MTXStorage: Add transactions

FossilOrigin-Name: 5d9f93730cae9ba8132f233ccab7d0aa1a47d26a6e56085558bd2b8a2b483ca5

src/MTXSQLite3Storage.m

@@ -82,6 +82,11 @@
 				 @")"];
 }
 
+- (void)transactionWithBlock: (mtx_storage_transaction_block_t)block
+{
+	[_conn transactionWithBlock: block];
+}
+
 - (void)setNextBatch: (OFString *)nextBatch
 	 forDeviceID: (OFString *)deviceID
 {

src/MTXStorage.h

@@ -24,10 +24,23 @@
 
 OF_ASSUME_NONNULL_BEGIN
 
+/**
+ * @brief A block which will be treated as a single transaction for the storage.
+ *
+ * @return Whether the transaction should be committed (`true`) or rolled back
+ *	   (`false`).
+ */
+typedef bool (^mtx_storage_transaction_block_t)(void);
+
 /**
  * @brief A protocol for a storage to be used by @ref MTXClient.
  */
 @protocol MTXStorage <OFObject>
+/**
+ * @brief Performs all operations inside the block as a transaction.
+ */
+- (void)transactionWithBlock: (mtx_storage_transaction_block_t)block;
+
 /**
  * @brief Stores the next batch for the specified device.
  *