wpcom-markdown.php

<?php
/**
 * Copyright (c) Automattic. All rights reserved.
 *
 * Released under the GPL license
 * http://www.opensource.org/licenses/gpl-license.php
 *
 * This is an add-on for WordPress
 * http://wordpress.org/
 *
 * **********************************************************************
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * **********************************************************************
 */

class WPCom_Markdown {


  const POST_OPTION = 'wpcom_publish_posts_with_markdown';
  const COMMENT_OPTION = 'wpcom_publish_comments_with_markdown';
  const POST_TYPE_SUPPORT = 'wpcom-markdown';
  const IS_MD_META = '_wpcom_is_markdown';

  private static $parser;
  private static $instance;

  // to ensure that our munged posts over xml-rpc are removed from the cache
  public $posts_to_uncache = array();
  private $monitoring = array( 'post' => array(), 'parent' => array() );


  /**
   * Yay singletons!
   * @return object WPCom_Markdown instance
   */
  public static function get_instance() {
    if ( ! self::$instance )
      self::$instance = new self();
    return self::$instance;
  }

  /**
   * Kicks things off on `init` action
   * @return null
   */
  public function load() {
    $this->add_default_post_type_support();
    $this->maybe_load_actions_and_filters();
    if ( defined( 'REST_API_REQUEST' ) && REST_API_REQUEST ) {
      add_action( 'switch_blog', array( $this, 'maybe_load_actions_and_filters' ), 10, 2 );
    }
    add_action( 'admin_init', array( $this, 'register_setting' ) );
    add_action( 'admin_init', array( $this, 'maybe_unload_for_bulk_edit' ) );
    if ( current_theme_supports( 'o2' ) || class_exists( 'P2' ) ) {
      $this->add_o2_helpers();
    }
  }

  /**
   * If we're in a bulk edit session, unload so that we don't lose our markdown metadata
   * @return null
   */
  public function maybe_unload_for_bulk_edit() {
    if ( isset( $_REQUEST['bulk_edit'] ) && $this->is_posting_enabled() ) {
      $this->unload_markdown_for_posts();
    }
  }

  /**
   * Called on init and fires on switch_blog to decide if our actions and filters
   * should be running.
   * @param int|null $new_blog_id New blog ID
   * @param int|null $old_blog_id Old blog ID
   * @return null
   */
  public function maybe_load_actions_and_filters( $new_blog_id = null, $old_blog_id = null ) {
    // If this is a switch_to_blog call, and the blog isn't changing, we'll already be loaded
    if ( $new_blog_id && $new_blog_id === $old_blog_id ) {
      return;
    }

    if ( $this->is_posting_enabled() ) {
      $this->load_markdown_for_posts();

    } else {
      $this->unload_markdown_for_posts();
    }

    if ( $this->is_commenting_enabled() ) {
      $this->load_markdown_for_comments();
    } else {
      $this->unload_markdown_for_comments();
    }
  }

  /**
   * Set up hooks for enabling Markdown conversion on posts
   * @return null
   */
  public function load_markdown_for_posts() {
    add_action( 'wp_insert_post', array( $this, 'wp_insert_post' ) );
    add_filter( 'wp_insert_post_data', array( $this, 'wp_insert_post_data' ), 10, 2 );
    add_filter( 'edit_post_content', array( $this, 'edit_post_content' ), 10, 2 );
    add_filter( 'edit_post_content_filtered', array( $this, 'edit_post_content_filtered' ), 10, 2 );
    add_action( 'wp_restore_post_revision', array( $this, 'wp_restore_post_revision' ), 10, 2 );
    add_filter( '_wp_post_revision_fields', array( $this, '_wp_post_revision_fields' ) );
    add_action( 'xmlrpc_call', array( $this, 'xmlrpc_actions' ) );
    add_filter( 'content_save_pre', array( $this, 'preserve_code_blocks' ), 1 );
    if ( defined( 'XMLRPC_REQUEST' ) && XMLRPC_REQUEST ) {
      $this->check_for_early_methods();
    }
  }

