Since 31/01/22 all the download files become corrupted.

That could be the cause. Unfortunately, I'm using Magento 1 which is not more supported from Magento devs so I need to do this on my own.

However... you said "HTTP clients are expected to handle and decode this automatically...". How the HTTP client should handle it if it doesn't do automatically?

This is the Magento helper:

<?php
/**
 * Magento
 *
 * NOTICE OF LICENSE
 *
 * This source file is subject to the Open Software License (OSL 3.0)
 * that is bundled with this package in the file LICENSE.txt.
 * It is also available through the world-wide-web at this URL:
 * http://opensource.org/licenses/osl-3.0.php
 * If you did not receive a copy of the license and are unable to
 * obtain it through the world-wide-web, please send an email
 * to license@magento.com so we can send you a copy immediately.
 *
 * DISCLAIMER
 *
 * Do not edit or add to this file if you wish to upgrade Magento to newer
 * versions in the future. If you wish to customize Magento for your
 * needs please refer to http://www.magento.com for more information.
 *
 * @category    Mage
 * @package     Mage_Downloadable
 * @copyright  Copyright (c) 2006-2019 Magento, Inc. (http://www.magento.com)
 * @license    http://opensource.org/licenses/osl-3.0.php  Open Software License (OSL 3.0)
 */

/**
 * Downloadable Products Download Helper
 *
 * @category    Mage
 * @package     Mage_Downloadable
 * @author      Magento Core Team <core@magentocommerce.com>
 */
class Mage_Downloadable_Helper_Download extends Mage_Core_Helper_Abstract
{
    const LINK_TYPE_URL         = 'url';
    const LINK_TYPE_FILE        = 'file';

    const XML_PATH_CONTENT_DISPOSITION  = 'catalog/downloadable/content_disposition';

    /**
     * Type of link
     *
     *  string
     */
    protected $_linkType        = self::LINK_TYPE_FILE;

    /**
     * Resource file
     *
     *  string
     */
    protected $_resourceFile    = null;

    /**
     * Resource open handle
     *
     *  resource
     */
    protected $_handle          = null;

    /**
     * Remote server headers
     *
     *  array
     */
    protected $_urlHeaders      = array();

    /**
     * MIME Content-type for a file
     *
     *  string
     */
    protected $_contentType     = 'application/octet-stream';

    /**
     * File name
     *
     *  string
     */
    protected $_fileName        = 'download';

    /**
     * Retrieve Resource file handle (socket, file pointer etc)
     *
     * @return resource
     */
    protected function _getHandle()
    {
        if (!$this->_resourceFile) {
            Mage::throwException(Mage::helper('downloadable')->__('Please set resource file and link type.'));
        }

        if (is_null($this->_handle)) {
            if ($this->_linkType == self::LINK_TYPE_URL) {

                /**
                 * Validate URL
                 */
                $urlProp = parse_url($this->_resourceFile);
                if (!isset($urlProp['scheme'])
                    || strtolower($urlProp['scheme'] != 'http') && strtolower($urlProp['scheme'] != 'https')) {
                    Mage::throwException(Mage::helper('downloadable')->__('Invalid download URL scheme.'));
                }
                if (!isset($urlProp['host'])) {
                    Mage::throwException(Mage::helper('downloadable')->__('Invalid download URL host.'));
                }
                switch ($urlProp['scheme']) {
                    case 'https':
                        $scheme = 'ssl://';
                        $port = 443;
                        break;
                    case 'http':
                    default:
                        $scheme = '';
                        $port = 80;
                }
                $hostname = $scheme . $urlProp['host'];

                if (isset($urlProp['port'])) {
                    $port = (int)$urlProp['port'];
                }

                $path = '/';
                if (isset($urlProp['path'])) {
                    $path = $urlProp['path'];
                }
                $query = '';
                if (isset($urlProp['query'])) {
                    $query = '?' . $urlProp['query'];
                }

                try {
                    $this->_handle = fsockopen($hostname, $port, $errno, $errstr);
                }
                catch (Exception $e) {
                    throw $e;
                }

                if ($this->_handle === false) {
                    Mage::throwException(Mage::helper('downloadable')->__('Cannot connect to remote host, error: %s.', $errstr));
                }

