Add send feature to make Image implement Send

This uses a global mutex when ref counts are changed, which should
prevent data races, as long as `&mut` prevent other mutation.

pixman/Cargo.toml

@@ -28,3 +28,4 @@ image = "0.24.7"
 [features]
 default = []
 drm-fourcc = ["dep:drm-fourcc"]
+send = []

pixman/src/image/bits.rs

@@ -13,6 +13,16 @@ pub struct Image<'bits, 'alpha> {
     _alpha: PhantomData<&'alpha ()>,
 }
 
+// SAFETY: A reference to the image is only created by `set_alpha_map`.
+// Which returns a type with a lifetime bound, so `&mut self` methods cannot
+// be called while this additional reference is in use.
+//
+// Thus the only mutability allowed is reference counting, which is made
+// thread-safe with the `REF_COUNT_LOCK` mutex, used when calling
+// `pixman_image_unref`, or `pixman_image_set_alpha_map`.
+#[cfg(feature = "send")]
+unsafe impl<'bits, 'alpha> Send for Image<'bits, 'alpha> {}
+
 impl<'bits, 'alpha> std::ops::Deref for Image<'bits, 'alpha> {
     type Target = ImageRef;
 
@@ -140,6 +150,8 @@ impl<'bits, 'a> Image<'bits, 'a> {
         x: i16,
         y: i16,
     ) -> Image<'bits, 'alpha> {
+        #[cfg(feature = "send")]
+        let _lock = crate::REF_COUNT_LOCK.lock().unwrap();
         unsafe {
             ffi::pixman_image_set_alpha_map(self.as_ptr(), alpha_map.as_ptr(), x, y);
         }
@@ -152,6 +164,8 @@ impl<'bits, 'a> Image<'bits, 'a> {
 
     /// Clear a previously set alpha map
     pub fn clear_alpha_map(self) -> Image<'bits, 'static> {
+        #[cfg(feature = "send")]
+        let _lock = crate::REF_COUNT_LOCK.lock().unwrap();
         unsafe {
             ffi::pixman_image_set_alpha_map(self.as_ptr(), std::ptr::null_mut(), 0, 0);
         }

pixman/src/image/mod.rs

@@ -174,6 +174,8 @@ impl ImageRef {
 
 impl Drop for ImageRef {
     fn drop(&mut self) {
+        #[cfg(feature = "send")]
+        let _lock = crate::REF_COUNT_LOCK.lock().unwrap();
         unsafe {
             ffi::pixman_image_unref(self.0);
         }
@@ -197,6 +199,8 @@ macro_rules! image_type {
                 x: i16,
                 y: i16,
             ) -> $name<'alpha> {
+                #[cfg(feature = "send")]
+                let _lock = $crate::REF_COUNT_LOCK.lock().unwrap();
                 unsafe {
                     $crate::ffi::pixman_image_set_alpha_map(
                         self.as_ptr(),
@@ -213,6 +217,8 @@ macro_rules! image_type {
 
             /// Clear a previously set alpha map
             pub fn clear_alpha_map(self) -> $name<'static> {
+                #[cfg(feature = "send")]
+                let _lock = $crate::REF_COUNT_LOCK.lock().unwrap();
                 unsafe {
                     $crate::ffi::pixman_image_set_alpha_map(
                         self.as_ptr(),

pixman/src/lib.rs

@@ -66,6 +66,9 @@ pub use vector::*;
 #[error("The requested operation failed")]
 pub struct OperationFailed;
 
+#[cfg(feature = "send")]
+static REF_COUNT_LOCK: std::sync::Mutex<()> = std::sync::Mutex::new(());
+
 /// Blit the src into the dst with the specified values
 #[allow(clippy::too_many_arguments)]
 pub fn blit(