fix(ffi): use EventCache::clear_all_rooms() to clear the events caches

Clearing only the store backend, while not emptying the in-memory linked
chunks, will lead to out-of-sync state across the in-memory caches and
the database. As a result, it's safer to call the existing
`EventCacheInner::clear_all_rooms`, which will clear all the rooms
manually.

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

@@ -1208,13 +1208,14 @@ impl Client {
     ///   retention policy.
     pub async fn clear_caches(&self) -> Result<(), ClientError> {
         let closure = async || -> Result<_, EventCacheError> {
-            let store = self.inner.event_cache_store().lock().await?;
-
-            // Clear all the room chunks.
-            store.clear_all_rooms_chunks().await?;
-
             // Clean up the media cache according to the current media retention policy.
-            store.clean_up_media_cache().await?;
+            self.inner.event_cache_store().lock().await?.clean_up_media_cache().await?;
+
+            // Clear all the room chunks. It's important to *not* call
+            // `EventCacheStore::clear_all_rooms_chunks` here, because there might be live
+            // observers of the linked chunks, and that would cause some very bad state
+            // mismatch.
+            self.inner.event_cache().clear_all_rooms().await?;
 
             Ok(())
         };

crates/matrix-sdk-base/src/event_cache/store/traits.rs

@@ -101,6 +101,10 @@ pub trait EventCacheStore: AsyncTraitDeps {
     /// using the above [`Self::handle_linked_chunk_updates`] methods. It
     /// must *also* delete all the events' content, if they were stored in a
     /// separate table.
+    ///
+    /// ⚠ This is meant only for super specific use cases, where there shouldn't
+    /// be any live in-memory linked chunks. In general, prefer using
+    /// `EventCache::clear_all_rooms()` from the common SDK crate.
     async fn clear_all_rooms_chunks(&self) -> Result<(), Self::Error>;
 
     /// Given a set of event IDs, return the duplicated events along with their

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

@@ -361,6 +361,13 @@ impl EventCache {
         Ok((room, drop_handles))
     }
 
+    /// Cleanly clear all the rooms' event caches.
+    ///
+    /// This will notify any live observers that the room has been cleared.
+    pub async fn clear_all_rooms(&self) -> Result<()> {
+        self.inner.clear_all_rooms().await
+    }
+
     /// Add an initial set of events to the event cache, reloaded from a cache.
     ///
     /// TODO: temporary for API compat, as the event cache should take care of