crypto: MemoryStore uses backup versions to track which sessions are backed up

Author: andybalaam

crates/matrix-sdk-crypto/CHANGELOG.md

@@ -25,6 +25,9 @@ Breaking changes:
 
 Additions:
 
+- Expose new method `BackupMachine::backup_version`.
+  ([#3320](https://github.com/matrix-org/matrix-rust-sdk/pull/3320))
+
 - Expose new method `OlmMachine::device_creation_time`.
   ([#3275](https://github.com/matrix-org/matrix-rust-sdk/pull/3275))
 

crates/matrix-sdk-crypto/src/backups/mod.rs

@@ -419,6 +419,12 @@ impl BackupMachine {
         Ok(())
     }
 
+    /// Provide the `backup_version` of the current `backup_key`, or None if there is no current
+    /// key, or the key is not used with any backup version.
+    pub async fn backup_version(&self) -> Option<String> {
+        self.backup_key.read().await.as_ref().and_then(|k| k.backup_version())
+    }
+
     /// Store the backup decryption key in the crypto store.
     ///
     /// This is useful if the client wants to support gossiping of the backup