  /**
   * Removes hooks to disable Markdown conversion on posts
   * @return null
   */
  public function unload_markdown_for_posts() {
    remove_action( 'wp_insert_post', array( $this, 'wp_insert_post' ) );
    remove_filter( 'wp_insert_post_data', array( $this, 'wp_insert_post_data' ), 10, 2 );
    remove_filter( 'edit_post_content', array( $this, 'edit_post_content' ), 10, 2 );
    remove_filter( 'edit_post_content_filtered', array( $this, 'edit_post_content_filtered' ), 10, 2 );
    remove_action( 'wp_restore_post_revision', array( $this, 'wp_restore_post_revision' ), 10, 2 );
    remove_filter( '_wp_post_revision_fields', array( $this, '_wp_post_revision_fields' ) );
    remove_action( 'xmlrpc_call', array( $this, 'xmlrpc_actions' ) );
    remove_filter( 'content_save_pre', array( $this, 'preserve_code_blocks' ), 1 );
  }

  /**
   * Set up hooks for enabling Markdown conversion on comments
   * @return null
   */
  protected function load_markdown_for_comments() {
    // Use priority 9 so that Markdown runs before KSES, which can clean up
    // any munged HTML.
    /* TODO DAVID: This is how you can transform other markdown */
    add_filter( 'pre_comment_content', array( $this, 'pre_comment_content' ), 9 );
  }

  /**
   * Removes hooks to disable Markdown conversion
   * @return null
   */
  protected function unload_markdown_for_comments() {
    remove_filter( 'pre_comment_content', array( $this, 'pre_comment_content' ), 9 );
  }

  /**
   * o2 does some of what we do. Let's take precedence.
   * @return null
   */
  public function add_o2_helpers() {
    if ( $this->is_posting_enabled() ) {
      add_filter( 'content_save_pre', array( $this, 'o2_escape_lists' ), 1 );
    }

    add_filter( 'o2_preview_post', array( $this, 'o2_preview_post' ) );
    add_filter( 'o2_preview_comment', array( $this, 'o2_preview_comment' ) );

    add_filter( 'wpcom_markdown_transform_pre', array( $this, 'o2_unescape_lists' ) );
    add_filter( 'wpcom_untransformed_content', array( $this, 'o2_unescape_lists' ) );
  }

  /**
   * If Markdown is enabled for posts on this blog, filter the text for o2 previews
   * @param  string $text Post text
   * @return string       Post text transformed through the magic of Markdown
   */
  public function o2_preview_post( $text ) {
    if ( $this->is_posting_enabled() ) {
      $text = $this->transform( $text, array( 'unslash' => false ) );
    }
    return $text;
  }

  /**
   * If Markdown is enabled for comments on this blog, filter the text for o2 previews
   * @param  string $text Comment text
   * @return string       Comment text transformed through the magic of Markdown
   */
  public function o2_preview_comment( $text ) {
    if ( $this->is_commenting_enabled() ) {
      $text = $this->transform( $text, array( 'unslash' => false ) );
    }
    return $text;
  }

  /**
   * Escapes lists so that o2 doesn't trounce them
   * @param  string $text Post/comment text
   * @return string       Text escaped with HTML entity for asterisk
   */
  public function o2_escape_lists( $text ) {
    return preg_replace( '/^\\* /um', '&#42; ', $text );
  }

  /**
   * Unescapes the token we inserted on o2_escape_lists
   * @param  string $text Post/comment text with HTML entities for asterisks
   * @return string       Text with the HTML entity removed
   */
  public function o2_unescape_lists( $text ) {
    return preg_replace( '/^[&]\#042; /um', '* ', $text );
  }

  /**
   * Preserve code blocks from being munged by KSES before they have a chance
   * @param  string $text post content
   * @return string       post content with code blocks escaped
   */
  public function preserve_code_blocks( $text ) {
    return $this->get_parser()->codeblock_preserve( $text );
  }