                $headers = 'GET ' . $path . $query . ' HTTP/1.0' . "\r\n"
                    . 'Host: ' . $urlProp['host'] . "\r\n"
                    . 'User-Agent: Magento ver/' . Mage::getVersion() . "\r\n"
                    . 'Connection: close' . "\r\n"
                    . "\r\n";
                fwrite($this->_handle, $headers);

                while (!feof($this->_handle)) {
                    $str = fgets($this->_handle, 1024);
                    if ($str == "\r\n") {
                        break;
                    }
                    $match = array();
                    if (preg_match('#^([^:]+): (.*)\s+$#', $str, $match)) {
                        $k = strtolower($match[1]);
                        if ($k == 'set-cookie') {
                            continue;
                        }
                        else {
                            $this->_urlHeaders[$k] = trim($match[2]);
                        }
                    }
                    elseif (preg_match('#^HTTP/[0-9\.]+ (\d+) (.*)\s$#', $str, $match)) {
                        $this->_urlHeaders['code'] = $match[1];
                        $this->_urlHeaders['code-string'] = trim($match[2]);
                    }
                }

                if (!isset($this->_urlHeaders['code']) || $this->_urlHeaders['code'] != 200) {
                    Mage::throwException(Mage::helper('downloadable')->__('An error occurred while getting the requested content. Please contact the store owner.'));
                }
            }
            elseif ($this->_linkType == self::LINK_TYPE_FILE) {
                $this->_handle = new Varien_Io_File();
                if (!is_file($this->_resourceFile)) {
                    Mage::helper('core/file_storage_database')->saveFileToFilesystem($this->_resourceFile);
                }
                $this->_handle->open(array('path'=>Mage::getBaseDir('var')));
                if (!$this->_handle->fileExists($this->_resourceFile, true)) {
                    Mage::throwException(Mage::helper('downloadable')->__('The file does not exist.'));
                }
                $this->_handle->streamOpen($this->_resourceFile, 'r');
            }
            else {
                Mage::throwException(Mage::helper('downloadable')->__('Invalid download link type.'));
            }
        }
        return $this->_handle;
    }

    /**
     * Retrieve file size in bytes
     */
    public function getFilesize()
    {
        $handle = $this->_getHandle();
        if ($this->_linkType == self::LINK_TYPE_FILE) {
            return $handle->streamStat('size');
        }
        elseif ($this->_linkType == self::LINK_TYPE_URL) {
            if (isset($this->_urlHeaders['content-length'])) {
                return $this->_urlHeaders['content-length'];
            }
        }
        return null;
    }

    public function getContentType()
    {
        $handle = $this->_getHandle();
        if ($this->_linkType == self::LINK_TYPE_FILE) {
            if (function_exists('mime_content_type') && ($contentType = mime_content_type($this->_resourceFile))) {
                return $contentType;
            } else {
                return Mage::helper('downloadable/file')->getFileType($this->_resourceFile);
            }
        }
        elseif ($this->_linkType == self::LINK_TYPE_URL) {
            if (isset($this->_urlHeaders['content-type'])) {
                $contentType = explode('; ', $this->_urlHeaders['content-type']);
                return $contentType[0];
            }
        }
        return $this->_contentType;
    }

    public function getFilename()
    {
        $handle = $this->_getHandle();
        if ($this->_linkType == self::LINK_TYPE_FILE) {
            return pathinfo($this->_resourceFile, PATHINFO_BASENAME);
        }
        elseif ($this->_linkType == self::LINK_TYPE_URL) {
            if (isset($this->_urlHeaders['content-disposition'])) {
                $contentDisposition = explode('; ', $this->_urlHeaders['content-disposition']);
                if (!empty($contentDisposition[1]) && strpos($contentDisposition[1], 'filename=') !== false) {
                    return substr($contentDisposition[1], 9);
                }
            }
            if ($fileName = @pathinfo($this->_resourceFile, PATHINFO_BASENAME)) {
                return $fileName;
            }
        }
        return $this->_fileName;
    }

    /**
     * Set resource file for download
     *
     * @param string $resourceFile
     * @param string $linkType
     * @return Mage_Downloadable_Helper_Download
     */
    public function setResource($resourceFile, $linkType = self::LINK_TYPE_FILE)
    {
        if (self::LINK_TYPE_FILE == $linkType) {
            //check LFI protection
            /**  $helper Mage_Core_Helper_Data */
            $helper = Mage::helper('core');
            $helper->checkLfiProtection($resourceFile);
        }

        $this->_resourceFile    = $resourceFile;
        $this->_linkType        = $linkType;

        return $this;
    }

    /**
     * Retrieve Http Request Object
     *
     * @return Mage_Core_Controller_Request_Http
     */
    public function getHttpRequest()
    {
        return Mage::app()->getFrontController()->getRequest();
    }

