att_db: validate prepared writes before execution. Additional att_write_callback with ATT_TRANSACTION_MODE_VALIDATE

src/ble/att_db.c

@@ -986,6 +986,20 @@ static void att_notify_write_callbacks(att_connection_t * att_connection, uint16
     (*att_write_callback)(att_connection->con_handle, 0, transaction_mode, 0, NULL, 0);
 }
 
+// returns first reported error or 0
+static uint8_t att_validate_prepared_write(att_connection_t * att_connection){
+    btstack_linked_list_iterator_t it;
+    btstack_linked_list_iterator_init(&it, &service_handlers);
+    while (btstack_linked_list_iterator_has_next(&it)){
+        att_service_handler_t * handler = (att_service_handler_t*) btstack_linked_list_iterator_next(&it);
+        if (!handler->write_callback) continue;
+        uint8_t error_code = (*handler->write_callback)(att_connection->con_handle, 0, ATT_TRANSACTION_MODE_VALIDATE, 0, NULL, 0);
+        if (error_code) return error_code;
+    }
+    if (!att_write_callback) return 0;
+    return (*att_write_callback)(att_connection->con_handle, 0, ATT_TRANSACTION_MODE_VALIDATE, 0, NULL, 0);
+}
+
 /*
  * @brief transcation queue of prepared writes, e.g., after disconnect
  */
@@ -1003,6 +1017,10 @@ static uint16_t handle_execute_write_request(att_connection_t * att_connection,
     
     uint8_t request_type = ATT_EXECUTE_WRITE_REQUEST;
     if (request_buffer[1]) {
+        // validate queued write
+        if (att_prepare_write_error_code == 0){
+            att_prepare_write_error_code = att_validate_prepared_write(att_connection);
+        }
         // deliver queued errors
         if (att_prepare_write_error_code){
             att_clear_transaction_queue(att_connection);

src/ble/att_db.h

@@ -76,15 +76,22 @@ typedef uint16_t (*att_read_callback_t)(hci_con_handle_t con_handle, uint16_t at
 // ATT Client Write Callback for Dynamic Data
 // @param con_handle of hci le connection
 // @param attribute_handle to be written
-// @param transaction - ATT_TRANSACTION_MODE_NONE for regular writes, ATT_TRANSACTION_MODE_ACTIVE for prepared writes and ATT_TRANSACTION_MODE_EXECUTE
+// @param transaction - ATT_TRANSACTION_MODE_NONE for regular writes. For prepared writes: ATT_TRANSACTION_MODE_ACTIVE, ATT_TRANSACTION_MODE_VALIDATE, ATT_TRANSACTION_MODE_EXECUTE, ATT_TRANSACTION_MODE_CANCEL
 // @param offset into the value - used for queued writes and long attributes
 // @param buffer 
 // @param buffer_size
 // @param signature used for signed write commmands
 // @returns 0 if write was ok, ATT_ERROR_PREPARE_QUEUE_FULL if no space in queue, ATT_ERROR_INVALID_OFFSET if offset is larger than max buffer
+//
+// Each Prepared Write Request triggers a callback with transaction mode ATT_TRANSACTION_MODE_ACTIVE.
+// On Execute Write, the callback will be called with ATT_TRANSACTION_MODE_VALIDATE and allows to validate all queued writes and return an application error.
+// If none of the registered callbacks return an error for ATT_TRANSACTION_MODE_VALIDATE and the callback will be called with ATT_TRANSACTION_MODE_EXECUTE.
+// Otherwise, all callbacks will be called with ATT_TRANSACTION_MODE_CANCEL.
+//
+// If the additional validation step is not needed, just return 0 for all callbacks with transaction mode ATT_TRANSACTION_MODE_VALIDATE.
+//
 typedef int (*att_write_callback_t)(hci_con_handle_t con_handle, uint16_t attribute_handle, uint16_t transaction_mode, uint16_t offset, uint8_t *buffer, uint16_t buffer_size);
 
-
 // Read & Write Callbacks for handle range
 typedef struct att_service_handler {
   btstack_linked_item_t * item;

src/bluetooth.h

@@ -1081,13 +1081,14 @@ typedef enum {
 #define ATT_PROPERTY_AUTHORIZATION_REQUIRED  0x800
 // Encryption key size stored in upper 4 bits, 0 == no encryption, encryption key size - 1 otherwise
 
-// ATT Transaction Timeout of 30 seconds for Command/Response or Incidationc/Confirmation
+// ATT Transaction Timeout of 30 seconds for Command/Response or Indication/Confirmation
 #define ATT_TRANSACTION_TIMEOUT_MS     30000
 
 #define ATT_TRANSACTION_MODE_NONE      0x0
 #define ATT_TRANSACTION_MODE_ACTIVE    0x1
 #define ATT_TRANSACTION_MODE_EXECUTE   0x2
 #define ATT_TRANSACTION_MODE_CANCEL    0x3
+#define ATT_TRANSACTION_MODE_VALIDATE  0x4
 
 // MARK: GATT UUIDs
 #define GATT_PRIMARY_SERVICE_UUID                   0x2800