The _wp_upload_dir function that tries to do too much (and fails when you need it most).

_wp_upload_dir-wp.php

/**
 * A non-filtered, non-cached version of wp_upload_dir() that doesn't check the path.
 *
 * @since 4.5.0
 * @access private
 *
 * @param string $time Optional. Time formatted in 'yyyy/mm'. Default null.
 * @return array See wp_upload_dir()
 */
function _wp_upload_dir( $time = null ) {
  $siteurl     = get_option( 'siteurl' );
  $upload_path = trim( get_option( 'upload_path' ) );

  if ( empty( $upload_path ) || 'wp-content/uploads' == $upload_path ) {
    $dir = WP_CONTENT_DIR . '/uploads';
  } elseif ( 0 !== strpos( $upload_path, ABSPATH ) ) {
    // $dir is absolute, $upload_path is (maybe) relative to ABSPATH
    $dir = path_join( ABSPATH, $upload_path );
  } else {
    $dir = $upload_path;
  }
  echo $upload_path;
  if ( ! $url = get_option( 'upload_url_path' ) ) {
    if ( empty( $upload_path ) || ( 'wp-content/uploads' == $upload_path ) || ( $upload_path == $dir ) ) {
      $url = WP_CONTENT_URL . '/uploads';
    } else {
      $url = trailingslashit( $siteurl ) . $upload_path;
    }
  }

  /*
   * Honor the value of UPLOADS. This happens as long as ms-files rewriting is disabled.
   * We also sometimes obey UPLOADS when rewriting is enabled -- see the next block.
   */
  if ( defined( 'UPLOADS' ) && ! ( is_multisite() && get_site_option( 'ms_files_rewriting' ) ) ) {
    $dir = ABSPATH . UPLOADS;
    $url = trailingslashit( $siteurl ) . UPLOADS;
  }

  // If multisite (and if not the main site in a post-MU network)
  if ( is_multisite() && ! ( is_main_network() && is_main_site() && defined( 'MULTISITE' ) ) ) {

    if ( ! get_site_option( 'ms_files_rewriting' ) ) {
      /*
       * If ms-files rewriting is disabled (networks created post-3.5), it is fairly
       * straightforward: Append sites/%d if we're not on the main site (for post-MU
       * networks). (The extra directory prevents a four-digit ID from conflicting with
       * a year-based directory for the main site. But if a MU-era network has disabled
       * ms-files rewriting manually, they don't need the extra directory, as they never
       * had wp-content/uploads for the main site.)
       */

      if ( defined( 'MULTISITE' ) ) {
        $ms_dir = '/sites/' . get_current_blog_id();
      } else {
        $ms_dir = '/' . get_current_blog_id();
      }

      $dir .= $ms_dir;
      $url .= $ms_dir;

    } elseif ( defined( 'UPLOADS' ) && ! ms_is_switched() ) {
      /*
       * Handle the old-form ms-files.php rewriting if the network still has that enabled.
       * When ms-files rewriting is enabled, then we only listen to UPLOADS when:
       * 1) We are not on the main site in a post-MU network, as wp-content/uploads is used
       *    there, and
       * 2) We are not switched, as ms_upload_constants() hardcodes these constants to reflect
       *    the original blog ID.
       *
       * Rather than UPLOADS, we actually use BLOGUPLOADDIR if it is set, as it is absolute.
       * (And it will be set, see ms_upload_constants().) Otherwise, UPLOADS can be used, as
       * as it is relative to ABSPATH. For the final piece: when UPLOADS is used with ms-files
       * rewriting in multisite, the resulting URL is /files. (#WP22702 for background.)
       */

      if ( defined( 'BLOGUPLOADDIR' ) ) {
        $dir = untrailingslashit( BLOGUPLOADDIR );
      } else {
        $dir = ABSPATH . UPLOADS;
      }
      $url = trailingslashit( $siteurl ) . 'files';
    }
  }

  $basedir = $dir;
  $baseurl = $url;

  $subdir = '';
  if ( get_option( 'uploads_use_yearmonth_folders' ) ) {
    // Generate the yearly and monthly dirs
    if ( ! $time ) {
      $time = current_time( 'mysql' );
    }
    $y      = substr( $time, 0, 4 );
    $m      = substr( $time, 5, 2 );
    $subdir = "/$y/$m";
  }

  $dir .= $subdir;
  $url .= $subdir;

