Split X-Magento-Tags HTTP header into multiple headers

app/code/Magento/PageCache/Model/HTTP/Header/XMagentoTags.php

@@ -0,0 +1,110 @@
+<?php
+
+namespace Magento\PageCache\Model\HTTP\Header;
+
+use Zend\Http\Header\MultipleHeaderInterface;
+use Zend\Http\Header\GenericHeader;
+use Zend\Http\Header\HeaderValue;
+use Zend\Http\Header\Exception;
+
+class XMagentoTags implements MultipleHeaderInterface
+{
+    /**
+     * @var string
+     */
+    protected $value;
+
+    /**
+     * XMagentoTags constructor.
+     *
+     * @param string|null $value
+     */
+    public function __construct($value = null)
+    {
+        if ($value) {
+            HeaderValue::assertValid($value);
+            $this->value = $value;
+        }
+    }
+
+    /**
+     * Create X-Magento-Tags header from a given header line
+     *
+     * @param string $headerLine The header line to parse.
+     *
+     * @return self
+     * @throws Exception\InvalidArgumentException If the name field in the given header line does not match.
+     */
+    public static function fromString($headerLine)
+    {
+        list($name, $value) = GenericHeader::splitHeaderLine($headerLine);
+
+        // check to ensure proper header type for this factory
+        if (strtolower($name) !== 'x-magento-tags') {
+            throw new Exception\InvalidArgumentException(
+                sprintf(
+                    'Invalid header line for X-Magento-Tags string: "%s"',
+                    $name
+                )
+            );
+        }
+
+        // @todo implementation details
+        $header = new static($value);
+
+        return $header;
+    }
+
+    /**
+     * Cast multiple header objects to a single string header
+     *
+     * @param  array $headers
+     * @throws Exception\InvalidArgumentException
+     * @return string
+     */
+    public function toStringMultipleHeaders(array $headers)
+    {
+        $name = $this->getFieldName();
+        $values = array($this->getFieldValue());
+        foreach ($headers as $header) {
+            if (!$header instanceof static) {
+                throw new Exception\InvalidArgumentException(
+                    'This method toStringMultipleHeaders was expecting an array of headers of the same type'
+                );
+            }
+            $values[] = $header->getFieldValue();
+        }
+
+        return $name . ': ' . implode(',', $values) . "\r\n";
+    }
+
+    /**
+     * Get the header name
+     *
+     * @return string
+     */
+    public function getFieldName()
+    {
+        return 'X-Magento-Tags';
+    }
+
+    /**
+     * Get the header value
+     *
+     * @return string
+     */
+    public function getFieldValue()
+    {
+        return $this->value;
+    }
+
+    /**
+     * Return the header as a string
+     *
+     * @return string
+     */
+    public function toString()
+    {
+        return 'X-Magento-Tags: ' . $this->getFieldValue();
+    }
+}

app/code/Magento/PageCache/Model/HTTP/PhpEnvironment/ResponsePlugin.php

@@ -0,0 +1,93 @@
+<?php
+
+namespace Magento\PageCache\Model\HTTP\PhpEnvironment;
+
+use Magento\Framework\HTTP\PhpEnvironment\Response as Subject;
+use Magento\PageCache\Model\Config;
+use Zend\Http\HeaderLoader;
+use Magento\PageCache\Model\HTTP\Header\XMagentoTags;
+
+class ResponsePlugin
+{
+    /**
+     * Approximately 8kb in length
+     *
+     * @var int
+     */
+    private $requestSize = 8000;
+
+    /**
+     * PageCache configuration
+     *
+     * @var Config
+     */
+    private $config;
+
+    /**
+     * ResponsePlugin constructor.
+     *
+     * @param Config $config
+     */
+    public function __construct(
+        Config $config
+    ) {
+        $this->config = $config;
+    }
+
+    /**
+     * Special case for handling X-Magento-Tags header
+     * splits very long header into multiple headers
+     *
+     * @param Subject  $subject
+     * @param \Closure $proceed
+     * @param string   $name
+     * @param string   $value
+     * @param bool     $replace
+     *
+     * @return Subject|mixed
+     */
+    public function aroundSetHeader(Subject $subject, \Closure $proceed, $name, $value, $replace = false)
+    {
+        //if varnish isn't enabled, don't do anything
+        if (!$this->config->isEnabled() || $this->config->getType() != Config::VARNISH) {
+            return $proceed($name, $value, $replace);
+        }
+
+        $this->addHeaderToStaticMap();
+
+        if ($name == 'X-Magento-Tags') {
+            $headerLength = 0;
+            $value = (string)$value;
+            $tags = explode(',', $value);
+
+            $newTags = [];
+            foreach ($tags as $tag) {
+                if ($headerLength + strlen($tag) > $this->requestSize - count($tags) - 1) {
+                    $tagString = implode(',', $tags);
+                    $subject->getHeaders()->addHeaderLine($name, $tagString);
+                    $newTags = [];
+                    $headerLength = 0;
+                }
+                $headerLength += strlen($tag);
+                $newTags[] = $tag;
+            }
+
+            return $subject;
+        }
+
+        return $proceed($name, $value, $replace);
+    }
+
+    /**
+     * Add X-Magento-Tags header to HeaderLoader static map
+     */
+    private function addHeaderToStaticMap()
+    {
+        HeaderLoader::addStaticMap(
+            [
+                'xmagentotags' => XMagentoTags::class,
+            ]
+        );
+    }
+
+}

app/code/Magento/PageCache/etc/frontend/di.xml

@@ -25,4 +25,7 @@
     <type name="Magento\Framework\App\Response\Http">
         <plugin name="response-http-page-cache" type="Magento\PageCache\Model\App\Response\HttpPlugin"/>
     </type>
+    <type name="Magento\Framework\HTTP\PhpEnvironment\Response">
+        <plugin name="response-split-x-magento-tags-headers" type="Magento\PageCache\Model\HTTP\PhpEnvironment\ResponsePlugin" />
+    </type>
 </config>

app/code/Magento/PageCache/etc/varnish4.vcl

@@ -169,6 +169,10 @@ sub vcl_backend_response {
         set beresp.ttl = 120s;
         set beresp.uncacheable = true;
     }
+
+    # collect all x-magento-tags headers
+    std.collect(beresp.http.X-Magento-Tags);
+
     return (deliver);
 }
 

app/code/Magento/PageCache/etc/varnish5.vcl

@@ -170,6 +170,10 @@ sub vcl_backend_response {
         set beresp.ttl = 120s;
         set beresp.uncacheable = true;
     }
+
+    # collect all x-magento-tags headers
+    std.collect(beresp.http.X-Magento-Tags);
+
     return (deliver);
 }