    /**
     * Retrieve Http Response Object
     *
     * @return Mage_Core_Controller_Response_Http
     */
    public function getHttpResponse()
    {
        return Mage::app()->getFrontController()->getResponse();
    }

    public function output()
    {
        $handle = $this->_getHandle();
        if ($this->_linkType == self::LINK_TYPE_FILE) {
            while ($buffer = $handle->streamRead()) {
                print $buffer;
            }
        }
        elseif ($this->_linkType == self::LINK_TYPE_URL) {
            while (!feof($handle)) {
                print fgets($handle, 1024);
            }
        }
    }

    /**
     * Use Content-Disposition: attachment
     *
     * @param mixed $store
     * @return bool
     */
    public function getContentDisposition($store = null)
    {
        return Mage::getStoreConfig(self::XML_PATH_CONTENT_DISPOSITION, $store);
    }
}

and this is the Controller:

<?php
/**
 * Magento
 *
 * NOTICE OF LICENSE
 *
 * This source file is subject to the Open Software License (OSL 3.0)
 * that is bundled with this package in the file LICENSE.txt.
 * It is also available through the world-wide-web at this URL:
 * http://opensource.org/licenses/osl-3.0.php
 * If you did not receive a copy of the license and are unable to
 * obtain it through the world-wide-web, please send an email
 * to license@magento.com so we can send you a copy immediately.
 *
 * DISCLAIMER
 *
 * Do not edit or add to this file if you wish to upgrade Magento to newer
 * versions in the future. If you wish to customize Magento for your
 * needs please refer to http://www.magento.com for more information.
 *
 * @category    Mage
 * @package     Mage_Downloadable
 * @copyright  Copyright (c) 2006-2019 Magento, Inc. (http://www.magento.com)
 * @license    http://opensource.org/licenses/osl-3.0.php  Open Software License (OSL 3.0)
 */

/**
 * Download controller
 *
 * @category    Mage
 * @package     Mage_Downloadable
 * @author      Magento Core Team <core@magentocommerce.com>
 */
class Mage_Downloadable_DownloadController extends Mage_Core_Controller_Front_Action
{

    /**
     * Return core session object
     *
     * @return Mage_Core_Model_Session
     */
    protected function _getSession()
    {
        return Mage::getSingleton('core/session');
    }

    /**
     * Return customer session object
     *
     * @return Mage_Customer_Model_Session
     */
    protected function _getCustomerSession()
    {
        return Mage::getSingleton('customer/session');
    }

    protected function _processDownload($resource, $resourceType)
    {
        $helper = Mage::helper('downloadable/download');
        /*  $helper Mage_Downloadable_Helper_Download */

        $helper->setResource($resource, $resourceType);

        $fileName       = $helper->getFilename();
        $contentType    = $helper->getContentType();

        $this->getResponse()
            ->setHttpResponseCode(200)
            ->setHeader('Pragma', 'public', true)
            ->setHeader('Cache-Control', 'must-revalidate, post-check=0, pre-check=0', true)
            ->setHeader('Content-type', $contentType, true);

        if ($fileSize = $helper->getFilesize()) {
            $this->getResponse()
                ->setHeader('Content-Length', $fileSize);
        }

        if ($contentDisposition = $helper->getContentDisposition()) {
            $this->getResponse()
                ->setHeader('Content-Disposition', $contentDisposition . '; filename='.$fileName);
        }

        $this->getResponse()
            ->clearBody();
        $this->getResponse()
            ->sendHeaders();

        session_write_close();
        $helper->output();
    }

    /**
     * Download sample action
     *
     */
    public function sampleAction()
    {
        $sampleId = $this->getRequest()->getParam('sample_id', 0);
        $sample = Mage::getModel('downloadable/sample')->load($sampleId);
        if (
            $sample->getId()
            && Mage::helper('catalog/product')
                ->getProduct((int) $sample->getProductId(), Mage::app()->getStore()->getId(), 'id')
                ->isAvailable()
        ) {
            $resource = '';
            $resourceType = '';
            if ($sample->getSampleType() == Mage_Downloadable_Helper_Download::LINK_TYPE_URL) {
                $resource = $sample->getSampleUrl();
                $resourceType = Mage_Downloadable_Helper_Download::LINK_TYPE_URL;
            } elseif ($sample->getSampleType() == Mage_Downloadable_Helper_Download::LINK_TYPE_FILE) {
                $resource = Mage::helper('downloadable/file')->getFilePath(
                    Mage_Downloadable_Model_Sample::getBasePath(), $sample->getSampleFile()
                );
                $resourceType = Mage_Downloadable_Helper_Download::LINK_TYPE_FILE;
            }
            try {
                $this->_processDownload($resource, $resourceType);
                exit(0);
            } catch (Mage_Core_Exception $e) {
                $this->_getSession()->addError(Mage::helper('downloadable')->__('Sorry, there was an error getting requested content. Please contact the store owner.'));
            }
        }
        return $this->_redirectReferer();
    }