  /**
   * Remove KSES if it's there. Store the result to manually invoke later if needed.
   * @return null
   */
  public function maybe_remove_kses() {
    // Filters return true if they existed before you removed them
    if ( $this->is_posting_enabled() )
      $this->kses = remove_filter( 'content_filtered_save_pre', 'wp_filter_post_kses' ) && remove_filter( 'content_save_pre', 'wp_filter_post_kses' );
  }

  /**
   * Add our Writing and Discussion settings.
   * @return null
   */
  public function register_setting() {
    add_settings_field( self::POST_OPTION, __( 'Markdown', 'jetpack' ), array( $this, 'post_field' ), 'writing' );
    register_setting( 'writing', self::POST_OPTION, array( $this, 'sanitize_setting') );
    add_settings_field( self::COMMENT_OPTION, __( 'Markdown', 'jetpack' ), array( $this, 'comment_field' ), 'discussion' );
    register_setting( 'discussion', self::COMMENT_OPTION, array( $this, 'sanitize_setting') );
  }

  /**
   * Sanitize setting. Don't really want to store "on" value, so we'll store "1" instead!
   * @param  string $input Value received by settings API via $_POST
   * @return bool          Cast to boolean.
   */
  public function sanitize_setting( $input ) {
    return (bool) $input;
  }

  /**
   * Prints HTML for the Writing setting
   * @return null
   */
  public function post_field() {
    printf(
      '<label><input name="%s" id="%s" type="checkbox"%s /> %s</label><p class="description">%s</p>',
      self::POST_OPTION,
      self::POST_OPTION,
      checked( $this->is_posting_enabled(), true, false ),
      esc_html__( 'Use Markdown for posts and pages.', 'jetpack' ),
      sprintf( '<a href="%s">%s</a>', esc_url( $this->get_support_url() ), esc_html__( 'Learn more about Markdown.', 'jetpack' ) )
    );
  }

  /**
   * Prints HTML for the Discussion setting
   * @return null
   */
  public function comment_field() {
    printf(
      '<label><input name="%s" id="%s" type="checkbox"%s /> %s</label><p class="description">%s</p>',
      self::COMMENT_OPTION,
      self::COMMENT_OPTION,
      checked( $this->is_commenting_enabled(), true, false ),
      esc_html__( 'Use Markdown for comments.', 'jetpack' ),
      sprintf( '<a href="%s">%s</a>', esc_url( $this->get_support_url() ), esc_html__( 'Learn more about Markdown.', 'jetpack' ) )
    );
  }

  /**
   * Get the support url for Markdown
   * @uses   apply_filters
   * @return string support url
   */
  protected function get_support_url() {
    /**
     * Filter the Markdown support URL.
     *
     * @module markdown
     *
     * @since 2.8.0
     *
     * @param string $url Markdown support URL.
     */
    return apply_filters( 'easy_markdown_support_url', 'http://en.support.wordpress.com/markdown-quick-reference/' );
  }

  /**
   * Is Mardown conversion for posts enabled?
   * @return boolean
   */
  public function is_posting_enabled() {
    return true;
  }

  /**
   * Is Markdown conversion for comments enabled?
   * @return boolean
   */
  public function is_commenting_enabled() {
    return (bool) get_option( self::COMMENT_OPTION, '' );
  }

  /**
   * Check if a $post_id has Markdown enabled
   * @param  int  $post_id A post ID.
   * @return boolean
   */
  public function is_markdown( $post_id ) {
    return get_metadata( 'post', $post_id, self::IS_MD_META, true );
  }

  /**
   * Set Markdown as enabled on a post_id. We skip over update_postmeta so we
   * can sneakily set metadata on post revisions, which we need.
   * @param int    $post_id A post ID.
   * @return bool  The metadata was successfully set.
   */
  protected function set_as_markdown( $post_id ) {
    return update_metadata( 'post', $post_id, self::IS_MD_META, true );
  }

