feat: add support for empty immutable shared arrays

[mszabo-wikia]

Why:

• Since PHP 7.3, it's possible for extensions to create zvals backed by an immutable shared hashtable via the ZVAL_EMPTY_ARRAY macro.
• This helps avoid redundant hashtable allocations when returning empty arrays back to userland PHP code, and could likewise be beneficial for Rust extensions too.

What:

• Add ZendHashTable::new_empty_immutable() to obtain a ZendHashTable that is actually an empty immutable shared hashtable.
• Add ZendHashTable::is_immutable(). Use it to avoid attempting to free the immutable shared hashtable on drop, and to set appropriate type flags when initializing a zval with a ZendHashTable.
• Make ZendHashTable's TryFrom implementations from Vec and HashMap return an empty immutable shared hashtable if the input collection was empty. Although this would allow every user to automatically benefit from this optimization, I'm not convinced about this part because this is technically a breaking change for consumers that construct ZendHashTables via these converters in their Rust code. Maybe it'd be better to not change the converters and let projects explicitly use ::new_empty_immutable if they deem the optimization would be useful.

[Xenira]

@mszabo-wikia Thank you for the contribution! I have not that much time, but will try to have a look soon.

From a quick look docsrs_bindings.rs needs to be updated.

PHP 8.2 also seems to fail. Edit: might be because the docs bindings.

[mszabo-wikia]

Thanks, I've regenerated the bindings using a PHP 8.2 debug build. It seems as though the bindings might not have been updated in a while, as there were some other unrelated changes. Or do they need to be generated using 8.0?

[Xenira]

@mszabo-wikia pretty sure its php 8.3, but no one seems to know #334 (comment)

[mszabo-wikia]

@Xenira ah thanks, this is valuable context. I've regenerated the bindings using PHP 8.3.

[Xenira]

Make ZendHashTable's TryFrom implementations from Vec and HashMap return an empty immutable shared hashtable if the input collection was empty

I think we should not do this, as returning a immutable hashtable is not the expected behaviour here

[mszabo-wikia]

Make ZendHashTable's TryFrom implementations from Vec and HashMap return an empty immutable shared hashtable if the input collection was empty

I think we should not do this, as returning a immutable hashtable is not the expected behaviour here

Yeah, I had doubts about that myself. I've removed this logic.

[Xenira]

@mszabo-wikia fixed the issue in nightly tests (#357). Could you rebase onto master.

You also need to update your commit message, as the new pipeline enforces conventional commits.

[mszabo-wikia]

Perfect, thanks—I was wondering about that since I didn't experience the same error locally, but I have stable Rust installed which explains it.

[Xenira]

This should probably not return a mutable vallue, as that allows mutable access to the immutable hashtable.

    let mut ht = ZendHashTable::new_empty_immutable();
    ht.insert("key", 1).unwrap();

would be valid but results in signal: 11 (SIGSEGV) (core dumped)

[mszabo-wikia]

Sorry for the delayed update—yeah, this definitely isn't ideal.

std::ptr only supports mutable pointers, so making the backing ptr immutable wouldn't play well with ZBox.

Maybe the right way to go here is to have a separate helper similar to the RETURN_EMPTY_ARRAY C macro in the Zend API, since really the only major use case for this in extensions is to return an empty array to userland without overhead. Then particularly performance-sensitive extensions can opt into using this as needed.

[Xenira]

Maybe just have a struct ZendEmptyArray; that implements IntoZval?

[mszabo-wikia]

That seems like a good solution, thanks.

[Xenira]

Regarding the docsrs_bindings.rs merge conflict probably best just to regenerate after rebasing.

Really need to find a way to just automate that.