Defer Panic Recovery

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func createPost(db *gorm.DB) error {
    tx := db.Begin()
    defer tx.Rollback()
    
    if err := tx.Create(&Post{Author: ""}).Error; err != nil {
        return err
    }
    
    return tx.Commit().Error
}

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func main() {
	for i := 0; i < 5; i++ {
		defer fmt.Println(i)
	}
}

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0

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func testDeferValue() {
	startedAt := time.Now()
	defer fmt.Println(time.Since(startedAt))

	time.Sleep(time.Second)
}

func main() {
	testDeferValue()
}

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151ns

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func test() (a int) {
	a = 5
	defer println("direct", a)
	defer func() {
		println("func", a)
	}()
	a = 10
	return a
}

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func 10
direct 5

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type _defer struct {
	siz     int32 // includes both arguments and results
	started bool
	heap    bool
	// openDefer indicates that this _defer is for a frame with open-coded
	// defers. We have only one defer record for the entire frame (which may
	// currently have 0, 1, or more defers active).
	openDefer bool
	sp        uintptr  // sp at time of defer
	pc        uintptr  // pc at time of defer
	fn        *funcval // can be nil for open-coded defers
	_panic    *_panic  // panic that is running defer
	link      *_defer

	// If openDefer is true, the fields below record values about the stack
	// frame and associated function that has the open-coded defer(s). sp
	// above will be the sp for the frame, and pc will be address of the
	// deferreturn call in the function.
	fd   unsafe.Pointer // funcdata for the function associated with the frame
	varp uintptr        // value of varp for the stack frame
	// framepc is the current pc associated with the stack frame. Together,
	// with sp above (which is the sp associated with the stack frame),
	// framepc/sp can be used as pc/sp pair to continue a stack trace via
	// gentraceback().
	framepc uintptr
}

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case ODEFER:
		if Debug_defer > 0 {
			var defertype string
			if s.hasOpenDefers {
				defertype = "open-coded"
			} else if n.Esc == EscNever {
				defertype = "stack-allocated"
			} else {
				defertype = "heap-allocated"
			}
			Warnl(n.Pos, "%s defer", defertype)
		}
		if s.hasOpenDefers {
			s.openDeferRecord(n.Left) // 开放编码
		} else {
			d := callDefer // 堆分配
			if n.Esc == EscNever {
				d = callDeferStack // 栈分配
			}
			s.callResult(n.Left, d)
		}

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func (s *state) call(n *Node, k callKind, returnResultAddr bool) *ssa.Value {
	...
	var call *ssa.Value
	if k == callDeferStack {
		// 在栈上初始化 defer 结构体
		...
	} else {
		...
		switch {
		case k == callDefer:
			aux := ssa.StaticAuxCall(deferproc, ACArgs, ACResults)
			call = s.newValue1A(ssa.OpStaticCall, types.TypeMem, aux, s.mem())
		...
		}
		call.AuxInt = stksize
	}
	s.vars[&memVar] = call
	...
}

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func (s *state) exit() *ssa.Block {
   if s.hasdefer {
   	...
   	s.rtcall(Deferreturn, true, nil)
   }
   ...
}

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func deferproc(siz int32, fn *funcval) {
	sp := getcallersp()
	argp := uintptr(unsafe.Pointer(&fn)) + unsafe.Sizeof(fn)
	callerpc := getcallerpc()

	d := newdefer(siz)
	if d._panic != nil {
		throw("deferproc: d.panic != nil after newdefer")
	}
	d.fn = fn
	d.pc = callerpc
	d.sp = sp
	switch siz {
	case 0:
	case sys.PtrSize:
		*(*uintptr)(deferArgs(d)) = *(*uintptr)(unsafe.Pointer(argp))
	default:
		memmove(deferArgs(d), unsafe.Pointer(argp), uintptr(siz))
	}

	return0()
}

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func newdefer(siz int32) *_defer {
	var d *_defer
	sc := deferclass(uintptr(siz))
	gp := getg()
	if sc < uintptr(len(p{}.deferpool)) {
		pp := gp.m.p.ptr()
		if len(pp.deferpool[sc]) == 0 && sched.deferpool[sc] != nil {
			for len(pp.deferpool[sc]) < cap(pp.deferpool[sc])/2 && sched.deferpool[sc] != nil {
				d := sched.deferpool[sc]
				sched.deferpool[sc] = d.link
				pp.deferpool[sc] = append(pp.deferpool[sc], d)
			}
		}
		if n := len(pp.deferpool[sc]); n > 0 {
			d = pp.deferpool[sc][n-1]
			pp.deferpool[sc][n-1] = nil
			pp.deferpool[sc] = pp.deferpool[sc][:n-1]
		}
	}
	if d == nil {
		total := roundupsize(totaldefersize(uintptr(siz)))
		d = (*_defer)(mallocgc(total, deferType, true))
	}
	d.siz = siz
	d.link = gp._defer
	gp._defer = d
	return d
}

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func deferreturn(arg0 uintptr) {
	gp := getg()
	d := gp._defer
	if d == nil {
		return
	}
	sp := getcallersp()
	...

	switch d.siz {
	case 0:
	case sys.PtrSize:
		*(*uintptr)(unsafe.Pointer(&arg0)) = *(*uintptr)(deferArgs(d))
	default:
		memmove(unsafe.Pointer(&arg0), deferArgs(d), uintptr(d.siz))
	}
	fn := d.fn
	gp._defer = d.link
	freedefer(d)
	jmpdefer(fn, uintptr(unsafe.Pointer(&arg0)))
}

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func (s *state) call(n *Node, k callKind) *ssa.Value {
	...
	var call *ssa.Value
	if k == callDeferStack {
		// 在栈上创建 _defer 结构体
		t := deferstruct(stksize)
		...

