runtime: incorrect unwind in panicwrap

[rsc]

the panicwrap code does not clean up properly on panic (ironically).

http://play.golang.org/p/7bR-7ZA1VU

package main

import "runtime"

func main() {
    test11()
}

type Tiny struct{}

func (Tiny) M() {
    panic("asdf")
}

// calling i.M increments panicwrap, but then i.M panics, and panicwrap
// is never restored. because panicwrap is now wrong, the recover
// in T3 after the call to badstate does not see the panic that it should.
func badstate() {
    defer func() {
        println(recover())
    }()
    var i I = Tiny{}
    i.M()
}

// -- code below this line copied from test/recover.go except marked line--

// tiny receiver, so basic wrapper in i.M()
type T3 struct{}

func (T3) M() {
    badstate() // THIS IS NEW!
    mustRecoverBody(doubleRecover(), recover(), recover(), 11)
}

func test11() {
    var i I = T3{}
    defer i.M()
    panic(11)
}

type I interface {
    M()
}

func mustRecoverBody(v1, v2, v3, x interface{}) {
    v := v1
    if v != nil {
        println("spurious recover", v)
        die()
    }
    v = v2
    if v == nil {
        println("missing recover")
        die() // panic is useless here
    }
    if v != x {
        println("wrong value", v, x)
        die()
    }

    // the value should be gone now regardless
    v = v3
    if v != nil {
        println("recover didn't recover")
        die()
    }
}

func doubleRecover() interface{} {
    return recover()
}

func mustRecover(x interface{}) {
    mustRecoverBody(doubleRecover(), recover(), recover(), x)
}

func die() {
    runtime.Breakpoint() // can't depend on panic
}

[dvyukov]

Comment 1:

I am not sure whether it's the same bug, but I've just discovered and fixed memory
corruption and memory leak in recursive panic handling. I will send a fix soon.
Reproducer is:
func main() {
    func() {
        defer func() {
            fmt.Println(recover())
        }()
        var x [8192]byte
        func(x [8192]byte) {
            defer func() {
                if err := recover(); err != nil {
                    panic("1: " + err.(string))
                }
            }()
            func(x [8192]byte) {
                defer func() {
                    if err := recover(); err != nil {
                        panic("2: " + err.(string))
                    }
                }()
                panic("bad")
            }(x)
        }(x)
    }()
    panic("again")
}

[dvyukov]

Comment 2:

maybe it's a different one, I do not see how panicwrap is involved in my bug

[rsc]

Comment 3:

This is too uncommon a case to try to fix for Go 1.3 at this point. It has literally
never come up in practice.

Labels changed: added release-go1.4, removed release-go1.3.

[gopherbot]

Comment 4:

CL https://golang.org/cl/135490044 mentions this issue.

[rsc]

Comment 5:

This issue was closed by revision 8473695.

Status changed to Fixed.