use mutex on suspend and not synchronized

matrix-sdk-android/src/main/java/org/matrix/android/sdk/internal/crypto/actions/EnsureOlmSessionsForDevicesAction.kt

@@ -16,6 +16,8 @@
 
 package org.matrix.android.sdk.internal.crypto.actions
 
+import kotlinx.coroutines.sync.Mutex
+import kotlinx.coroutines.sync.withLock
 import kotlinx.coroutines.withContext
 import org.matrix.android.sdk.api.MatrixCoroutineDispatchers
 import org.matrix.android.sdk.api.logger.LoggerTag
@@ -39,16 +41,18 @@ internal class EnsureOlmSessionsForDevicesAction @Inject constructor(
         private val coroutineDispatchers: MatrixCoroutineDispatchers,
         private val oneTimeKeysForUsersDeviceTask: ClaimOneTimeKeysForUsersDeviceTask) {
 
+    private val ensureMutex = Mutex()
+
     /**
      * We want to synchronize a bit here, because we are iterating to check existing olm session and
      * also adding some
      */
-    @Synchronized
     suspend fun handle(devicesByUser: Map<String, List<CryptoDeviceInfo>>, force: Boolean = false): MXUsersDevicesMap<MXOlmSessionResult> {
+        ensureMutex.withLock {
+            val results = MXUsersDevicesMap<MXOlmSessionResult>()
             val deviceList = devicesByUser.flatMap { it.value }
             Timber.tag(loggerTag.value)
                     .d("ensure olm forced:$force for ${deviceList.joinToString { it.shortDebugString() }}")
-        val results = MXUsersDevicesMap<MXOlmSessionResult>()
             val devicesToCreateSessionWith = mutableListOf<CryptoDeviceInfo>()
             if (force) {
                 // we take all devices and will query otk for them
@@ -113,9 +117,9 @@ internal class EnsureOlmSessionsForDevicesAction @Inject constructor(
                     }
                 }
             }
-
             return results
         }
+    }
 
     private fun verifyKeyAndStartSession(oneTimeKey: MXKey, userId: String, deviceInfo: CryptoDeviceInfo): String? {
         var sessionId: String? = null