  /**
   * Get our Markdown parser object, optionally requiring all of our needed classes and
   * instantiating our parser.
   * @return object WPCom_GHF_Markdown_Parser instance.
   */
  public function get_parser() {

    if ( ! self::$parser ) {

      self::$parser = new WPCom_GHF_Markdown_Parser;
    }

    return self::$parser;
  }

  /**
   * We don't want Markdown conversion all over the place.
   * @return null
   */
  public function add_default_post_type_support() {
    add_post_type_support( 'post', self::POST_TYPE_SUPPORT );
    add_post_type_support( 'page', self::POST_TYPE_SUPPORT );
    add_post_type_support( 'snippet', self::POST_TYPE_SUPPORT );
    add_post_type_support( 'portfolio', self::POST_TYPE_SUPPORT );
    add_post_type_support( 'revision', self::POST_TYPE_SUPPORT );
  }

  /**
   * Figure out the post type of the post screen we're on
   * @return string Current post_type
   */
  protected function get_post_screen_post_type() {
    global $pagenow;
    if ( 'post-new.php' === $pagenow )
      return ( isset( $_GET['post_type'] ) ) ? $_GET['post_type'] : 'post';
    if ( isset( $_GET['post'] ) ) {
      $post = get_post( (int) $_GET['post'] );
      if ( is_object( $post ) && isset( $post->post_type ) )
        return $post->post_type;
    }
    return 'post';
  }

  /**
   * Swap post_content and post_content_filtered for editing
   * @param  string $content Post content
   * @param  int $id         post ID
   * @return string          Swapped content
   */
  public function edit_post_content( $content, $id ) {
    if ( $this->is_markdown( $id ) ) {
      $post = get_post( $id );
      if ( $post && ! empty( $post->post_content_filtered ) ) {
        $post = $this->swap_for_editing( $post );
        return $post->post_content;
      }
    }
    return $content;
  }

  /**
   * Swap post_content_filtered and post_content for editing
   * @param  string $content Post content_filtered
   * @param  int $id         post ID
   * @return string          Swapped content
   */
  public function edit_post_content_filtered( $content, $id ) {
    // if markdown was disabled, let's turn this off
    if ( ! $this->is_posting_enabled() && $this->is_markdown( $id ) ) {
      $post = get_post( $id );
      if ( $post && ! empty( $post->post_content_filtered ) )
        $content = '';
    }
    return $content;
  }

  /**
   * Magic happens here. Markdown is converted and stored on post_content. Original Markdown is stored
   * in post_content_filtered so that we can continue editing as Markdown.
   * @param  array $post_data  The post data that will be inserted into the DB. Slashed.
   * @param  array $postarr    All the stuff that was in $_POST.
   * @return array             $post_data with post_content and post_content_filtered modified
   */
  public function wp_insert_post_data( $post_data, $postarr ) {
    // $post_data array is slashed!
    $post_id = isset( $postarr['ID'] ) ? $postarr['ID'] : false;
    // bail early if markdown is disabled or this post type is unsupported.
    if ( ! $this->is_posting_enabled() || ! post_type_supports( $post_data['post_type'], self::POST_TYPE_SUPPORT ) ) {
      // it's disabled, but maybe this *was* a markdown post before.
      if ( $this->is_markdown( $post_id ) && ! empty( $post_data['post_content_filtered'] ) ) {
        $post_data['post_content_filtered'] = '';
      }
      // we have no context to determine supported post types in the `post_content_pre` hook,
      // which already ran to sanitize code blocks. Undo that.
      $post_data['post_content'] = $this->get_parser()->codeblock_restore( $post_data['post_content'] );
      return $post_data;
    }
    // rejigger post_content and post_content_filtered
    // revisions are already in the right place, except when we're restoring, but that's taken care of elsewhere
    // also prevent quick edit feature from overriding already-saved markdown (issue https://github.com/Automattic/jetpack/issues/636)
    if ( 'revision' !== $post_data['post_type'] && ! isset( $_POST['_inline_edit'] ) ) {
      /**
       * Filter the original post content passed to Markdown.
       *
       * @module markdown
       *
       * @since 2.8.0
       *
       * @param string $post_data['post_content'] Untransformed post content.
       */
      $post_data['post_content_filtered'] = apply_filters( 'wpcom_untransformed_content', $post_data['post_content'] );
      $post_data['post_content'] = $this->transform( $post_data['post_content'], array( 'id' => $post_id ) );
      /** This filter is already documented in core/wp-includes/default-filters.php */
      $post_data['post_content'] = apply_filters( 'content_save_pre', $post_data['post_content'] );
    } elseif ( 0 === strpos( $post_data['post_name'], $post_data['post_parent'] . '-autosave' ) ) {
      // autosaves for previews are weird
      $post_data['post_content'] = $this->transform( $post_data['post_content'], array( 'id' => $post_data['post_parent'] ) );
      /** This filter is already documented in core/wp-includes/default-filters.php */
      $post_data['post_content'] = apply_filters( 'content_save_pre', $post_data['post_content'] );
    }

