Allow usage of Reader without GooglePlayPodcast extension

Custom implementations of `ExtensionManagerInterface` may not have _new
core_ extensions present. As a result, when upgrading, an exception is
thrown due to inability to load the new extension.

This was discovered with the 2.10 release, when we added a new core
extension, GooglePlayPodcast; see #80 and https://www.drupal.org/project/drupal/issues/2976335
for more details.

As such, we now check for _new_ core extensions before registering them.
When we discover they are not in the extension manager, we emit an
`E_USER_NOTICE` indicating the user should update their
`ExtensionManagerInterface` implementation to add entries for the new
extension, but then continue on without registering the extension. This
ensures no exception is thrown.

src/Reader/Reader.php

@@ -577,19 +577,24 @@ public static function getExtensionManager()
      */
     public static function registerExtension($name)
     {
+        $manager   = static::getExtensionManager();
         $feedName  = $name . '\Feed';
         $entryName = $name . '\Entry';
-        $manager   = static::getExtensionManager();
+
         if (static::isRegistered($name)) {
             if ($manager->has($feedName) || $manager->has($entryName)) {
                 return;
             }
         }
 
-        if (! $manager->has($feedName) && ! $manager->has($entryName)) {
-            throw new Exception\RuntimeException('Could not load extension: ' . $name
-                . ' using Plugin Loader. Check prefix paths are configured and extension exists.');
+        if (! static::hasExtension($name)) {
+            throw new Exception\RuntimeException(sprintf(
+                'Could not load extension "%s" using Plugin Loader.'
+                . ' Check prefix paths are configured and extension exists.',
+                $name
+            ));
         }
+
         if ($manager->has($feedName)) {
             static::$extensions['feed'][] = $feedName;
         }
@@ -672,7 +677,18 @@ protected static function registerCoreExtensions()
         static::registerExtension('WellFormedWeb');
         static::registerExtension('Thread');
         static::registerExtension('Podcast');
-        static::registerExtension('GooglePlayPodcast');
+
+        // Added in 2.10.0; check for it conditionally
+        static::hasExtension('GooglePlayPodcast')
+            ? static::registerExtension('GooglePlayPodcast')
+            : trigger_error(
+                sprintf(
+                    'Please update your %1$s\ExtensionManagerInterface implementation to add entries for'
+                    . ' %1$s\Extension\GooglePlayPodcast\Entry and %1$s\Extension\GooglePlayPodcast\Feed.',
+                    __NAMESPACE__
+                ),
+                \E_USER_NOTICE
+            );
     }
 
     /**
@@ -693,4 +709,28 @@ public static function arrayUnique(array $array)
         }
         return $array;
     }
+
+    /**
+     * Does the extension manager have the named extension?
+     *
+     * This method exists to allow us to test if an extension is present in the
+     * extension manager. It may be used by registerExtension() to determine if
+     * the extension has items present in the manager, or by
+     * registerCoreExtension() to determine if the core extension has entries
+     * in the extension manager. In the latter case, this can be useful when
+     * adding new extensions in a minor release, as custom extension manager
+     * implementations may not yet have an entry for the extension, which would
+     * then otherwise cause registerExtension() to fail.
+     *
+     * @param string $name
+     * @return bool
+     */
+    protected static function hasExtension($name)
+    {
+        $feedName  = $name . '\Feed';
+        $entryName = $name . '\Entry';
+        $manager   = static::getExtensionManager();
+
+        return $manager->has($feedName) || $manager->has($entryName);
+    }
 }

test/Reader/ReaderTest.php

@@ -353,6 +353,35 @@ public function testSetHttpClientThrowsException()
         Reader\Reader::setHttpClient(new stdClass);
     }
 
+    public function testReaderEmitsNoticeDuringFeedImportWhenGooglePlayPodcastExtensionUnavailable()
+    {
+        Reader\Reader::setExtensionManager(new TestAsset\CustomExtensionManager());
+
+        $notices = (object) [
+            'messages' => [],
+        ];
+
+        set_error_handler(function ($errno, $errstr) use ($notices) {
+            $notices->messages[] = $errstr;
+        }, \E_USER_NOTICE);
+        $feed = Reader\Reader::importFile(
+            dirname(__FILE__) . '/Entry/_files/Atom/title/plain/atom10.xml'
+        );
+        restore_error_handler();
+
+        $message = array_reduce($notices->messages, function ($toReturn, $message) {
+            if ('' !== $toReturn) {
+                return $toReturn;
+            }
+            return false === strstr($message, 'GooglePlayPodcast') ? '' : $message;
+        }, '');
+
+        $this->assertNotEmpty(
+            $message,
+            'GooglePlayPodcast extension was present in extension manager, but was not expected to be'
+        );
+    }
+
     // @codingStandardsIgnoreStart
     protected function _getTempDirectory()
     {

test/Reader/TestAsset/CustomExtensionManager.php

@@ -0,0 +1,57 @@
+<?php
+/**
+ * @see       https://github.com/zendframework/zend-feed for the canonical source repository
+ * @copyright Copyright (c) 2018 Zend Technologies USA Inc. (https://www.zend.com)
+ * @license   https://github.com/zendframework/zend-feed/blob/master/LICENSE.md New BSD License
+ */
+
+namespace ZendTest\Feed\Reader\TestAsset;
+
+use Zend\Feed\Reader\Exception\InvalidArgumentException;
+use Zend\Feed\Reader\Extension;
+use Zend\Feed\Reader\ExtensionManagerInterface;
+
+/**
+ * Standalone extension manager that omits any extensions added after the 2.9 series.
+ */
+class CustomExtensionManager implements ExtensionManagerInterface
+{
+    private $extensions = [
+        'Atom\Entry'            => Extension\Atom\Entry::class,
+        'Atom\Feed'             => Extension\Atom\Feed::class,
+        'Content\Entry'         => Extension\Content\Entry::class,
+        'CreativeCommons\Entry' => Extension\CreativeCommons\Entry::class,
+        'CreativeCommons\Feed'  => Extension\CreativeCommons\Feed::class,
+        'DublinCore\Entry'      => Extension\DublinCore\Entry::class,
+        'DublinCore\Feed'       => Extension\DublinCore\Feed::class,
+        'Podcast\Entry'         => Extension\Podcast\Entry::class,
+        'Podcast\Feed'          => Extension\Podcast\Feed::class,
+        'Slash\Entry'           => Extension\Slash\Entry::class,
+        'Syndication\Feed'      => Extension\Syndication\Feed::class,
+        'Thread\Entry'          => Extension\Thread\Entry::class,
+        'WellFormedWeb\Entry'   => Extension\WellFormedWeb\Entry::class,
+    ];
+
+    /**
+     * Do we have the extension?
+     *
+     * @param  string $extension
+     * @return bool
+     */
+    public function has($extension)
+    {
+        return array_key_exists($extension, $this->extensions);
+    }
+
+    /**
+     * Retrieve the extension
+     *
+     * @param  string $extension
+     * @return Extension\AbstractEntry|Extension\AbstractFeed
+     */
+    public function get($extension)
+    {
+        $class = $this->extensions[$extension];
+        return new $class();
+    }
+}