  $arr = array(
    'path'    => $dir,
    'url'     => $url,
    'subdir'  => $subdir,
    'basedir' => $basedir,
    'baseurl' => $baseurl,
    'error'   => false,
  );
  echo '<pre>';
  var_dump( $arr );
  echo '</pre>';
  return $arr;
}

wp-upload-dir-alt.php

<?php

/**
 * A non-filtered, non-cached version of wp_upload_dir() that doesn't check the path.
 *
 * @since 4.5.0
 * @access private
 *
 * @param string $time Optional. Time formatted in 'yyyy/mm'. Default null.
 * @return array See wp_upload_dir()
 */
function _wp_upload_dir( $time = null ) {

  // Define the path to the root of the site ($site_root_path goes to the core).
  if ( ! defined( 'SITE_ROOT_PATH' ) )
  {
    define( 'SITE_ROOT_PATH', rtrim( ABSPATH, '/e' ) );
  }

  // Define the site media directory here, if not already defined in config.
  if ( ! defined( 'SITE_MEDIA_DIR' ) )
  {
    // replaces the hard coded "uploads".
    define( 'SITE_MEDIA_DIR', '/media' );
  }

  // replace ABSPATH with $site_root_path in this function.

  // The media url is reference to the "root" directory
  $siteurl = get_option( 'siteurl' );

  $siteurl = rtrim( $siteurl, '/e' );

  $upload_path = trim( get_option( 'upload_path' ) );

  if ( empty( $upload_path ) || 'wp-content/uploads' == $upload_path ) {
    $dir = WP_CONTENT_DIR . SITE_MEDIA_DIR;
  } elseif ( 0 !== strpos( $upload_path, SITE_ROOT_PATH ) ) {
    // $dir is absolute, $upload_path is (maybe) relative to ABSPATH
    $dir = path_join( SITE_ROOT_PATH, $upload_path );
  } else {
    $dir = $upload_path;
  }

  if ( ! $url = get_option( 'upload_url_path' ) ) {
    if ( empty( $upload_path ) || ( 'wp-content/uploads' == $upload_path ) || ( $upload_path == $dir ) ) {
      $url = WP_CONTENT_URL . SITE_MEDIA_DIR;
    } else {
      $url = trailingslashit( $siteurl ) . $upload_path;
    }
  }

  /*
   * Honor the value of UPLOADS. This happens as long as ms-files rewriting is disabled.
   * We also sometimes obey UPLOADS when rewriting is enabled -- see the next block.
   */
  if ( defined( 'UPLOADS' ) && ! ( is_multisite() && get_site_option( 'ms_files_rewriting' ) ) ) {
    $dir = SITE_ROOT_PATH . UPLOADS;
    $url = trailingslashit( $siteurl ) . UPLOADS;
  }

  // If multisite (and if not the main site in a post-MU network)
  if ( is_multisite() && ! ( is_main_network() && is_main_site() && defined( 'MULTISITE' ) ) ) {

    if ( ! get_site_option( 'ms_files_rewriting' ) ) {
      /*
       * If ms-files rewriting is disabled (networks created post-3.5), it is fairly
       * straightforward: Append sites/%d if we're not on the main site (for post-MU
       * networks). (The extra directory prevents a four-digit ID from conflicting with
       * a year-based directory for the main site. But if a MU-era network has disabled
       * ms-files rewriting manually, they don't need the extra directory, as they never
       * had wp-content/uploads for the main site.)
       */

      if ( defined( 'MULTISITE' ) ) {
        $ms_dir = '/sites/' . get_current_blog_id();
      } else {
        $ms_dir = '/' . get_current_blog_id();
      }

      $dir .= $ms_dir;
      $url .= $ms_dir;

    } elseif ( defined( 'UPLOADS' ) && ! ms_is_switched() ) {
      /*
       * Handle the old-form ms-files.php rewriting if the network still has that enabled.
       * When ms-files rewriting is enabled, then we only listen to UPLOADS when:
       * 1) We are not on the main site in a post-MU network, as wp-content/uploads is used
       *    there, and
       * 2) We are not switched, as ms_upload_constants() hardcodes these constants to reflect
       *    the original blog ID.
       *
       * Rather than UPLOADS, we actually use BLOGUPLOADDIR if it is set, as it is absolute.
       * (And it will be set, see ms_upload_constants().) Otherwise, UPLOADS can be used, as
       * as it is relative to ABSPATH. For the final piece: when UPLOADS is used with ms-files
       * rewriting in multisite, the resulting URL is /files. (#WP22702 for background.)
       */

      if ( defined( 'BLOGUPLOADDIR' ) ) {
        $dir = untrailingslashit( BLOGUPLOADDIR );
      } else {
        $dir = $site_root_path . UPLOADS;
      }
      $url = trailingslashit( $siteurl ) . 'files';
    }
  }