		ACArgs = append(ACArgs, ssa.Param{Type: types.Types[TUINTPTR], Offset: int32(Ctxt.FixedFrameSize())})
		aux := ssa.StaticAuxCall(deferprocStack, ACArgs, ACResults) // 调用 deferprocStack
		arg0 := s.constOffPtrSP(types.Types[TUINTPTR], Ctxt.FixedFrameSize())
		s.store(types.Types[TUINTPTR], arg0, addr)
		call = s.newValue1A(ssa.OpStaticCall, types.TypeMem, aux, s.mem())
		call.AuxInt = stksize
	} else {
		...
	}
	s.vars[&memVar] = call
	...
}

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func deferprocStack(d *_defer) {
	gp := getg()
	d.started = false
	d.heap = false // 栈上分配的 _defer
	d.openDefer = false
	d.sp = getcallersp()
	d.pc = getcallerpc()
	d.framepc = 0
	d.varp = 0
	*(*uintptr)(unsafe.Pointer(&d._panic)) = 0
	*(*uintptr)(unsafe.Pointer(&d.fd)) = 0
	*(*uintptr)(unsafe.Pointer(&d.link)) = uintptr(unsafe.Pointer(gp._defer))
	*(*uintptr)(unsafe.Pointer(&gp._defer)) = uintptr(unsafe.Pointer(d))

	return0()
}

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const maxOpenDefers = 8

func walkstmt(n *Node) *Node {
	switch n.Op {
	case ODEFER:
		Curfn.Func.SetHasDefer(true)
		Curfn.Func.numDefers++
		if Curfn.Func.numDefers > maxOpenDefers {
			Curfn.Func.SetOpenCodedDeferDisallowed(true)
		}
		if n.Esc != EscNever {
			Curfn.Func.SetOpenCodedDeferDisallowed(true)
		}
		fallthrough
	...
	}
}

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func buildssa(fn *Node, worker int) *ssa.Func {
	...
	s.hasOpenDefers = s.hasdefer && !s.curfn.Func.OpenCodedDeferDisallowed()
	...
	if s.hasOpenDefers &&
		s.curfn.Func.numReturns*s.curfn.Func.numDefers > 15 {
		s.hasOpenDefers = false
	}
	...
}

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func buildssa(fn *Node, worker int) *ssa.Func {
	...
	if s.hasOpenDefers {
		deferBitsTemp := tempAt(src.NoXPos, s.curfn, types.Types[TUINT8]) // 初始化延迟比特
		s.deferBitsTemp = deferBitsTemp
		startDeferBits := s.entryNewValue0(ssa.OpConst8, types.Types[TUINT8])
		s.vars[&deferBitsVar] = startDeferBits
		s.deferBitsAddr = s.addr(deferBitsTemp)
		s.store(types.Types[TUINT8], s.deferBitsAddr, startDeferBits)
		s.vars[&memVar] = s.newValue1Apos(ssa.OpVarLive, types.TypeMem, deferBitsTemp, s.mem(), false)
	}
}

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deferBits := 0 // 初始化 deferBits

_f1, _a1 := f1, a1  // 保存函数以及参数
deferBits |= 1 << 0 // 将 deferBits 最后一位置位 1

if condition {
    _f2, _a2 := f2, a2  // 保存函数以及参数
    deferBits |= 1 << 1 // 将 deferBits 倒数第二位置位 1
}
exit:

if deferBits & 1 << 1 != 0 {
    deferBits &^= 1 << 1
    _f2(a2)
}

if deferBits & 1 << 0 != 0 {
    deferBits &^= 1 << 0
    _f1(a1)
}

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type openDeferInfo struct {
	n           *Node
	closure     *ssa.Value
	closureNode *Node
	rcvr        *ssa.Value
	rcvrNode    *Node
	argVals     []*ssa.Value
	argNodes    []*Node
}

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func deferreturn(arg0 uintptr) {
	gp := getg()
	d := gp._defer
	sp := getcallersp()
	if d.openDefer {
		runOpenDeferFrame(gp, d)
		gp._defer = d.link
		freedefer(d)
		return
	}
	...
}

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func runOpenDeferFrame(gp *g, d *_defer) bool {
	fd := d.fd

