ffi: duplicate the tokio runtime hack for Encryption :melting:

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

@@ -1,24 +1,44 @@
-use std::sync::Arc;
+use std::{mem::ManuallyDrop, sync::Arc};
 
 use futures_util::StreamExt;
 use matrix_sdk::{
     encryption,
     encryption::{backups, recovery},
 };
 use thiserror::Error;
+use tracing::info;
 use zeroize::Zeroize;
 
 use super::RUNTIME;
 use crate::{error::ClientError, task_handle::TaskHandle};
 
 #[derive(uniffi::Object)]
 pub struct Encryption {
-    inner: matrix_sdk::encryption::Encryption,
+    inner: ManuallyDrop<matrix_sdk::encryption::Encryption>,
+}
+
+impl Drop for Encryption {
+    fn drop(&mut self) {
+        // This holds a reference to the `Client`, so it must duplicate the hack on the
+        // FFI's `Client` data structure, since it may be the last reference to
+        // a `Client`.
+        //
+        // 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 {
+            info!("Dropping an Encryption");
+            ManuallyDrop::drop(&mut self.inner);
+        }
+    }
 }
 
 impl From<matrix_sdk::encryption::Encryption> for Encryption {
     fn from(value: matrix_sdk::encryption::Encryption) -> Self {
-        Self { inner: value }
+        Self { inner: ManuallyDrop::new(value) }
     }
 }