How to enable BLE long read on ESP32

If you have tried sending more than 22 bytes using the esp_ble_gatts_send_response API, you may have encountered the warning.

BT_GATT: attribute value too long, to be truncated to 22

The generic attribute protocol only supports packet sizes of 23 by default.
Excluding ATT protocol headers this leaves 22 bytes of payload.
In order to send bigger payloads, your application needs to support the long read command which sends your payload in multiple packets.
Within the ESP32 bluedroid framework, both read and read long commands trigger the ESP_GATTS_READ_EVT event.
Read long commands can be differentiated using the is_long and offset fields in read command params.

    struct gatts_read_evt_param {
        uint16_t conn_id;               /*!< Connection id */
        uint32_t trans_id;              /*!< Transfer id */
        esp_bd_addr_t bda;              /*!< The bluetooth device address which been read */
        uint16_t handle;                /*!< The attribute handle */
        uint16_t offset;                /*!< Offset of the value, if the value is too long */
        bool is_long;                   /*!< The value is too long or not */
        bool need_rsp;                  /*!< The read operation need to do response */
    } read;

For a long read, the ESP_GATTS_READ_EVT is fired multiple times with increasing offsets.
So to tie everything together, use the offset parameter to locate the correct segment within the send buffer. When buffer has been read to entirety send an empty segment.

typedef struct {
    uint8_t               *buf;
    int                   len;
} partial_buf;

static void gatts_profile_event_handler(esp_gatts_cb_event_t event, esp_gatt_if_t gatts_if, esp_ble_gatts_cb_param_t *param) {

   // Other events

   case ESP_GATTS_READ_EVT: {
   	/* struct partial_buf buf contains data to transmit */
        if (param->read.is_long && buf.buf) {
           rsp.attr_value.handle = param->read.handle;
	   int remaining_read = (buf.len - param->read.offset);
	   rsp.attr_value.len = (remaining_read) > 22 ? 22: remaining_read;
	   memcpy(rsp.attr_value.value, buf.buf + param->read.offset, rsp.attr_value.len);
	   if (offset + rsp.attr_value.len >= buf.len) {
	      if (buf.buf) {
	      	 free(buf.buf);
		 buf.buf = NULL;
	      }
	      buf.len = 0;
	   }
	}
	esp_ble_gatts_send_response(gatts_if, param->read.conn_id, param->read.trans_id,
                                    ESP_GATT_OK, &rsp);
        break;
   }
   // Other events
}

Sankalp Bose