Set the availability of the VideoPress extension for Gutenberg (#11321)

It sets the `jetpack/videopress` extension availability, so we can check it on the editor when deciding if we need to enhance the core video block with VideoPress support. This will also allow us in the future to display upgrade nudges if we detect we're missing a plan.

The extension is set available:
- On Jetpack sites having the VideoPress module activated (included in Premium and above subscriptions).
- On WordPress.com sites having the video upgrade (included in Premium and above plans).

modules/videopress/class.videopress-gutenberg.php

@@ -11,29 +11,104 @@
 class VideoPress_Gutenberg {
 
 	/**
-	 * Initialize the VideoPress Gutenberg extension
+	 * Singleton
 	 */
 	public static function init() {
-		// Should not initialize if Gutenberg is not available or if Jetpack is not active.
+		static $instance = false;
+
+		if ( ! $instance ) {
+			$instance = new VideoPress_Gutenberg();
+		}
+
+		return $instance;
+	}
+
+	/**
+	 * VideoPress_Gutenberg constructor.
+	 *
+	 * Initialize the VideoPress Gutenberg extension
+	 */
+	private function __construct() {
+		add_action( 'init', array( $this, 'register_video_block_with_videopress' ) );
+		add_action( 'jetpack_register_gutenberg_extensions', array( $this, 'set_extension_availability' ) );
+	}
+
+	/**
+	 * Used to check whether VideoPress is enabled for given site.
+	 *
+	 * @todo Create a global `jetpack_check_module_availability( $module )` helper so we can re-use it on other modules.
+	 *       This global helper should be created in a file synced with WordPress.com so we can use it there too.
+	 * @see https://github.com/Automattic/jetpack/pull/11321#discussion_r255477815
+	 *
+	 * @return array Associative array indicating if the module is available (key `available`) and the reason why it is
+	 * unavailable (key `unavailable_reason`)
+	 */
+	public function check_videopress_availability() {
+		// It is available on Simple Sites having the appropriate a plan.
 		if (
-			( function_exists( 'register_block_type' ) )
-			&& (
-				( method_exists( 'Jetpack', 'is_active' ) && Jetpack::is_active() )
-				|| ( defined( 'IS_WPCOM' ) && IS_WPCOM )
-			)
+			defined( 'IS_WPCOM' ) && IS_WPCOM
+			&& method_exists( 'Store_Product_List', 'get_site_specific_features_data' )
 		) {
-			add_action( 'init', array( __CLASS__, 'register_video_block_with_videopress' ) );
+			$features = Store_Product_List::get_site_specific_features_data();
+			if ( in_array( 'videopress', $features['active'], true ) ) {
+				return array( 'available' => true );
+			} else {
+				return array(
+					'available'          => false,
+					'unavailable_reason' => 'missing_plan',
+				);
+			}
 		}
+
+		// It is available on Jetpack Sites having the module active.
+		if (
+			method_exists( 'Jetpack', 'is_active' ) && Jetpack::is_active()
+			&& method_exists( 'Jetpack', 'is_module_active' )
+			&& method_exists( 'Jetpack', 'active_plan_supports' )
+		) {
+			if ( Jetpack::is_module_active( 'videopress' ) ) {
+				return array( 'available' => true );
+			} elseif ( ! Jetpack::active_plan_supports( 'videopress' ) ) {
+				return array(
+					'available'          => false,
+					'unavailable_reason' => 'missing_plan',
+				);
+			} else {
+				return array(
+					'available'          => false,
+					'unavailable_reason' => 'missing_module',
+				);
+			}
+		}
+
+		return array(
+			'available'          => false,
+			'unavailable_reason' => 'unknown',
+		);
 	}
 
 	/**
-	 * Register the Jetpack Gutenberg extension that adds VideoPress support to the core video block.
+	 * Set the Jetpack Gutenberg extension availability.
 	 */
-	public static function register_video_block_with_videopress() {
+	public function set_extension_availability() {
+		$availability = $this->check_videopress_availability();
+		if ( $availability['available'] ) {
+			Jetpack_Gutenberg::set_extension_available( 'jetpack/videopress' );
+		} else {
+			Jetpack_Gutenberg::set_extension_unavailable( 'jetpack/videopress', $availability['unavailable_reason'] );
+		}
+	}
+
+	/**
+	 * Register the core video block as a dynamic block.
+	 *
+	 * It defines a server-side rendering that adds VideoPress support to the core video block.
+	 */
+	public function register_video_block_with_videopress() {
 		jetpack_register_block_type(
 			'core/video',
 			array(
-				'render_callback' => array( __CLASS__, 'render_video_block_with_videopress' ),
+				'render_callback' => array( $this, 'render_video_block_with_videopress' ),
 			)
 		);
 	}
@@ -46,17 +121,27 @@ public static function register_video_block_with_videopress() {
 	 *
 	 * @return string
 	 */
-	public static function render_video_block_with_videopress( $attributes, $content ) {
+	public function render_video_block_with_videopress( $attributes, $content ) {
 		if ( ! isset( $attributes['id'] ) || isset( $attributes['guid'] ) ) {
 			return $content;
 		}
 
 		if ( defined( 'IS_WPCOM' ) && IS_WPCOM ) {
 			$blog_id = get_current_blog_id();
-		} else {
+		} elseif ( method_exists( 'Jetpack', 'is_active' ) && Jetpack::is_active() ) {
+			/**
+			 * We're intentionally not using `get_current_blog_id` because it was returning unexpected values.
+			 *
+			 * @see https://github.com/Automattic/jetpack/pull/11193#issuecomment-457883886
+			 * @see https://github.com/Automattic/jetpack/pull/11193/commits/215cf789f3d8bd03ff9eb1bbdb693acb8831d273
+			 */
 			$blog_id = Jetpack_Options::get_option( 'id' );
 		}
 
+		if ( ! isset( $blog_id ) ) {
+			return $content;
+		}
+
 		$post_id         = absint( $attributes['id'] );
 		$videopress_id   = video_get_info_by_blogpostid( $blog_id, $post_id )->guid;
 		$videopress_data = videopress_get_video_details( $videopress_id );