use super::*; use crate::ComInterface; use std::sync::atomic::{AtomicIsize, Ordering}; #[doc(hidden)] #[repr(transparent)] #[derive(Default)] pub struct WeakRefCount(AtomicIsize); impl WeakRefCount { pub fn new() -> Self { Self(AtomicIsize::new(1)) } pub fn add_ref(&self) -> u32 { self.0.fetch_update(Ordering::Relaxed, Ordering::Relaxed, |count_or_pointer| then_some(!is_weak_ref(count_or_pointer), count_or_pointer + 1)).map(|u| u as u32 + 1).unwrap_or_else(|pointer| unsafe { TearOff::decode(pointer).strong_count.add_ref() }) } pub fn release(&self) -> u32 { self.0.fetch_update(Ordering::Release, Ordering::Relaxed, |count_or_pointer| then_some(!is_weak_ref(count_or_pointer), count_or_pointer - 1)).map(|u| u as u32 - 1).unwrap_or_else(|pointer| unsafe { let tear_off = TearOff::decode(pointer); let remaining = tear_off.strong_count.release(); // If this is the last strong reference, we can release the weak reference implied by the strong reference. // There may still be weak references, so the WeakRelease is called to handle such possibilities. if remaining == 0 { TearOff::WeakRelease(&mut tear_off.weak_vtable as *mut _ as _); } remaining }) } /// # Safety pub unsafe fn query(&self, iid: *const crate::GUID, object: *mut std::ffi::c_void) -> *mut std::ffi::c_void { if *iid != IWeakReferenceSource::IID { return std::ptr::null_mut(); } let mut count_or_pointer = self.0.load(Ordering::Relaxed); if is_weak_ref(count_or_pointer) { return TearOff::from_encoding(count_or_pointer); } let tear_off = TearOff::new(object, count_or_pointer as u32); let tear_off_ptr: *mut std::ffi::c_void = std::mem::transmute_copy(&tear_off); let encoding: usize = ((tear_off_ptr as usize) >> 1) | (1 << (std::mem::size_of::() * 8 - 1)); loop { match self.0.compare_exchange_weak(count_or_pointer, encoding as isize, Ordering::AcqRel, Ordering::Relaxed) { Ok(_) => { let result: *mut std::ffi::c_void = std::mem::transmute(tear_off); TearOff::from_strong_ptr(result).strong_count.add_ref(); return result; } Err(pointer) => count_or_pointer = pointer, } if is_weak_ref(count_or_pointer) { return TearOff::from_encoding(count_or_pointer); } TearOff::from_strong_ptr(tear_off_ptr).strong_count.0.store(count_or_pointer as i32, Ordering::SeqCst); } } } fn is_weak_ref(value: isize) -> bool { value < 0 } #[repr(C)] struct TearOff { strong_vtable: *const IWeakReferenceSource_Vtbl, weak_vtable: *const IWeakReference_Vtbl, object: *mut std::ffi::c_void, strong_count: RefCount, weak_count: RefCount, } impl TearOff { #[allow(clippy::new_ret_no_self)] unsafe fn new(object: *mut std::ffi::c_void, strong_count: u32) -> IWeakReferenceSource { std::mem::transmute(std::boxed::Box::new(TearOff { strong_vtable: &Self::STRONG_VTABLE, weak_vtable: &Self::WEAK_VTABLE, object, strong_count: RefCount::new(strong_count), weak_count: RefCount::new(1), })) } unsafe fn from_encoding(encoding: isize) -> *mut std::ffi::c_void { let tear_off = TearOff::decode(encoding); tear_off.strong_count.add_ref(); tear_off as *mut _ as *mut _ } const STRONG_VTABLE: IWeakReferenceSource_Vtbl = IWeakReferenceSource_Vtbl { base__: crate::IUnknown_Vtbl { QueryInterface: Self::StrongQueryInterface, AddRef: Self::StrongAddRef, Release: Self::StrongRelease }, GetWeakReference: Self::StrongDowngrade, }; const WEAK_VTABLE: IWeakReference_Vtbl = IWeakReference_Vtbl { base__: crate::IUnknown_Vtbl { QueryInterface: Self::WeakQueryInterface, AddRef: Self::WeakAddRef, Release: Self::WeakRelease }, Resolve: Self::WeakUpgrade, }; unsafe fn from_strong_ptr<'a>(this: *mut std::ffi::c_void) -> &'a mut Self { &mut *(this as *mut *mut std::ffi::c_void as *mut Self) } unsafe fn from_weak_ptr<'a>(this: *mut std::ffi::c_void) -> &'a mut Self { &mut *((this as *mut *mut std::ffi::c_void).sub(1) as *mut Self) } unsafe fn decode<'a>(value: isize) -> &'a mut Self { std::mem::transmute(value << 1) } unsafe fn query_interface(&self, iid: *const crate::GUID, interface: *mut *mut std::ffi::c_void) -> crate::HRESULT { ((*(*(self.object as *mut *mut crate::IUnknown_Vtbl))).QueryInterface)(self.object, iid, interface) } unsafe extern "system" fn StrongQueryInterface(ptr: *mut std::ffi::c_void, iid: *const crate::GUID, interface: *mut *mut std::ffi::c_void) -> crate::HRESULT { let this = Self::from_strong_ptr(ptr); if iid.is_null() || interface.is_null() { return ::windows_core::HRESULT(-2147467261); // E_POINTER } // Only directly respond to queries for the the tear-off's strong interface. This is // effectively a self-query. if *iid == IWeakReferenceSource::IID { *interface = ptr; this.strong_count.add_ref(); return crate::HRESULT(0); } // As the tear-off is sharing the identity of the object, simply delegate any remaining // queries to the object. this.query_interface(iid, interface) } unsafe extern "system" fn WeakQueryInterface(ptr: *mut std::ffi::c_void, iid: *const crate::GUID, interface: *mut *mut std::ffi::c_void) -> crate::HRESULT { let this = Self::from_weak_ptr(ptr); if iid.is_null() || interface.is_null() { return ::windows_core::HRESULT(-2147467261); // E_POINTER } // While the weak vtable is packed into the same allocation as the strong vtable and // tear-off, it represents a distinct COM identity and thus does not share or delegate to // the object. *interface = if *iid == IWeakReference::IID || *iid == crate::IUnknown::IID || *iid == IAgileObject::IID { ptr } else { std::ptr::null_mut() }; // TODO: implement IMarshal if (*interface).is_null() { E_NOINTERFACE } else { this.weak_count.add_ref(); crate::HRESULT(0) } } unsafe extern "system" fn StrongAddRef(ptr: *mut std::ffi::c_void) -> u32 { let this = Self::from_strong_ptr(ptr); // Implement `AddRef` directly as we own the strong reference. this.strong_count.add_ref() } unsafe extern "system" fn WeakAddRef(ptr: *mut std::ffi::c_void) -> u32 { let this = Self::from_weak_ptr(ptr); // Implement `AddRef` directly as we own the weak reference. this.weak_count.add_ref() } unsafe extern "system" fn StrongRelease(ptr: *mut std::ffi::c_void) -> u32 { let this = Self::from_strong_ptr(ptr); // Forward strong `Release` to the object so that it can destroy itself. It will then // decrement its weak reference and allow the tear-off to be released as needed. ((*(*(this.object as *mut *mut crate::IUnknown_Vtbl))).Release)(this.object) } unsafe extern "system" fn WeakRelease(ptr: *mut std::ffi::c_void) -> u32 { let this = Self::from_weak_ptr(ptr); // Implement `Release` directly as we own the weak reference. let remaining = this.weak_count.release(); // If there are no remaining references, it means that the object has already been // destroyed. Go ahead and destroy the tear-off. if remaining == 0 { let _ = std::boxed::Box::from_raw(this); } remaining } unsafe extern "system" fn StrongDowngrade(ptr: *mut std::ffi::c_void, interface: *mut *mut std::ffi::c_void) -> crate::HRESULT { let this = Self::from_strong_ptr(ptr); // The strong vtable hands out a reference to the weak vtable. This is always safe and // straightforward since a strong reference guarantees there is at least one weak // reference. *interface = &mut this.weak_vtable as *mut _ as _; this.weak_count.add_ref(); crate::HRESULT(0) } unsafe extern "system" fn WeakUpgrade(ptr: *mut std::ffi::c_void, iid: *const crate::GUID, interface: *mut *mut std::ffi::c_void) -> crate::HRESULT { let this = Self::from_weak_ptr(ptr); this.strong_count .0 .fetch_update(Ordering::Acquire, Ordering::Relaxed, |count| { // Attempt to acquire a strong reference count to stabilize the object for the duration // of the `QueryInterface` call. then_some(count != 0, count + 1) }) .map(|_| { // Let the object respond to the upgrade query. let result = this.query_interface(iid, interface); // Decrement the temporary reference account used to stabilize the object. this.strong_count.0.fetch_sub(1, Ordering::Relaxed); // Return the result of the query. result }) .unwrap_or_else(|_| { *interface = std::ptr::null_mut(); crate::HRESULT(0) }) } }