cairo/user_data.rs
1// Take a look at the license at the top of the repository in the LICENSE file.
2
3use std::marker::PhantomData;
4
5use crate::ffi;
6
7pub struct UserDataKey<T> {
8 pub(crate) ffi: ffi::cairo_user_data_key_t,
9 marker: PhantomData<*const T>,
10}
11
12unsafe impl<T> Sync for UserDataKey<T> {}
13
14impl<T> UserDataKey<T> {
15 pub const fn new() -> Self {
16 Self {
17 ffi: ffi::cairo_user_data_key_t { unused: 0 },
18 marker: PhantomData,
19 }
20 }
21}
22
23impl<T> Default for UserDataKey<T> {
24 fn default() -> Self {
25 Self::new()
26 }
27}
28
29// In a safe API for user data we can’t make `get_user_data`
30// transfer full ownership of the value to the caller (e.g. by returning `Box<T>`)
31// because `self` still has a pointer to that value
32// and `get_user_data` could be called again with the same key.
33//
34// We also can’t return a `&T` reference that borrows from `self`
35// because the value could be removed with `remove_user_data` or replaced with `set_user_data`
36// while the borrow still needs to be valid.
37// (Borrowing with `&mut self` would not help as `Self` can be itself reference-counted.)
38//
39// Therefore, the value must be reference-counted.
40//
41// We use `Rc` over `Arc` because the types implementing these methods are `!Send` and `!Sync`.
42// See <https://github.com/gtk-rs/cairo/issues/256>
43
44macro_rules! user_data_methods {
45 ($ffi_get_user_data: path, $ffi_set_user_data: path,) => {
46 /// Attach user data to `self` for the given `key`.
47 pub fn set_user_data<T: 'static>(
48 &self,
49 key: &'static crate::UserDataKey<T>,
50 value: std::rc::Rc<T>,
51 ) -> Result<(), crate::Error> {
52 unsafe extern "C" fn destructor<T>(ptr: *mut libc::c_void) {
53 let ptr: *const T = ptr as _;
54 drop(std::rc::Rc::from_raw(ptr))
55 }
56 // Safety:
57 //
58 // The destructor’s cast and `from_raw` are symmetric
59 // with the `into_raw` and cast below.
60 // They both transfer ownership of one strong reference:
61 // neither of them touches the reference count.
62 let ptr: *const T = std::rc::Rc::into_raw(value);
63 let ptr = ptr as *mut T as *mut libc::c_void;
64 let status = crate::utils::status_to_result(unsafe {
65 $ffi_set_user_data(self.to_raw_none(), &key.ffi, ptr, Some(destructor::<T>))
66 });
67
68 if status.is_err() {
69 // Safety:
70 //
71 // On errors the user data is leaked by cairo and needs to be freed here.
72 unsafe {
73 destructor::<T>(ptr);
74 }
75 }
76
77 status
78 }
79
80 /// Return the user data previously attached to `self` with the given `key`, if any.
81 pub fn user_data<T: 'static>(
82 &self,
83 key: &'static crate::UserDataKey<T>,
84 ) -> Option<std::rc::Rc<T>> {
85 let ptr = self.user_data_ptr(key)?.as_ptr();
86
87 // Safety:
88 //
89 // `Rc::from_raw` would normally take ownership of a strong reference for this pointer.
90 // But `self` still has a copy of that pointer and `get_user_data` can be called again
91 // with the same key.
92 // We use `ManuallyDrop` to avoid running the destructor of that first `Rc`,
93 // and return a cloned one (which increments the reference count).
94 unsafe {
95 let rc = std::mem::ManuallyDrop::new(std::rc::Rc::from_raw(ptr));
96 Some(std::rc::Rc::clone(&rc))
97 }
98 }
99
100 /// Return the user data previously attached to `self` with the given `key`, if any,
101 /// without incrementing the reference count.
102 ///
103 /// The pointer is valid when it is returned from this method,
104 /// until the cairo object that `self` represents is destroyed
105 /// or `remove_user_data` or `set_user_data` is called with the same key.
106 pub fn user_data_ptr<T: 'static>(
107 &self,
108 key: &'static crate::UserDataKey<T>,
109 ) -> Option<std::ptr::NonNull<T>> {
110 // Safety:
111 //
112 // If `ffi_get_user_data` returns a non-null pointer,
113 // there was a previous call to `ffi_set_user_data` with a key with the same address.
114 // Either:
115 //
116 // * This was a call to a Rust `Self::set_user_data` method.
117 // Because that method takes a `&'static` reference,
118 // the key used then must live at that address until the end of the process.
119 // Because `UserDataKey<T>` has a non-zero size regardless of `T`,
120 // no other `UserDataKey<U>` value can have the same address.
121 // Therefore, the `T` type was the same then at it is now and `cast` is type-safe.
122 //
123 // * Or, it is technically possible that the `set` call was to the C function directly,
124 // with a `cairo_user_data_key_t` in heap-allocated memory that was then freed,
125 // then `Box::new(UserDataKey::new()).leak()` was used to create a `&'static`
126 // that happens to have the same address because the allocator for `Box`
127 // reused that memory region.
128 // Since this involves a C (or FFI) call *and* is so far out of “typical” use
129 // of the user data functionality, we consider this a misuse of an unsafe API.
130 unsafe {
131 let ptr = $ffi_get_user_data(self.to_raw_none(), &key.ffi);
132 Some(std::ptr::NonNull::new(ptr)?.cast())
133 }
134 }
135
136 /// Unattached from `self` the user data associated with `key`, if any.
137 /// If there is no other `Rc` strong reference, the data is destroyed.
138 pub fn remove_user_data<T: 'static>(
139 &self,
140 key: &'static crate::UserDataKey<T>,
141 ) -> Result<(), crate::Error> {
142 let status = unsafe {
143 $ffi_set_user_data(self.to_raw_none(), &key.ffi, std::ptr::null_mut(), None)
144 };
145 crate::utils::status_to_result(status)
146 }
147 };
148}