Fix s2b

[YenForYang]

Ensure len(b) <= cap(b) at all times. Additionally, use runtime.KeepAlive() to prevent s from being GC-ed.

A likely better solution (which requires Go 1.17) would use:

b = unsafe.Slice((*byte)(unsafe.Pointer((*reflect.StringHeader)(unsafe.Pointer(&s)).Data)), len(s))

[erikdubbelboer]

I'm not completely sure if runtime.KeepAlive is really required as s is an argument so it should be kept alive by the caller right?

But it can't hurt either since it's a no-op (only does something compile time).

The len(b) <= cap(b) might not do anything either right now either, does the garbage collector look at this? But it is more correct and might prevent some future bugs.

I'm curious how you came up with these fixes?

[zhangyunhao116]

AFAIK, the GC will recognize the unsafe.Pointer instead of other steps. For example if copy stack happens, the runtime will move the slice to a new stack, because it see the unsafe.Pointer. Looks like the runtime.KeepAlive is meaningless, because this function return immediately.

from doc:
Note: KeepAlive should only be used to prevent finalizers from running prematurely. In particular, when used with unsafe.Pointer, the rules for valid uses of unsafe.Pointer still apply.

ref: https://pkg.go.dev/runtime#KeepAlive