    /**
     * Download link's sample action
     *
     */
    public function linkSampleAction()
    {
        $linkId = $this->getRequest()->getParam('link_id', 0);
        $link = Mage::getModel('downloadable/link')->load($linkId);
        if (
            $link->getId()
            && Mage::helper('catalog/product')
                ->getProduct((int) $link->getProductId(), Mage::app()->getStore()->getId(), 'id')
                ->isAvailable()
        ) {
            $resource = '';
            $resourceType = '';
            if ($link->getSampleType() == Mage_Downloadable_Helper_Download::LINK_TYPE_URL) {
                $resource = $link->getSampleUrl();
                $resourceType = Mage_Downloadable_Helper_Download::LINK_TYPE_URL;
            } elseif ($link->getSampleType() == Mage_Downloadable_Helper_Download::LINK_TYPE_FILE) {
                $resource = Mage::helper('downloadable/file')->getFilePath(
                    Mage_Downloadable_Model_Link::getBaseSamplePath(), $link->getSampleFile()
                );
                $resourceType = Mage_Downloadable_Helper_Download::LINK_TYPE_FILE;
            }
            try {
                $this->_processDownload($resource, $resourceType);
                exit(0);
            } catch (Mage_Core_Exception $e) {
                $this->_getCustomerSession()->addError(Mage::helper('downloadable')->__('Sorry, there was an error getting requested content. Please contact the store owner.'));
            }
        }
        return $this->_redirectReferer();
    }

    /**
     * Download link action
     */
    public function linkAction()
    {
        $id = $this->getRequest()->getParam('id', 0);
        $linkPurchasedItem = Mage::getModel('downloadable/link_purchased_item')->load($id, 'link_hash');
        if (! $linkPurchasedItem->getId() ) {
            $this->_getCustomerSession()->addNotice(Mage::helper('downloadable')->__("Requested link does not exist."));
            return $this->_redirect('*/customer/products');
        }
        if (!Mage::helper('downloadable')->getIsShareable($linkPurchasedItem)) {
            $customerId = $this->_getCustomerSession()->getCustomerId();
            if (!$customerId) {
                $product = Mage::getModel('catalog/product')->load($linkPurchasedItem->getProductId());
                if ($product->getId()) {
                    $notice = Mage::helper('downloadable')->__('Please log in to download your product or purchase <a href="%s">%s</a>.', $product->getProductUrl(), $product->getName());
                } else {
                    $notice = Mage::helper('downloadable')->__('Please log in to download your product.');
                }
                $this->_getCustomerSession()->addNotice($notice);
                $this->_getCustomerSession()->authenticate($this);
                $this->_getCustomerSession()->setBeforeAuthUrl(Mage::getUrl('downloadable/customer/products/'),
                    array('_secure' => true)
                );
                return ;
            }
            $linkPurchased = Mage::getModel('downloadable/link_purchased')->load($linkPurchasedItem->getPurchasedId());
            if ($linkPurchased->getCustomerId() != $customerId) {
                $this->_getCustomerSession()->addNotice(Mage::helper('downloadable')->__("Requested link does not exist."));
                return $this->_redirect('*/customer/products');
            }
        }
        $downloadsLeft = $linkPurchasedItem->getNumberOfDownloadsBought()
            - $linkPurchasedItem->getNumberOfDownloadsUsed();

        $status = $linkPurchasedItem->getStatus();
        if ($status == Mage_Downloadable_Model_Link_Purchased_Item::LINK_STATUS_AVAILABLE
            && ($downloadsLeft || $linkPurchasedItem->getNumberOfDownloadsBought() == 0)
        ) {
            $resource = '';
            $resourceType = '';
            if ($linkPurchasedItem->getLinkType() == Mage_Downloadable_Helper_Download::LINK_TYPE_URL) {
                $resource = $linkPurchasedItem->getLinkUrl();
                $resourceType = Mage_Downloadable_Helper_Download::LINK_TYPE_URL;
            } elseif ($linkPurchasedItem->getLinkType() == Mage_Downloadable_Helper_Download::LINK_TYPE_FILE) {
                $resource = Mage::helper('downloadable/file')->getFilePath(
                    Mage_Downloadable_Model_Link::getBasePath(), $linkPurchasedItem->getLinkFile()
                );
                $resourceType = Mage_Downloadable_Helper_Download::LINK_TYPE_FILE;
            }
            try {
                $this->_processDownload($resource, $resourceType);
                $linkPurchasedItem->setNumberOfDownloadsUsed($linkPurchasedItem->getNumberOfDownloadsUsed() + 1);

