chore(ffi): introduce AsyncRuntimeDropped helper

This avoids proliferation of `ManuallyDrop` in the code base, by having
a single type that's used for dropping under an async runtime.

Author: bnjbvr

bindings/matrix-sdk-ffi/src/client.rs

@@ -1,7 +1,6 @@
 use std::{
     collections::HashMap,
     fmt::Debug,
-    mem::ManuallyDrop,
     path::Path,
     sync::{Arc, RwLock},
 };
@@ -79,6 +78,7 @@ use crate::{
     ruma::AuthData,
     sync_service::{SyncService, SyncServiceBuilder},
     task_handle::TaskHandle,
+    utils::AsyncRuntimeDropped,
     ClientError,
 };
 
@@ -184,26 +184,12 @@ impl From<matrix_sdk::TransmissionProgress> for TransmissionProgress {
 
 #[derive(uniffi::Object)]
 pub struct Client {
-    pub(crate) inner: ManuallyDrop<MatrixClient>,
+    pub(crate) inner: AsyncRuntimeDropped<MatrixClient>,
     delegate: RwLock<Option<Arc<dyn ClientDelegate>>>,
     session_verification_controller:
         Arc<tokio::sync::RwLock<Option<SessionVerificationController>>>,
 }
 
-impl Drop for Client {
-    fn drop(&mut self) {
-        // Dropping the inner OlmMachine must happen within a tokio context
-        // because deadpool drops sqlite connections in the DB pool on tokio's
-        // blocking threadpool to avoid blocking async worker threads.
-        let _guard = RUNTIME.enter();
-        // SAFETY: self.inner is never used again, which is the only requirement
-        //         for ManuallyDrop::drop to be used safely.
-        unsafe {
-            ManuallyDrop::drop(&mut self.inner);
-        }
-    }
-}
-
 impl Client {
     pub async fn new(
         sdk_client: MatrixClient,
@@ -224,7 +210,7 @@ impl Client {
         });
 
         let client = Client {
-            inner: ManuallyDrop::new(sdk_client),
+            inner: AsyncRuntimeDropped::new(sdk_client),
             delegate: RwLock::new(None),
             session_verification_controller,
         };

bindings/matrix-sdk-ffi/src/room_list.rs

@@ -1,12 +1,6 @@
 #![allow(deprecated)]
 
-use std::{
-    fmt::Debug,
-    mem::{ManuallyDrop, MaybeUninit},
-    ptr::addr_of_mut,
-    sync::Arc,
-    time::Duration,
-};
+use std::{fmt::Debug, mem::MaybeUninit, ptr::addr_of_mut, sync::Arc, time::Duration};
 
 use eyeball_im::VectorDiff;
 use futures_util::{pin_mut, StreamExt, TryFutureExt};
@@ -34,6 +28,7 @@ use crate::{
     room_preview::RoomPreview,
     timeline::{EventTimelineItem, Timeline},
     timeline_event_filter::TimelineEventTypeFilter,
+    utils::AsyncRuntimeDropped,
     TaskHandle, RUNTIME,
 };
 
@@ -635,7 +630,7 @@ impl RoomListItem {
 
         let room_preview = client.get_room_preview(&room_or_alias_id, server_names).await?;
 
-        Ok(RoomPreview::new(ManuallyDrop::new(client), room_preview))
+        Ok(RoomPreview::new(AsyncRuntimeDropped::new(client), room_preview))
     }
 
     /// Build a full `Room` FFI object, filling its associated timeline.

bindings/matrix-sdk-ffi/src/room_preview.rs

@@ -1,33 +1,16 @@
-use std::mem::ManuallyDrop;
-
 use anyhow::Context as _;
-use async_compat::TOKIO1 as RUNTIME;
 use matrix_sdk::{room_preview::RoomPreview as SdkRoomPreview, Client};
 use ruma::space::SpaceRoomJoinRule;
 use tracing::warn;
 
-use crate::{client::JoinRule, error::ClientError, room::Membership};
+use crate::{client::JoinRule, error::ClientError, room::Membership, utils::AsyncRuntimeDropped};
 
 /// A room preview for a room. It's intended to be used to represent rooms that
 /// aren't joined yet.
 #[derive(uniffi::Object)]
 pub struct RoomPreview {
     inner: SdkRoomPreview,
-    client: ManuallyDrop<Client>,
-}
-
-impl Drop for RoomPreview {
-    fn drop(&mut self) {
-        // Dropping the inner OlmMachine must happen within a tokio context
-        // because deadpool drops sqlite connections in the DB pool on tokio's
-        // blocking threadpool to avoid blocking async worker threads.
-        let _guard = RUNTIME.enter();
-        // SAFETY: self.client is never used again, which is the only requirement
-        //         for ManuallyDrop::drop to be used safely.
-        unsafe {
-            ManuallyDrop::drop(&mut self.client);
-        }
-    }
+    client: AsyncRuntimeDropped<Client>,
 }
 
 #[matrix_sdk_ffi_macros::export]
@@ -65,7 +48,7 @@ impl RoomPreview {
 }
 
 impl RoomPreview {
-    pub(crate) fn new(client: ManuallyDrop<Client>, inner: SdkRoomPreview) -> Self {
+    pub(crate) fn new(client: AsyncRuntimeDropped<Client>, inner: SdkRoomPreview) -> Self {
         Self { client, inner }
     }
 }

bindings/matrix-sdk-ffi/src/utils.rs

@@ -12,6 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use std::{mem::ManuallyDrop, ops::Deref};
+
+use async_compat::TOKIO1 as RUNTIME;
 use ruma::UInt;
 use tracing::warn;
 
@@ -21,3 +24,45 @@ pub(crate) fn u64_to_uint(u: u64) -> UInt {
         UInt::MAX
     })
 }
+
+/// Tiny wrappers for data types that must be dropped in the context of an async
+/// runtime.
+///
+/// This is useful whenever such a data type may transitively call some
+/// runtime's `block_on` function in their `Drop` impl (since we lack async drop
+/// at the moment), like done in some `deadpool` drop impls.
+pub(crate) struct AsyncRuntimeDropped<T>(ManuallyDrop<T>);
+
+impl<T> AsyncRuntimeDropped<T> {
+    /// Create a new wrapper for this type that will be dropped under an async
+    /// runtime.
+    pub fn new(val: T) -> Self {
+        Self(ManuallyDrop::new(val))
+    }
+}
+
+impl<T> Drop for AsyncRuntimeDropped<T> {
+    fn drop(&mut self) {
+        let _guard = RUNTIME.enter();
+        // SAFETY: self.inner is never used again, which is the only requirement
+        //         for ManuallyDrop::drop to be used safely.
+        unsafe {
+            ManuallyDrop::drop(&mut self.0);
+        }
+    }
+}
+
+// What is an `AsyncRuntimeDropped<T>`, if not a `T` in disguise?
+impl<T> Deref for AsyncRuntimeDropped<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<T: Clone> Clone for AsyncRuntimeDropped<T> {
+    fn clone(&self) -> Self {
+        Self(self.0.clone())
+    }
+}