extmod/modbluetooth: Add gatts_set_buffer.

- Adds an explicit way to set the size of a value's internal buffer,
  replacing `ble.gatts_write(handle, bytes(size))` (although that
  still works).
- Add an "append" mode for values, which means that remote writes
  will append to the buffer.

extmod/modbluetooth.c

@@ -557,6 +557,14 @@ STATIC mp_obj_t bluetooth_ble_gatts_notify(size_t n_args, const mp_obj_t *args)
 }
 STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(bluetooth_ble_gatts_notify_obj, 3, 4, bluetooth_ble_gatts_notify);
 
+STATIC mp_obj_t bluetooth_ble_gatts_set_buffer(size_t n_args, const mp_obj_t *args) {
+    mp_int_t value_handle = mp_obj_get_int(args[1]);
+    mp_int_t len = mp_obj_get_int(args[2]);
+    bool append = n_args >= 4 && mp_obj_is_true(args[3]);
+    return bluetooth_handle_errno(mp_bluetooth_gatts_set_buffer(value_handle, len, append));
+}
+STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(bluetooth_ble_gatts_set_buffer_obj, 3, 4, bluetooth_ble_gatts_set_buffer);
+
 // ----------------------------------------------------------------------------
 // Bluetooth object: GATTC (Central/Scanner role)
 // ----------------------------------------------------------------------------
@@ -626,6 +634,7 @@ STATIC const mp_rom_map_elem_t bluetooth_ble_locals_dict_table[] = {
     { MP_ROM_QSTR(MP_QSTR_gatts_read), MP_ROM_PTR(&bluetooth_ble_gatts_read_obj) },
     { MP_ROM_QSTR(MP_QSTR_gatts_write), MP_ROM_PTR(&bluetooth_ble_gatts_write_obj) },
     { MP_ROM_QSTR(MP_QSTR_gatts_notify), MP_ROM_PTR(&bluetooth_ble_gatts_notify_obj) },
+    { MP_ROM_QSTR(MP_QSTR_gatts_set_buffer), MP_ROM_PTR(&bluetooth_ble_gatts_set_buffer_obj) },
     #if MICROPY_PY_BLUETOOTH_ENABLE_CENTRAL_MODE
     // GATT Client (i.e. central/scanner role)
     { MP_ROM_QSTR(MP_QSTR_gattc_discover_services), MP_ROM_PTR(&bluetooth_ble_gattc_discover_services_obj) },

extmod/modbluetooth.h

@@ -183,6 +183,10 @@ int mp_bluetooth_gatts_notify_send(uint16_t conn_handle, uint16_t value_handle,
 // Indicate the central.
 int mp_bluetooth_gatts_indicate(uint16_t conn_handle, uint16_t value_handle);
 
+// Resize and enable/disable append-mode on a value.
+// Append-mode means that remote writes will append and local reads will clear after reading.
+int mp_bluetooth_gatts_set_buffer(uint16_t value_handle, size_t len, bool append);
+
 // Disconnect from a central or peripheral.
 int mp_bluetooth_gap_disconnect(uint16_t conn_handle);
 

extmod/modbluetooth_nimble.c

@@ -154,9 +154,14 @@ STATIC ble_addr_t create_nimble_addr(uint8_t addr_type, const uint8_t *addr) {
 #endif // MICROPY_PY_BLUETOOTH_ENABLE_CENTRAL_MODE
 
 typedef struct {
+    // Pointer to heap-allocated data.
     uint8_t *data;
+    // Allocated size of data.
     size_t data_alloc;
+    // Current bytes in use.
     size_t data_len;
+    // Whether new writes append or replace existing data (default false).
+    bool append;
 } gatts_db_entry_t;
 
 volatile int mp_bluetooth_nimble_ble_state = MP_BLUETOOTH_NIMBLE_BLE_STATE_OFF;
@@ -206,6 +211,7 @@ STATIC void create_gatts_db_entry(uint16_t handle) {
     entry->data = m_new(uint8_t, MP_BLUETOOTH_MAX_ATTR_SIZE);
     entry->data_alloc = MP_BLUETOOTH_MAX_ATTR_SIZE;
     entry->data_len = 0;
+    entry->append = false;
     elem->value = MP_OBJ_FROM_PTR(entry);
 }
 
@@ -430,8 +436,13 @@ static int characteristic_access_cb(uint16_t conn_handle, uint16_t value_handle,
                 return BLE_ATT_ERR_ATTR_NOT_FOUND;
             }
             entry = MP_OBJ_TO_PTR(elem->value);
-            entry->data_len = MIN(entry->data_alloc, OS_MBUF_PKTLEN(ctxt->om));
-            os_mbuf_copydata(ctxt->om, 0, entry->data_len, entry->data);
+
+            size_t offset = 0;
+            if (entry->append) {
+                offset = entry->data_len;
+            }
+            entry->data_len = MIN(entry->data_alloc, OS_MBUF_PKTLEN(ctxt->om) + offset);
+            os_mbuf_copydata(ctxt->om, 0, entry->data_len - offset, entry->data + offset);
 
             mp_bluetooth_gatts_on_write(conn_handle, value_handle);
 
@@ -547,6 +558,9 @@ int mp_bluetooth_gatts_read(uint16_t value_handle, uint8_t **value, size_t *valu
     gatts_db_entry_t *entry = MP_OBJ_TO_PTR(elem->value);
     *value = entry->data;
     *value_len = entry->data_len;
+    if (entry->append) {
+        entry->data_len = 0;
+    }
     return 0;
 }
 
@@ -587,6 +601,19 @@ int mp_bluetooth_gatts_indicate(uint16_t conn_handle, uint16_t value_handle) {
     return ble_hs_err_to_errno(ble_gattc_indicate(conn_handle, value_handle));
 }
 
+int mp_bluetooth_gatts_set_buffer(uint16_t value_handle, size_t len, bool append) {
+    mp_map_elem_t *elem = mp_map_lookup(MP_STATE_PORT(bluetooth_nimble_root_pointers)->gatts_db, MP_OBJ_NEW_SMALL_INT(value_handle), MP_MAP_LOOKUP);
+    if (!elem) {
+        return MP_EINVAL;
+    }
+    gatts_db_entry_t *entry = MP_OBJ_TO_PTR(elem->value);
+    entry->data = m_renew(uint8_t, entry->data, entry->data_alloc, len);
+    entry->data_alloc = len;
+    entry->data_len = 0;
+    entry->append = append;
+    return 0;
+}
+
 #if MICROPY_PY_BLUETOOTH_ENABLE_CENTRAL_MODE
 
 STATIC int gap_scan_cb(struct ble_gap_event *event, void *arg) {