                if ($linkPurchasedItem->getNumberOfDownloadsBought() != 0 && !($downloadsLeft - 1)) {
                    $linkPurchasedItem->setStatus(Mage_Downloadable_Model_Link_Purchased_Item::LINK_STATUS_EXPIRED);
                }
                $linkPurchasedItem->save();
                exit(0);
            }
            catch (Exception $e) {
                $this->_getCustomerSession()->addError(
                    Mage::helper('downloadable')->__('An error occurred while getting the requested content. Please contact the store owner.')
                );
            }
        } elseif ($status == Mage_Downloadable_Model_Link_Purchased_Item::LINK_STATUS_EXPIRED) {
            $this->_getCustomerSession()->addNotice(Mage::helper('downloadable')->__('The link has expired.'));
        } elseif ($status == Mage_Downloadable_Model_Link_Purchased_Item::LINK_STATUS_PENDING
            || $status == Mage_Downloadable_Model_Link_Purchased_Item::LINK_STATUS_PAYMENT_REVIEW
        ) {
            $this->_getCustomerSession()->addNotice(Mage::helper('downloadable')->__('The link is not available.'));
        } else {
            $this->_getCustomerSession()->addError(
                Mage::helper('downloadable')->__('An error occurred while getting the requested content. Please contact the store owner.')
            );
        }
        return $this->_redirect('*/customer/products');
    }

}

So, you said it should not get anymore the file size because you set "Transfer-Encoding: chunked". How should it handle without it?

Solved!

Just comment out the lines in the controller responsible for set the header "Content-Length" and then set the header  "Transfer-Encoding: chunked".

File: app/code/core/Mage/Downloadable/controllers/DownloadController.php

on _processDownload method on line 73 comment out these lines and add this other:

...
protected function _processDownload($resource, $resourceType)
        ....

        // if ($fileSize = $helper->getFilesize()) {
        //     $this->getResponse()
        //         ->setHeader('Content-Length', $fileSize);
        // }

        $this->getResponse()
        ->setHeader('Transfer-Encoding', 'chunked');

        ....
        }

Now it works but could you please tell me if there are some other headers to add or remove?

Thanks for following up. I'm glad to hear you got this working. I can't provide guidance on what other headers you should or shouldn't be setting though, as that depends on how the app works and what the app does.

Sure, I understand. However, did you guys change just that recently?

Yes, that update was deployed towards the end of January.

Thanks for your support.

Update: in order to temporarily accommodate clients that don’t properly support automatically handling “Transfer-Encoding: chunked”, we’re temporarily rolling back this change, so that these links will no longer use “Transfer-Encoding: chunked” and will instead return “Content-Length” on HTTP/1.1. We will begin rolling that out starting around 2/17. That will be in place until around 3/1. At that point, we will begin using “Transfer-Encoding: chunked” and no longer returning “Content-Length” on HTTP/1.1 again.

Going forward, please ensure that your clients are able to automatically handle both chunked encoding and non-chunked encoding automatically.

Then I'll set it like this:

if ($fileSize = $helper->getFilesize()) {
    $this->getResponse()->setHeader('Content-Length', $fileSize);
} else {
    $this->getResponse()->setHeader('Transfer-Encoding', 'chunked');
}

Update: the team has been able to complete some further updates to our infrastructure to be able to support the previous non-chunked behavior going forward indefinitely. That means that we plan to continue returning Content-Length (and not 'Transfer-Encoding: chunked') in the future and will not be reverting this as previously planned. (Regardless, for HTTP compatibility in general, we still recommend you make sure your HTTP clients support both types.) Hope this helps!

I noticed, since around September/October 2023, downloads really often (8 in 10 times) are interrupted at different size. And just now I remembered this issue.

Did you change something again?

I also opened this thread.

kamzata I moved your message to its own thread here: https://www.dropboxforum.com/t5/Dropbox-API-Support-Feedback/Re-Since-31-01-22-all-the-download-files-become-corrupted/td-p/754307

Please follow up there with the details so we can look into the current issue for you.