	...
	deferBitsOffset, fd := readvarintUnsafe(fd)
	nDefers, fd := readvarintUnsafe(fd)
	deferBits := *(*uint8)(unsafe.Pointer(d.varp - uintptr(deferBitsOffset)))

	for i := int(nDefers) - 1; i >= 0; i-- {
		var argWidth, closureOffset, nArgs uint32 // 读取函数的地址和参数信息
		argWidth, fd = readvarintUnsafe(fd)
		closureOffset, fd = readvarintUnsafe(fd)
		nArgs, fd = readvarintUnsafe(fd)
		if deferBits&(1<<i) == 0 {
			...
			continue
		}
		closure := *(**funcval)(unsafe.Pointer(d.varp - uintptr(closureOffset)))
		d.fn = closure

		...

		deferBits = deferBits &^ (1 << i)
		*(*uint8)(unsafe.Pointer(d.varp - uintptr(deferBitsOffset))) = deferBits
		p := d._panic
		reflectcallSave(p, unsafe.Pointer(closure), deferArgs, argWidth)
		if p != nil && p.aborted {
			break
		}
		d.fn = nil
		memclrNoHeapPointers(deferArgs, uintptr(argWidth))
		...
	}
	return done
}

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package main

import (
	"time"
)

func main() {
  println("start main")
	defer println("in main")
	testChan := make(chan int, 10)
	go func() {
		for {
			select {
			case val, ok := <-testChan:
				println(val, ok)
			}
		}
	}()
	go func() {
		defer println("in goroutine")
		panic("")
	}()

	time.Sleep(1 * time.Second)
}

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start main
in goroutine
panic: 

goroutine 6 [running]:
main.main.func2()
        /Users/hxsup/go/src/github.com/hTangle/test-go/main.go:20 +0x7b
created by main.main
        /Users/hxsup/go/src/github.com/hTangle/test-go/main.go:18 +0xd3

goroutine 1 [sleep]:
time.Sleep(0x3b9aca00)
        /usr/local/go/src/runtime/time.go:193 +0x12e
main.main()
        /Users/hxsup/go/src/github.com/hTangle/test-go/main.go:23 +0xdd

goroutine 5 [chan receive]:
main.main.func1()
        /Users/hxsup/go/src/github.com/hTangle/test-go/main.go:13 +0x27
created by main.main
        /Users/hxsup/go/src/github.com/hTangle/test-go/main.go:10 +0xc5

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// A _panic holds information about an active panic.
//
// A _panic value must only ever live on the stack.
//
// The argp and link fields are stack pointers, but don't need special
// handling during stack growth: because they are pointer-typed and
// _panic values only live on the stack, regular stack pointer
// adjustment takes care of them.
type _panic struct {
	argp      unsafe.Pointer // pointer to arguments of deferred call run during panic; cannot move - known to liblink
	arg       interface{}    // argument to panic
	link      *_panic        // link to earlier panic
	pc        uintptr        // where to return to in runtime if this panic is bypassed
	sp        unsafe.Pointer // where to return to in runtime if this panic is bypassed
	recovered bool           // whether this panic is over
	aborted   bool           // the panic was aborted
	goexit    bool
}

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func gopanic(e interface{}) {
	gp := getg()
	...
	var p _panic
	p.arg = e
	p.link = gp._panic
	gp._panic = (*_panic)(noescape(unsafe.Pointer(&p)))

	for {
		d := gp._defer
		if d == nil {
			break
		}

		d._panic = (*_panic)(noescape(unsafe.Pointer(&p)))

		reflectcall(nil, unsafe.Pointer(d.fn), deferArgs(d), uint32(d.siz), uint32(d.siz))

		d._panic = nil
		d.fn = nil
		gp._defer = d.link

		freedefer(d)
		if p.recovered {
			...
		}
	}

	fatalpanic(gp._panic)
	*(*int)(nil) = 0
}

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func fatalpanic(msgs *_panic) {
	pc := getcallerpc()
	sp := getcallersp()
	gp := getg()

	if startpanic_m() && msgs != nil {
		atomic.Xadd(&runningPanicDefers, -1)
		printpanics(msgs)
	}
	if dopanic_m(gp, pc, sp) {
		crash()
	}

	exit(2)
}

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func gorecover(argp uintptr) interface{} {
	gp := getg()
	p := gp._panic
	if p != nil && !p.recovered && argp == uintptr(p.argp) {
		p.recovered = true
		return p.arg
	}
	return nil
}

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func gopanic(e interface{}) {
	...

	for {
		// 执行延迟调用函数，可能会设置 p.recovered = true
		...

		pc := d.pc
		sp := unsafe.Pointer(d.sp)

		...
		if p.recovered {
			gp._panic = p.link
			for gp._panic != nil && gp._panic.aborted {
				gp._panic = gp._panic.link
			}
			if gp._panic == nil {
				gp.sig = 0
			}
			gp.sigcode0 = uintptr(sp)
			gp.sigcode1 = pc
			mcall(recovery)
			throw("recovery failed")
		}
	}
	...
}

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func deferproc(siz int32, fn *funcval) {
	...
	return0()
}