node/bindnode: using Unwrap on some bindnode nodes can panic

[mvdan]

For example, I just tried using it on the nodes returned by MapIterator.Next, and I get:

panic: reflect.Value.Addr of unaddressable value

goroutine 1 [running]:
reflect.Value.Addr({0x58f980?, 0xc00012ac10?, 0x3?})
	/home/mvdan/tip/src/reflect/value.go:273 +0x65
github.com/ipld/go-ipld-prime/node/bindnode.Unwrap({0x604140?, 0xc00010f200?})
	/home/mvdan/go/pkg/mod/github.com/ipld/go-ipld-prime@v0.14.4/node/bindnode/api.go:113 +0x89
main.main()
	/home/mvdan/src/genericstest/main.go:53 +0x2d4
exit status 2

This is most likely because the MapIterator code path doesn't create datamodel.Node values with addressable Go values underneath.

I see three possible fixes:

1. All code paths that create a node should ensure the Go value is addressable
2. Unwrap should make an addressable copy of the value if it's not addressable
3. Alter the API to show that some nodes can't be unwrapped

I don't like option 3, because at best we duplicate API, and at worst we'd break users. It's also harder to have to explain that some nodes aren't like others.

Option 2 would be the easiest, but also slightly inconsistent. If unwrapping one node twice returns two copies, but unwrapping another node doesn't make copies and returns the same pointer twice, then suddenly those two scenarios are very different.

So, option 1 seems best.

[rvagg]

I've tried pretty hard to reproduce this but can't. Various types of both addressable and unaddressable values and unwrapping sub-nodes within a larger data structure, but can't. So I'm going to close this issue after adding #431 which has my attempts at getting to this error.

If someone else manages to come up with a case that causes this problem then we can fix it then.