    // set as markdown on the wp_insert_post hook later
    if ( $post_id )
      $this->monitoring['post'][ $post_id ] = true;
    else
      $this->monitoring['content'] = wp_unslash( $post_data['post_content'] );
    if ( 'revision' === $postarr['post_type'] && $this->is_markdown( $postarr['post_parent'] ) )
      $this->monitoring['parent'][ $postarr['post_parent'] ] = true;

    return $post_data;
  }

  /**
   * Calls on wp_insert_post action, after wp_insert_post_data. This way we can
   * still set postmeta on our revisions after it's all been deleted.
   * @param  int $post_id The post ID that has just been added/updated
   * @return null
   */
  public function wp_insert_post( $post_id ) {
    $post_parent = get_post_field( 'post_parent', $post_id );
    // this didn't have an ID yet. Compare the content that was just saved.
    if ( isset( $this->monitoring['content'] ) && $this->monitoring['content'] === get_post_field( 'post_content', $post_id ) ) {
      unset( $this->monitoring['content'] );
      $this->set_as_markdown( $post_id );
    }
    if ( isset( $this->monitoring['post'][$post_id] ) ) {
      unset( $this->monitoring['post'][$post_id] );
      $this->set_as_markdown( $post_id );
    } elseif ( isset( $this->monitoring['parent'][$post_parent] ) ) {
      unset( $this->monitoring['parent'][$post_parent] );
      $this->set_as_markdown( $post_id );
    }
  }

  /**
   * Run a comment through Markdown. Easy peasy.
   * @param  string $content
   * @return string
   */
  public function pre_comment_content( $content ) {
    return $this->transform( $content, array(
      'id' => $this->comment_hash( $content ),
    ) );
  }

  protected function comment_hash( $content ) {
    return 'c-' . substr( md5( $content ), 0, 8 );
  }

  /**
   * Markdown conversion. Some DRYness for repetitive tasks.
   * @param  string $text  Content to be run through Markdown
   * @param  array  $args  Arguments, with keys:
   *                       id: provide a string to prefix footnotes with a unique identifier
   *                       unslash: when true, expects and returns slashed data
   *                       decode_code_blocks: when true, assume that text in fenced code blocks is already
   *                         HTML encoded and should be decoded before being passed to Markdown, which does
   *                         its own encoding.
   * @return string        Markdown-processed content
   */
  public function transform( $text, $args = array() ) {
    $args = wp_parse_args( $args, array(
      'id' => false,
      'unslash' => true,
      'decode_code_blocks' => ! $this->get_parser()->use_code_shortcode
    ) );
    // probably need to unslash
    if ( $args['unslash'] )
      $text = wp_unslash( $text );

