[网络协议]rt-thread http_ota通过WIFI下载固件文件的内存不足解决

rt-thead、SPI flash、FAL分区、AT框架
CPU通过ESP8266 AT指令使用AT框架连接http服务器，下载文件过程中发生内存不足的错误打印

1、造成这个报警原因：（1）spi flash单数据线，写入速率较慢。（2）对比以前通过NB IOT升级没问题的原因，也可能是WIFI的速率太快，系统缓存不过来
2、查看http_ota_fw_download函数源码，OTA先获取文件大小，再擦除相就的大小flash的空间，而且提示信息也是先有擦除消息打印，再有内存不足打印
3、将flash擦除移到HTTP通讯的前面，先擦除再通讯，问题解决

static int http_ota_fw_download(const char* uri)
{
    int ret = 0, resp_status;
    int file_size = 0, length, total_length = 0;
    rt_uint8_t *buffer_read = RT_NULL;
    struct webclient_session* session = RT_NULL;
    const struct fal_partition * dl_part = RT_NULL;
    
    /* Get download partition information and erase download partition data */
    if ((dl_part = fal_partition_find("fm_area")) == RT_NULL)
    {
        LOG_E("Firmware download failed! Partition (%s) find error!", "fm_area");
        ret = -RT_ERROR;
        goto __exit;
    }

    LOG_I("Start erase flash (%s) partition!", dl_part->name);

    if (fal_partition_erase(dl_part, 0, dl_part->len) < 0)
    {
        LOG_E("Firmware download failed! Partition (%s) erase error!", dl_part->name);
        ret = -RT_ERROR;
        goto __exit;
    }
    LOG_I("Erase flash (%s) partition success!", dl_part->name);

    /* create webclient session and set header response size */
    session = webclient_session_create(GET_HEADER_BUFSZ);
    if (!session)
    {
        LOG_E("open uri failed.");
        ret = -RT_ERROR;
        goto __exit;
    }

    /* send GET request by default header */
    if ((resp_status = webclient_get(session, uri)) != 200)
    {
        LOG_E("webclient GET request failed, response(%d) error.", resp_status);
        ret = -RT_ERROR;
        goto __exit;
    }

    file_size = webclient_content_length_get(session);
    rt_kprintf("http file_size:%d\n",file_size);

    if (file_size == 0)
    {
        LOG_E("Request file size is 0!");
        ret = -RT_ERROR;
        goto __exit;
    }
    else if (file_size < 0)
    {
        LOG_E("webclient GET request type is chunked.");
        ret = -RT_ERROR;
        goto __exit;
    }

    buffer_read = web_malloc(HTTP_OTA_BUFF_LEN);
    if (buffer_read == RT_NULL)
    {
        LOG_E("No memory for http ota!");
        ret = -RT_ERROR;
        goto __exit;
    }
    memset(buffer_read, 0x00, HTTP_OTA_BUFF_LEN);

    LOG_I("OTA file size is (%d)", file_size);

    do
    {
        length = webclient_read(session, buffer_read, file_size - total_length > HTTP_OTA_BUFF_LEN ?
                            HTTP_OTA_BUFF_LEN : file_size - total_length);   
        if (length > 0)
        {
            /* Write the data to the corresponding partition address */
            if (fal_partition_write(dl_part, total_length, buffer_read, length) < 0)
            {
                LOG_E("Firmware download failed! Partition (%s) write data error!", dl_part->name);
                ret = -RT_ERROR;
                goto __exit;
            }
            total_length += length;

            print_progress(total_length, file_size);
        }
        else
        {
            LOG_E("Exit: server return err (%d)!", length);
            ret = -RT_ERROR;
            goto __exit;
        }

    } while(total_length != file_size);

    ret = RT_EOK;

    if (total_length == file_size)
    {
        if (session != RT_NULL)
        {
            webclient_close(session);
            session = RT_NULL;
        }
        if (buffer_read != RT_NULL)
        {
            web_free(buffer_read);
            buffer_read = RT_NULL;
        }

        LOG_I("Download firmware to flash success.");
        LOG_I("System now will restart...");

        rt_thread_delay(rt_tick_from_millisecond(5));

        /* Reset the device, Start new firmware */
        extern void rt_hw_cpu_reset(void);
        rt_hw_cpu_reset();
    }

__exit:
    if (session != RT_NULL)
        webclient_close(session);
    if (buffer_read != RT_NULL)
        web_free(buffer_read);

    return ret;
}