cairo/user_data.rs
1use std::marker::PhantomData;
2
3use ffi::cairo_user_data_key_t;
4
5pub struct UserDataKey<T> {
6 pub(crate) ffi: cairo_user_data_key_t,
7 marker: PhantomData<*const T>,
8}
9
10unsafe impl<T> Sync for UserDataKey<T> {}
11
12impl<T> UserDataKey<T> {
13 pub const fn new() -> Self {
14 UserDataKey {
15 ffi: cairo_user_data_key_t { unused: 0 },
16 marker: PhantomData,
17 }
18 }
19}
20
21// In a safe API for user data we can’t make `get_user_data`
22// transfer full ownership of the value to the caller (e.g. by returning `Box<T>`)
23// because `self` still has a pointer to that value
24// and `get_user_data` could be called again with the same key.
25//
26// We also can’t return a `&T` reference that borrows from `self`
27// because the value could be removed with `remove_user_data` or replaced with `set_user_data`
28// while the borrow still needs to be valid.
29// (Borrowing with `&mut self` would not help as `Self` can be itself reference-counted.)
30//
31// Therefore the value must be reference-counted.
32//
33// We use `Rc` over `Arc` because the types implementing these methods are `!Send` and `!Sync`.
34// See <https://github.com/gtk-rs/cairo/issues/256>
35
36macro_rules! user_data_methods {
37 ($ffi_get_user_data: path, $ffi_set_user_data: path,) => {
38 /// Attach user data to `self` for the given `key`.
39 pub fn set_user_data<T: 'static>(&self, key: &'static crate::UserDataKey<T>,
40 value: std::rc::Rc<T>)
41 {
42 unsafe extern "C" fn destructor<T>(ptr: *mut libc::c_void) {
43 let ptr: *const T = ptr as _;
44 drop(std::rc::Rc::from_raw(ptr))
45 }
46 // Safety:
47 //
48 // The destructor’s cast and `from_raw` are symetric
49 // with the `into_raw` and cast below.
50 // They both transfer ownership of one strong reference:
51 // neither of them touches the reference count.
52 let ptr: *const T = std::rc::Rc::into_raw(value);
53 let ptr = ptr as *mut T as *mut libc::c_void;
54 let result = unsafe {
55 $ffi_set_user_data(self.to_raw_none(), &key.ffi, ptr, Some(destructor::<T>))
56 };
57 Status::from(result).ensure_valid()
58 }
59
60 /// Return the user data previously attached to `self` with the given `key`, if any.
61 pub fn get_user_data<T: 'static>(&self, key: &'static crate::UserDataKey<T>)
62 -> Option<std::rc::Rc<T>>
63 {
64 let ptr = self.get_user_data_ptr(key)?.as_ptr();
65
66 // Safety:
67 //
68 // `Rc::from_raw` would normally take ownership of a strong reference for this pointer.
69 // But `self` still has a copy of that pointer and `get_user_data` can be called again
70 // with the same key.
71 // We use `ManuallyDrop` to avoid running the destructor of that first `Rc`,
72 // and return a cloned one (which increments the reference count).
73 unsafe {
74 let rc = std::mem::ManuallyDrop::new(std::rc::Rc::from_raw(ptr));
75 Some(std::rc::Rc::clone(&rc))
76 }
77 }
78
79 /// Return the user data previously attached to `self` with the given `key`, if any,
80 /// without incrementing the reference count.
81 ///
82 /// The pointer is valid when it is returned from this method,
83 /// until the cairo object that `self` represents is destroyed
84 /// or `remove_user_data` or `set_user_data` is called with the same key.
85 pub fn get_user_data_ptr<T: 'static>(&self, key: &'static crate::UserDataKey<T>)
86 -> Option<std::ptr::NonNull<T>>
87 {
88 // Safety:
89 //
90 // If `ffi_get_user_data` returns a non-null pointer,
91 // there was a previous call to `ffi_set_user_data` with a key with the same address.
92 // Either:
93 //
94 // * This was a call to a Rust `Self::set_user_data` method.
95 // Because that method takes a `&'static` reference,
96 // the key used then must live at that address until the end of the process.
97 // Because `UserDataKey<T>` has a non-zero size regardless of `T`,
98 // no other `UserDataKey<U>` value can have the same address.
99 // Therefore the `T` type was the same then at it is now and `cast` is type-safe.
100 //
101 // * Or, it is technically possible that the `set` call was to the C function directly,
102 // with a `cairo_user_data_key_t` in heap-allocated memory that was then freed,
103 // then `Box::new(UserDataKey::new()).leak()` was used to create a `&'static`
104 // that happens to have the same address because the allocator for `Box`
105 // reused that memory region.
106 // Since this involves a C (or FFI) call *and* is so far out of “typical” use
107 // of the user data functionality, we consider this a misuse of an unsafe API.
108 unsafe {
109 let ptr = $ffi_get_user_data(self.to_raw_none(), &key.ffi);
110 Some(std::ptr::NonNull::new(ptr)?.cast())
111 }
112 }
113
114 /// Unattach from `self` the user data associated with `key`, if any.
115 /// If there is no other `Rc` strong reference, the data is destroyed.
116 pub fn remove_user_data<T: 'static>(&self, key: &'static crate::UserDataKey<T>) {
117 let result = unsafe {
118 $ffi_set_user_data(self.to_raw_none(), &key.ffi, std::ptr::null_mut(), None)
119 };
120 Status::from(result).ensure_valid()
121 }
122 };
123}