    /**
     * Filter the content to be run through Markdown, before it's transformed by Markdown.
     *
     * @module markdown
     *
     * @since 2.8.0
     *
     * @param string $text Content to be run through Markdown
     * @param array $args Array of Markdown options.
     */
    $text = apply_filters( 'wpcom_markdown_transform_pre', $text, $args );
    // ensure our paragraphs are separated
    $text = str_replace( array( '</p><p>', "</p>\n<p>" ), "</p>\n\n<p>", $text );
    // visual editor likes to add <p>s. Buh-bye.
    $text = $this->get_parser()->unp( $text );
    // sometimes we get an encoded > at start of line, breaking blockquotes
    $text = preg_replace( '/^&gt;/m', '>', $text );
    // DAVIDS EDITS. Fix broken arrow functions
    $text = preg_replace( '/=&gt;/m', '=>', $text );
    // prefixes are because we need to namespace footnotes by post_id
    $this->get_parser()->fn_id_prefix = $args['id'] ? $args['id'] . '-' : '';
    // If we're not using the code shortcode, prevent over-encoding.
    if ( $args['decode_code_blocks'] ) {
      $text = $this->get_parser()->codeblock_restore( $text );
    }
    // Transform it!
    $text = $this->get_parser()->transform( $text );
    // Fix footnotes - kses doesn't like the : IDs it supplies
    $text = preg_replace( '/((id|href)="#?fn(ref)?):/', "$1-", $text );
    // Markdown inserts extra spaces to make itself work. Buh-bye.
    $text = rtrim( $text );
    /**
     * Filter the content to be run through Markdown, after it was transformed by Markdown.
     *
     * @module markdown
     *
     * @since 2.8.0
     *
     * @param string $text Content to be run through Markdown
     * @param array $args Array of Markdown options.
     */
    $text = apply_filters( 'wpcom_markdown_transform_post', $text, $args );

    // probably need to re-slash
    if ( $args['unslash'] )
      $text = wp_slash( $text );

    return $text;
  }

  /**
   * Shows Markdown in the Revisions screen, and ensures that post_content_filtered
   * is maintained on revisions
   * @param  array $fields  Post fields pertinent to revisions
   * @return array          Modified array to include post_content_filtered
   */
  public function _wp_post_revision_fields( $fields ) {
    $fields['post_content_filtered'] = __( 'Markdown content', 'jetpack' );
    return $fields;
  }

  /**
   * Do some song and dance to keep all post_content and post_content_filtered content
   * in the expected place when a post revision is restored.
   * @param  int $post_id        The post ID have a restore done to it
   * @param  int $revision_id    The revision ID being restored
   * @return null
   */
  public function wp_restore_post_revision( $post_id, $revision_id ) {
    if ( $this->is_markdown( $revision_id ) ) {
      $revision = get_post( $revision_id, ARRAY_A );
      $post = get_post( $post_id, ARRAY_A );
      $post['post_content'] = $revision['post_content_filtered']; // Yes, we put it in post_content, because our wp_insert_post_data() expects that
      // set this flag so we can restore the post_content_filtered on the last revision later
      $this->monitoring['restore'] = true;
      // let's not make a revision of our fixing update
      add_filter( 'wp_revisions_to_keep', '__return_false', 99 );
      wp_update_post( $post );
      $this->fix_latest_revision_on_restore( $post_id );
      remove_filter( 'wp_revisions_to_keep', '__return_false', 99 );
    }
  }

  /**
   * We need to ensure the last revision has Markdown, not HTML in its post_content_filtered
   * column after a restore.
   * @param  int $post_id The post ID that was just restored.
   * @return null
   */
  protected function fix_latest_revision_on_restore( $post_id ) {
    global $wpdb;
    $post = get_post( $post_id );
    $last_revision = $wpdb->get_row( $wpdb->prepare( "SELECT * FROM $wpdb->posts WHERE post_type = 'revision' AND post_parent = %d ORDER BY ID DESC", $post->ID ) );
    $last_revision->post_content_filtered = $post->post_content_filtered;
    wp_insert_post( (array) $last_revision );
  }

  /**
   * Kicks off magic for an XML-RPC session. We want to keep editing Markdown
   * and publishing HTML.
   * @param  string $xmlrpc_method The current XML-RPC method
   * @return null
   */
  public function xmlrpc_actions( $xmlrpc_method ) {
    switch ( $xmlrpc_method ) {
      case 'metaWeblog.getRecentPosts':
      case 'wp.getPosts':
      case 'wp.getPages':
        add_action( 'parse_query', array( $this, 'make_filterable' ), 10, 1 );
        break;
      case 'wp.getPost':
        $this->prime_post_cache();
        break;
    }
  }

