lter', 10, 3 );
	add_filter( 'get_post_metadata', '_wp_preview_post_thumbnail_filter', 10, 3 );

	return $post;
}

/**
 * Filters the latest content for preview from the post autosave.
 *
 * @since 2.7.0
 * @access private
 */
function _show_post_preview() {
	if ( isset( $_GET['preview_id'] ) && isset( $_GET['preview_nonce'] ) ) {
		$id = (int) $_GET['preview_id'];

		if ( false === wp_verify_nonce( $_GET['preview_nonce'], 'post_preview_' . $id ) ) {
			wp_die( __( 'Sorry, you are not allowed to preview drafts.' ), 403 );
		}

		add_filter( 'the_preview', '_set_preview' );
	}
}

/**
 * Filters terms lookup to set the post format.
 *
 * @since 3.6.0
 * @access private
 *
 * @param array  $terms
 * @param int    $post_id
 * @param string $taxonomy
 * @return array
 */
function _wp_preview_terms_filter( $terms, $post_id, $taxonomy ) {
	$post = get_post();
	if ( ! $post ) {
		return $terms;
	}

	if ( empty( $_REQUEST['post_format'] ) || $post->ID != $post_id
		|| 'post_format' !== $taxonomy || 'revision' === $post->post_type
	) {
		return $terms;
	}

	if ( 'standard' === $_REQUEST['post_format'] ) {
		$terms = array();
	} else {
		$term = get_term_by( 'slug', 'post-format-' . sanitize_key( $_REQUEST['post_format'] ), 'post_format' );
		if ( $term ) {
			$terms = array( $term ); // Can only have one post format.
		}
	}

	return $terms;
}

/**
 * Filters post thumbnail lookup to set the post thumbnail.
 *
 * @since 4.6.0
 * @access private
 *
 * @param null|array|string $value    The value to return - a single metadata value, or an array of values.
 * @param int               $post_id  Post ID.
 * @param string            $meta_key Meta key.
 * @return null|array The default return value or the post thumbnail meta array.
 */
function _wp_preview_post_thumbnail_filter( $value, $post_id, $meta_key ) {
	$post = get_post();
	if ( ! $post ) {
		return $value;
	}

	if ( empty( $_REQUEST['_thumbnail_id'] ) ||
		empty( $_REQUEST['preview_id'] ) ||
		$post->ID != $post_id ||
		'_thumbnail_id' !== $meta_key ||
		'revision' === $post->post_type ||
		$post_id != $_REQUEST['preview_id'] ) {

		return $value;
	}

	$thumbnail_id = (int) $_REQUEST['_thumbnail_id'];
	if ( $thumbnail_id <= 0 ) {
		return '';
	}

	return (string) $thumbnail_id;
}

/**
 * Gets the post revision version.
 *
 * @since 3.6.0
 * @access private
 *
 * @param WP_Post $revision
 * @return int|false
 */
function _wp_get_post_revision_version( $revision ) {
	if ( is_object( $revision ) ) {
		$revision = get_object_vars( $revision );
	} elseif ( ! is_array( $revision ) ) {
		return false;
	}

	if ( preg_match( '/^\d+-(?:autosave|revision)-v(\d+)$/', $revision['post_name'], $matches ) ) {
		return (int) $matches[1];
	}

	return 0;
}

/**
 * Upgrade the revisions author, add the current post as a revision and set the revisions version to 1
 *
 * @since 3.6.0
 * @access private
 *
 * @global wpdb $wpdb WordPress database abstraction object.
 *
 * @param WP_Post $post      Post object
 * @param array   $revisions Current revisions of the post
 * @return bool true if the revisions were upgraded, false if problems
 */
function _wp_upgrade_revisions_of_post( $post, $revisions ) {
	global $wpdb;

	// Add post option exclusively.
	$lock   = "revision-upgrade-{$post->ID}";
	$now    = time();
	$result = $wpdb->query( $wpdb->prepare( "INSERT IGNORE INTO `$wpdb->options` (`option_name`, `option_value`, `autoload`) VALUES (%s, %s, 'no') /* LOCK */", $lock, $now ) );
	if ( ! $result ) {
		// If we couldn't get a lock, see how old the previous lock is.
		$locked = get_option( $lock );
		if ( ! $locked ) {
			// Can't write to the lock, and can't read the lock.
			// Something broken has happened.
			return false;
		}

		if ( $locked > $now - 3600 ) {
			// Lock is not too old: some other process may be upgrading this post. Bail.
			return false;
		}

		// Lock is too old - update it (below) and continue.
	}

	// If we could get a lock, re-"add" the option to fire all the correct filters.
	update_option( $lock, $now );

	reset( $revisions );
	$add_last = true;

	do {
		$this_revision = current( $revisions );
		$prev_revision = next( $revisions );

		$this_revision_version = _wp_get_post_revision_version( $this_revision );

		// Something terrible happened.
		if ( false === $this_revision_version ) {
			continue;
		}

		// 1 is the latest revision version, so we're already up to date.
		// No need to add a copy of the post as latest revision.
		if ( 0 < $this_revision_version ) {
			$add_last = false;
			continue;
		}

		// Always update the revision version.
		$update = array(
			'post_name' => preg_replace( '/^(\d+-(?:autosave|revision))[\d-]*$/', '$1-v1', $this_revision->post_name ),
		);

		/*
		 * If this revision is the oldest revision of the post, i.e. no $prev_revision,
		 * the correct post_author is probably $post->post_author, but that's only a good guess.
		 * Update the revision version only and Leave the author as-is.
		 */
		if ( $prev_revision ) {
			$prev_revision_version = _wp_get_post_revision_version( $prev_revision );

			// If the previous revision is already up to date, it no longer has the information we need :(
			if ( $prev_revision_version < 1 ) {
				$update['post_author'] = $prev_revision->post_author;
			}
		}

		// Upgrade this revision.
		$result = $wpdb->update( $wpdb->posts, $update, array( 'ID' => $this_revision->ID ) );

		if ( $result ) {
			wp_cache_delete( $this_revision->ID, 'posts' );
		}
	} while ( $prev_revision );

	delete_option( $lock );

	// Add a copy of the post as latest revision.
	if ( $add_last ) {
		wp_save_post_revision( $post->ID );
	}

	return true;
}