[gnovm] chain Panics

[jaekwon]

Description

I don't think the current machine.Panic() handles recursive panics as it should.

// machine.go
func (m *Machine) Panic(ex TypedValue) {
        // TODO: chain exceptions if preexisting unrecovered exception.
        m.Exception = &ex
        m.PopUntilLastCallFrame()
        m.PushOp(OpPanic2)
        m.PushOp(OpReturnCallDefers)
}

vs https://groups.google.com/g/golang-nuts/c/HOXNBQu5c-Q/m/ltQ-QHBrw9gJ

The golang nuts link includes examples that should be added to our filetests, and failure there will prove this issue.

Recommend discussing spec before implementation for feedback!

[tbruyelle]

I noticed that and it was on my TODO list :)

About the examples in the golang nuts forum, I noticed this one, is it the one you're talking about ?

func f1() (success bool) {
	defer func() { success = recover() == nil }()
	defer f2()
	panic("panicking")
}

func f2() {
	defer func() { _ = recover() }()
}

func f3() { _ = recover() }

Does f1() return true or false? I'd expect false, since it wouldn't
make much sense for a panic to be caught several levels up the call
stack. But then, if you replace f2 with f3, I'd expect true, since f2
is at the same depth as a closure would be, so it it's a matter of
depth wrt the panicker, it should be equivalent. However, if it's
something that gets wrapped by the closure, I'd expect false.

Essentially, how does pancking propogate upward?

[jaekwon]

The first post from Rob had a few examples.
https://groups.google.com/g/golang-nuts/c/HOXNBQu5c-Q/m/Je0qo1hbxIsJ

But we should run that too and others to verify the problem and test the solution!

[tbruyelle]

Thanks I'll work on that!

[tbruyelle]

Somehow unrelated with the TODO, but in Rob's examples I found something that isn't supported in Gno :

https://go.dev/play/p/CcMGgY606O-

Basically a recover() that is invoked in a deferred closure that is absent from the stack at the time of the panic(), should return nil. In Gno it returns the panic argument. I'll prepare a PR to add this in the tests/challenges folder, with other stuffs.

[tbruyelle]

@jaekwon FYI the PR that resolves this issue is ready for review : #771