Refactor Zend strings

[davidcole1340]

Zend strings now come in two forms - the borrowed &ZendStr and the owned ZendString, which can be compared to &str and String.

[davidcole1340]

Forgot to rebase this properly, but I'm going to delete example/skel soon as it's just my testbench at this point

[vodik]

For the sake of producing a safe and correct wrapper around this, the persistent field probably should be hoisted into the type. It is not safe to provide request-arena allocated strings, for example, to a function that expects persisted ones - while the opposite is likely fine. We have covariance.

I don't think this can be easily expressed via lifetimes, but should be expressible with type variables:

struct ZendStringBase<Alloc>;

pub type ZendString = ZendStringBase<Request>;

pub mod persistent {
    pub type ZendString = ZendStringBase<Persistent>;
}

[vodik]

As I understand it, the above should be perfectly fine so long as we're content with only using rust to extend PHP.

However, I'm not super familiar with how request allocated memory interacts with embedded PHP. I assume the lifetime of a request might be in the control of the hosting language.

If so, that opens the door to another safety issue: use after free after the request arena is deallocated. So another option might be to make ZendString::new always persistent and instead add some sort of token that represents the current request into the API - even if its largely an artificial abstraction.

This request object then must used to allocate per-request data and can be used to coordinate lifetimes

[vodik]

The detail I'm still not sure about, I've been digging through php-src to figure this out, is where does the emalloc heap get cleared.

[davidcole1340]

Forcing all owned Zend strings to be persistent might be the best way I think, calling zend_string_release once their GC cycle is zero still causes them to be deallocated, however we lose the memory leak checks of the ZMM.

I'm not sure how we could account for the user potentially keeping a request-bound ZendString past the request boundary, leaving a dangling pointer inside of ZendString. Maybe just forcing all strings to be persistent would be the safest option?

[vodik]

I'm not sure how we could account for the user potentially keeping a request-bound ZendString past the request boundary, leaving a dangling pointer inside of ZendString

Explicitly creating a token type that wraps the request is probably the way to go: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=c431c93f58529be80decb279e61589bc

In the example, even through TEST is static, if I grab it through fake_alloc I cannot get a borrow that outlives the Request token type.

Alt with a closure: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=8254a995ae5213f44de52aef2eaa87cf

[davidcole1340]

How would we represent the lifetime of the request though? A Request object could either live for the lifetime of the extern "C" wrapper function or 'static (or maybe I am missing something)?

The ZendString API is technically an 'internal' API, so perhaps it could be deemed unsafe (or default to persistent with an unsafe request-bound option)?

[davidcole1340]

So it seems that you aren't supposed to put persistent strings in zvals... When calling zend_ptr_dtor() (from the Drop impl or when called from PHP) it calls zend_string_destroy which only attempts to deallocate using the Zend memory manager.

[davidcole1340]

Going to merge this for now and can revisit later.