
}

// only for tests
func pinnerGetPinCounter(addr unsafe.Pointer) *uintptr {
	_, span, objIndex := findObject(uintptr(addr), 0, 0)
	offset := objIndex * span.elemsize
	t, exists := span.specialFindSplicePoint(offset, _KindSpecialPinCounter)
	if !exists {
		return nil
	}
	counter := (*specialPinCounter)(unsafe.Pointer(*t))
	return &counter.counter
}

// to be able to test that the GC panics when a pinned pointer is leaking, this
// panic function is a variable, that can be overwritten by a test.
var pinnerLeakPanic = func() {
	panic(errorString("runtime.Pinner: found leaking pinned pointer; forgot to call Unpin()?"))
}