ndk: Requre all off-thread dyn Fn* callback types to implement Send

In all these cases the NDK either documents or implements the callback
to be invoked from a single, separate thread.  For this to be allowed
by Rust thread safety, the closures and their (moved) contents are
effectively  "moved" to a different thread (`Send`) but not accessed
concurrently (`Sync`), but the type definitions were never requiring
this marker trait which could lead to undefined/invalid behaviour.

A request has been filed to solidify this "behavioural contract" in the
documentation where not done so yet:
https://issuetracker.google.com/issues/300602767#comment12

ndk/CHANGELOG.md

@@ -2,6 +2,15 @@
 
 - Move `MediaFormat` from `media::media_codec` to its own `media::media_format` module. (#442)
 - media_format: Expose `MediaFormat::copy()` and `MediaFormat::clear()` from API level 29. (#449)
+- **Breaking**: Require all `dyn Fn*` types to implement `Send` when the FFI implementation invokes them on a separate thread (#455)
+  - `audio::AudioStreamDataCallback`;
+  - `audio::AudioStreamErrorCallback`;
+  - `media::image_reader::BufferRemovedListener`;
+  - `media::image_reader::ImageListener`;
+  - `media::media_codec::ErrorCallback`;
+  - `media::media_codec::FormatChangedCallback`;
+  - `media::media_codec::InputAvailableCallback`;
+  - `media::media_codec::OutputAvailableCallback`.
 
 # 0.8.0 (2023-10-15)
 

ndk/src/audio.rs

@@ -500,9 +500,9 @@ impl fmt::Debug for AudioStreamBuilder {
 
 #[doc(alias = "AAudioStream_dataCallback")]
 pub type AudioStreamDataCallback =
-    Box<dyn FnMut(&AudioStream, *mut c_void, i32) -> AudioCallbackResult>;
+    Box<dyn FnMut(&AudioStream, *mut c_void, i32) -> AudioCallbackResult + Send>;
 #[doc(alias = "AAudioStream_errorCallback")]
-pub type AudioStreamErrorCallback = Box<dyn FnMut(&AudioStream, AudioError)>;
+pub type AudioStreamErrorCallback = Box<dyn FnMut(&AudioStream, AudioError) + Send>;
 
 impl AudioStreamBuilder {
     fn from_ptr(inner: NonNull<ffi::AAudioStreamBuilder>) -> Self {

ndk/src/media/image_reader.rs

@@ -44,10 +44,10 @@ pub enum ImageFormat {
     DEPTH_JPEG = ffi::AIMAGE_FORMATS::AIMAGE_FORMAT_DEPTH_JPEG.0,
 }
 
-pub type ImageListener = Box<dyn FnMut(&ImageReader)>;
+pub type ImageListener = Box<dyn FnMut(&ImageReader) + Send>;
 
 #[cfg(feature = "api-level-26")]
-pub type BufferRemovedListener = Box<dyn FnMut(&ImageReader, &HardwareBuffer)>;
+pub type BufferRemovedListener = Box<dyn FnMut(&ImageReader, &HardwareBuffer) + Send>;
 
 /// A native [`AImageReader *`]
 ///

ndk/src/media/media_codec.rs

@@ -94,10 +94,10 @@ impl fmt::Debug for AsyncNotifyCallback {
     }
 }
 
-pub type InputAvailableCallback = Box<dyn FnMut(usize)>;
-pub type OutputAvailableCallback = Box<dyn FnMut(usize, &BufferInfo)>;
-pub type FormatChangedCallback = Box<dyn FnMut(&MediaFormat)>;
-pub type ErrorCallback = Box<dyn FnMut(MediaError, ActionCode, &CStr)>;
+pub type InputAvailableCallback = Box<dyn FnMut(usize) + Send>;
+pub type OutputAvailableCallback = Box<dyn FnMut(usize, &BufferInfo) + Send>;
+pub type FormatChangedCallback = Box<dyn FnMut(&MediaFormat) + Send>;
+pub type ErrorCallback = Box<dyn FnMut(MediaError, ActionCode, &CStr) + Send>;
 
 impl MediaCodec {
     fn as_ptr(&self) -> *mut ffi::AMediaCodec {