  /**
   * metaWeblog.getPost and wp.getPage fire xmlrpc_call action *after* get_post() is called.
   * So, we have to detect those methods and prime the post cache early.
   * @return null
   */
  protected function check_for_early_methods() {
    global $HTTP_RAW_POST_DATA;
    if ( false === strpos( $HTTP_RAW_POST_DATA, 'metaWeblog.getPost' )
      && false === strpos( $HTTP_RAW_POST_DATA, 'wp.getPage' ) ) {
      return;
    }
    include_once( ABSPATH . WPINC . '/class-IXR.php' );
    $message = new IXR_Message( $HTTP_RAW_POST_DATA );
    $message->parse();
    $post_id_position = 'metaWeblog.getPost' === $message->methodName ?  0 : 1;
    $this->prime_post_cache( $message->params[ $post_id_position ] );
  }

  /**
   * Prime the post cache with swapped post_content. This is a sneaky way of getting around
   * the fact that there are no good hooks to call on the *.getPost xmlrpc methods.
   *
   * @return null
   */
  private function prime_post_cache( $post_id = false ) {
    global $wp_xmlrpc_server;
    if ( ! $post_id ) {
      $post_id = $wp_xmlrpc_server->message->params[3];
    }

    // prime the post cache
    if ( $this->is_markdown( $post_id ) ) {
      $post = get_post( $post_id );
      if ( ! empty( $post->post_content_filtered ) ) {
        wp_cache_delete( $post->ID, 'posts' );
        $post = $this->swap_for_editing( $post );
        wp_cache_add( $post->ID, $post, 'posts' );
        $this->posts_to_uncache[] = $post_id;
      }
    }
    // uncache munged posts if using a persistent object cache
    if ( wp_using_ext_object_cache() ) {
      add_action( 'shutdown', array( $this, 'uncache_munged_posts' ) );
    }
  }

  /**
   * Swaps `post_content_filtered` back to `post_content` for editing purposes.
   * @param  object $post WP_Post object
   * @return object       WP_Post object with swapped `post_content_filtered` and `post_content`
   */
  protected function swap_for_editing( $post ) {
    $markdown = $post->post_content_filtered;
    // unencode encoded code blocks
    $markdown = $this->get_parser()->codeblock_restore( $markdown );
    // restore beginning of line blockquotes
    $markdown = preg_replace( '/^&gt; /m', '> ', $markdown );
    $post->post_content_filtered = $post->post_content;
    $post->post_content = $markdown;
    return $post;
  }


  /**
   * We munge the post cache to serve proper markdown content to XML-RPC clients.
   * Uncache these after the XML-RPC session ends.
   * @return null
   */
  public function uncache_munged_posts() {
    // $this context gets lost in testing sometimes. Weird.
    foreach( WPCom_Markdown::get_instance()->posts_to_uncache as $post_id ) {
      wp_cache_delete( $post_id, 'posts' );
    }
  }

  /**
   * Since *.(get)?[Rr]ecentPosts calls get_posts with suppress filters on, we need to
   * turn them back on so that we can swap things for editing.
   * @param  object $wp_query WP_Query object
   * @return null
   */
  public function make_filterable( $wp_query ) {
    $wp_query->set( 'suppress_filters', false );
    add_action( 'the_posts', array( $this, 'the_posts' ), 10, 2 );
  }

  /**
   * Swaps post_content and post_content_filtered for editing.
   * @param  array  $posts     Posts returned by the just-completed query
   * @param  object $wp_query  Current WP_Query object
   * @return array             Modified $posts
   */
  public function the_posts( $posts, $wp_query ) {
    foreach ( $posts as $key => $post ) {
      if ( $this->is_markdown( $post->ID ) && ! empty( $posts[ $key ]->post_content_filtered ) ) {
        $markdown = $posts[ $key ]->post_content_filtered;
        $posts[ $key ]->post_content_filtered = $posts[ $key ]->post_content;
        $posts[ $key ]->post_content = $markdown;
      }
    }
    return $posts;
  }

  /**
   * Singleton silence is golden
   */
  private function __construct() {}
}

add_action( 'init', array( WPCom_Markdown::get_instance(), 'load' ) );