Fix #79364: When copy empty array, next key is unspecified

[cmb69]

We must not forget to keep the nNextFreeElement when duplicating
empty arrays.

[nikic]

I believe the correct behavior is the other way around: We should always reset the next free element when copying arrays.

[nikic]

Or maybe not, who knows. Is there a benefit for going in one direction or the other?

[nikic]

As we do copy the internal array pointer, which is a very similar bit of hidden state, I guess this is okay.

For master, we should maybe investigate how expensive it is to have an "always correct" next free element...

[cmb69]

Thanks! Applied as 2462f2d.

For master, we should maybe investigate how expensive it is to have an "always correct" next free element...

Well, I think the current behavior (modulo this bug, which is about a very special case, namely empty arrays with nNextFreeElement != 0) is quite desireable, because a copy (of an array) should be independent from the original. E.g. https://3v4l.org/872qn should always produce the same output.

[nikic]

@cmb69 Sorry, I was a bit unclear there: What I meant by "always correct" is to adjust the next free element on unset, so it actually stays the next free element, without holes.

[cmb69]

Ah, thanks! I agree that this is something worth to be investigated. :)