xpc-sys: Don't xpc_release NULLs

[mach-kernel]

I am not sure that implementing Default for XPCObject makes much sense. Options are:

• Get rid of the Default impl
• Return xpc_null_create()
• Leave as is and check for NULL xpc_object_t (probably a good idea)

Strangely enough nothing bad happens when doing this on Sonoma:

#[test]
fn release_null() {
    unsafe { xpc_release(null_mut()) }
}

running 1 test
test objects::xpc_object::tests::release_null ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 23 filtered out; finished in 0.00s

[hjmallon]

Interesting, I am on Sonoma and that is what is showing complaints (I added it to a test and ran cargo test).

If nullptr is never valid for xpc-object-t then you could make the ptr NonNull? I am not sure if null is ever useful?

[mach-kernel]

Re NonNull: The XPC bindings expect an xpc_object_t (*mut c_void), so it would change things a little, e.g.:

pub fn xpc_copy(xpc_object: NonNull<c_void>) -> Option<Self> {
    // -> *mut c_void
    let clone = unsafe { xpc_copy(xpc_object.as_ptr()) };
    NonNull::new(clone).map(Self::new)
}

What's confusing is that some of the Obj-C signatures use the _Nonnull extension like xpc_dictionary_create, but not xpc_copy:

xpc_object_t xpc_dictionary_create(const char *const _Nonnull *keys, xpc_object_t _Nullable const *values, size_t count);
xpc_object_t xpc_copy(xpc_object_t object);

Some of the documented XPC APIs return NULL xpc_dictionary_create_reply. I need to do some homework on FFI semantics + NonNull. Then go figure out which usages it makes most sense for (here is appetizing). It might be OK to use NonNull<c_void> instead of xpc_object_t in the Rust XPCObject.

At the very least this has convinced me to drop the current Default impl for XPCObject.

[hjmallon]

Ok so a good step one would be removing default and also merging this? Then nullptr xpcobjects work, but default doesn’t make them. That would work for what I am doing right not.

to do the more extreme nonnull path would involve more checking in more places and also working out whether it is ever useful to have a nullptr.

[mach-kernel]

Pardon the delay. I dropped impl Default and will cut a 0.5.0 for xpc-sys. Thanks!