feat(sdk): Ensure SlidingSync::sync drains its internal channel.

Something weird is happening with ElementX iOS. When
`SlidingSync::stop_sync` is called, the internal message `SyncLoopStop`
has time to be sent in the internal channel, but iOS decides to suspend
the code before the sync-loop processes it. When ElementX decides to
start the sync-loop again, it immediately processes the `SyncLoopStop`
message, and… stops the sync-loop.

This patch ensures that `SlidingSync::sync` drains the internal channel
before starting the sync-loop for real. `tokio::sync::mpsc::Receiver`
type has no `drain` method, so this patch implements its
own logic by calling `try_recv` in a loop, until it returns
`Err(TryRecvError::Empty)`.

crates/matrix-sdk/src/sliding_sync/mod.rs

@@ -52,7 +52,7 @@ use serde::{Deserialize, Serialize};
 use tokio::{
     select, spawn,
     sync::{
-        mpsc::{Receiver, Sender},
+        mpsc::{error::TryRecvError, Receiver, Sender},
         Mutex as AsyncMutex, RwLock as AsyncRwLock,
     },
 };
@@ -536,6 +536,24 @@ impl SlidingSync {
         let sync_span = Span::current();
 
         stream! {
+            // Drain the internal channel, in case some messages were still present in it.
+            // It's unlikely to happen in a closed Rust world, but it can happen with async
+            // operations over FFI. Let's make sure the internal channel is properly drained
+            // before starting the sync-loop.
+            {
+                let mut internal_channel_receiver_lock = self.inner.internal_channel.1.write().await;
+
+                // *Gurgle, gurgle, gurgle*
+                while internal_channel_receiver_lock.try_recv().is_ok() {}
+
+                // At this point, the receiver must be empty.
+                assert_eq!(
+                    internal_channel_receiver_lock.try_recv(),
+                    Err(TryRecvError::Empty),
+                    "Sliding Sync internal channel is not empty"
+                );
+            }
+
             loop {
                 sync_span.in_scope(|| {
                     debug!(?self.inner.extensions, ?self.inner.position,"Sync-loop is running");
@@ -645,7 +663,7 @@ impl SlidingSyncInner {
     }
 }
 
-#[derive(Debug)]
+#[derive(Debug, PartialEq)]
 enum SlidingSyncInternalMessage {
     /// Instruct the sync loop to stop.
     SyncLoopStop,