New CryptoMachine on each background operation

Author: Anderas

MatrixSDK/Background/Crypto/MXBackgroundCryptoV2.swift

@@ -27,29 +27,13 @@ class MXBackgroundCryptoV2: MXBackgroundCrypto {
         case missingCredentials
     }
 
-    private let machine: MXCryptoMachine
+    private let credentials: MXCredentials
+    private let restClient: MXRestClient
     private let log = MXNamedLog(name: "MXBackgroundCryptoV2")
 
-    init(credentials: MXCredentials, restClient: MXRestClient) throws {
-        guard
-            let userId = credentials.userId,
-            let deviceId = credentials.deviceId
-        else {
-            throw Error.missingCredentials
-        }
-        
-        // `MXCryptoMachine` will load the same store as the main application meaning that background and foreground
-        // sync services have access to the same data / keys. Possible race conditions are handled internally.
-        machine = try MXCryptoMachine(
-            userId: userId,
-            deviceId: deviceId,
-            restClient: restClient,
-            getRoomAction: { [log] _ in
-                log.error("The background crypto should not be accessing rooms")
-                return nil
-            }
-        )
-        
+    init(credentials: MXCredentials, restClient: MXRestClient) {
+        self.credentials = credentials
+        self.restClient = restClient
         log.debug("Initialized background crypto module")
     }
 
@@ -66,6 +50,7 @@ class MXBackgroundCryptoV2: MXBackgroundCrypto {
         log.debug(details)
 
         do {
+            let machine = try createMachine()
             _ = try await machine.handleSyncResponse(
                 toDevice: syncResponse.toDevice,
                 deviceLists: syncResponse.deviceLists,
@@ -100,6 +85,7 @@ class MXBackgroundCryptoV2: MXBackgroundCrypto {
         do {
             // Rust-sdk does not expose api to see if we have a given session key yet (will be added in the future)
             // so for the time being to find out if we can decrypt we simply perform the (more expensive) decryption
+            let machine = try createMachine()
             _ = try machine.decryptRoomEvent(event)
             log.debug("Event `\(eventId)` can be decrypted with session `\(sessionId)`")
             return true
@@ -117,6 +103,7 @@ class MXBackgroundCryptoV2: MXBackgroundCrypto {
         log.debug("Decrypting event `\(eventId)`")
 
         do {
+            let machine = try createMachine()
             let decrypted = try machine.decryptRoomEvent(event)
             let result = try MXEventDecryptionResult(event: decrypted)
             event.setClearData(result)
@@ -127,6 +114,30 @@ class MXBackgroundCryptoV2: MXBackgroundCrypto {
             throw error
         }
     }
+    
+    // `MXCryptoMachine` will load the same store as the main application meaning that background and foreground
+    // sync services have access to the same data / keys. The machine is not fully multi-thread and multi-process
+    // safe, and until this is resolved we open a new instance of `MXCryptoMachine` on each background operation
+    // to ensure we are always up-to-date with whatever has been written by the foreground process in the meanwhile.
+    // See https://github.com/matrix-org/matrix-rust-sdk/issues/1415 for more details.
+    private func createMachine() throws -> MXCryptoMachine {
+        guard
+            let userId = credentials.userId,
+            let deviceId = credentials.deviceId
+        else {
+            throw Error.missingCredentials
+        }
+         
+        return try MXCryptoMachine(
+            userId: userId,
+            deviceId: deviceId,
+            restClient: restClient,
+            getRoomAction: { [log] _ in
+                log.error("The background crypto should not be accessing rooms")
+                return nil
+            }
+        )
+    }
 }
 
 #endif

MatrixSDK/Background/MXBackgroundSyncService.swift

@@ -40,7 +40,7 @@ public enum MXBackgroundSyncServiceError: Error {
     private let processingQueue: DispatchQueue
     public let credentials: MXCredentials
     private let syncResponseStoreManager: MXSyncResponseStoreManager
-    private let crypto: MXBackgroundCrypto?
+    private let crypto: MXBackgroundCrypto
     private var store: MXStore
     private let restClient: MXRestClient
     private var pushRulesManager: MXBackgroundPushRulesManager
@@ -91,12 +91,7 @@ public enum MXBackgroundSyncServiceError: Error {
         crypto = {
             #if DEBUG
             if MXSDKOptions.sharedInstance().isCryptoSDKAvailable && MXSDKOptions.sharedInstance().enableCryptoSDK {
-                do {
-                    return try MXBackgroundCryptoV2(credentials: credentials, restClient: restClient)
-                } catch {
-                    MXLog.failure("Cannot create background crypto", context: error)
-                    return nil
-                }
+                return MXBackgroundCryptoV2(credentials: credentials, restClient: restClient)
             }
             #endif
             return MXLegacyBackgroundCrypto(credentials: credentials, resetBackgroundCryptoStore: resetBackgroundCryptoStore)
@@ -299,7 +294,7 @@ public enum MXBackgroundSyncServiceError: Error {
             }
 
             //  should decrypt it first
-            if let crypto = crypto, crypto.canDecryptEvent(event) {
+            if crypto.canDecryptEvent(event) {
                 //  we have keys to decrypt the event
                 MXLog.debug("[MXBackgroundSyncService] fetchEvent: Event needs to be decrpyted, and we have the keys to decrypt it.")
 
@@ -406,8 +401,7 @@ public enum MXBackgroundSyncServiceError: Error {
                     await self.handleSyncResponse(syncResponse, syncToken: eventStreamToken)
 
                     if let event = self.syncResponseStoreManager.event(withEventId: eventId, inRoom: roomId),
-                       let crypto = self.crypto,
-                       !crypto.canDecryptEvent(event),
+                       !self.crypto.canDecryptEvent(event),
                        (syncResponse.toDevice?.events ?? []).count > 0 {
                         //  we got the event but not the keys to decrypt it. continue to sync
                         self.launchBackgroundSync(forEventId: eventId, roomId: roomId, completion: completion)
@@ -446,7 +440,7 @@ public enum MXBackgroundSyncServiceError: Error {
         }
         syncResponseStoreManager.updateStore(with: syncResponse, syncToken: syncToken)
 
-        await crypto?.handleSyncResponse(syncResponse)
+        await crypto.handleSyncResponse(syncResponse)
 
         if MXSDKOptions.sharedInstance().autoAcceptRoomInvites,
            let invitedRooms = syncResponse.rooms?.invite {