  $basedir = $dir;
  $baseurl = $url;

  $subdir = '';
  if ( get_option( 'uploads_use_yearmonth_folders' ) ) {
    // Generate the yearly and monthly dirs
    if ( ! $time ) {
      $time = current_time( 'mysql' );
    }
    $y      = substr( $time, 0, 4 );
    $m      = substr( $time, 5, 2 );
    $subdir = "/$y/$m";
  }

  $dir .= $subdir;
  $url .= $subdir;

  $url = str_replace( '//b/', '/b/', $url );

  $baseurl = str_replace( '//b/', '/b/', $baseurl );

  $arr = array(
    'path'    => $dir,
    'url'     => $url,
    'subdir'  => $subdir,
    'basedir' => $basedir,
    'baseurl' => $baseurl,
    'error'   => false,
  );

  return $arr;
}

wp_upload_dir-alt.php

/**
 * Get an array containing the current upload directory's path and url.
 *
 * Checks the 'upload_path' option, which should be from the web root folder,
 * and if it isn't empty it will be used. If it is empty, then the path will be
 * 'WP_CONTENT_DIR/uploads'. If the 'UPLOADS' constant is defined, then it will
 * override the 'upload_path' option and 'WP_CONTENT_DIR/uploads' path.
 *
 * The upload URL path is set either by the 'upload_url_path' option or by using
 * the 'WP_CONTENT_URL' constant and appending '/uploads' to the path.
 *
 * If the 'uploads_use_yearmonth_folders' is set to true (checkbox if checked in
 * the administration settings panel), then the time will be used. The format
 * will be year first and then month.
 *
 * If the path couldn't be created, then an error will be returned with the key
 * 'error' containing the error message. The error suggests that the parent
 * directory is not writable by the server.
 *
 * On success, the returned array will have many indices:
 * 'path' - base directory and sub directory or full path to upload directory.
 * 'url' - base url and sub directory or absolute URL to upload directory.
 * 'subdir' - sub directory if uploads use year/month folders option is on.
 * 'basedir' - path without subdir.
 * 'baseurl' - URL path without subdir.
 * 'error' - false or error message.
 *
 * @since 2.0.0
 * @uses _wp_upload_dir()
 *
 * @staticvar array $cache
 * @staticvar array $tested_paths
 *
 * @param string $time Optional. Time formatted in 'yyyy/mm'. Default null.
 * @param bool   $create_dir Optional. Whether to check and create the uploads directory.
 *                           Default true for backward compatibility.
 * @param bool   $refresh_cache Optional. Whether to refresh the cache. Default false.
 * @return array See above for description.
 */
function wp_upload_dir( $time = null, $create_dir = true, $refresh_cache = false ) {

  // Define the path to the root of the site (Note the WordPress core files).
  if ( ! defined( 'SITE_ROOT_PATH' ) )
  {
    define( 'SITE_ROOT_PATH', rtrim( ABSPATH, '/e' ) );
  }

  // Define the site media directory, if not already defined in the config file.
  if ( ! defined( 'SITE_MEDIA_DIR' ) )
  {
    // replace the hard coded "uploads" with the more neutral word, "media".
    define( 'SITE_MEDIA_DIR', '/media' );
  }

  static $cache = array(), $tested_paths = array();

  $key = sprintf( '%d-%s', get_current_blog_id(), (string) $time );

  if ( $refresh_cache || empty( $cache[ $key ] ) ) {
    $cache[ $key ] = _wp_upload_dir( $time );
  }

  /**
   * Filters the uploads directory data.
   *
   * @since 2.0.0
   *
   * @param array $uploads Array of upload directory data with keys of 'path',
   *                       'url', 'subdir, 'basedir', and 'error'.
   */
  $uploads = apply_filters( 'upload_dir', $cache[ $key ] );

  if ( $create_dir ) {
    $path = $uploads['path'];

    if ( array_key_exists( $path, $tested_paths ) ) {
      $uploads['error'] = $tested_paths[ $path ];
    } else {
      if ( ! wp_mkdir_p( $path ) ) {
        if ( 0 === strpos( $uploads['basedir'], SITE_ROOT_PATH ) ) {
          $error_path = str_replace( SITE_ROOT_PATH, '', $uploads['basedir'] ) . $uploads['subdir'];
        } else {
          $error_path = basename( $uploads['basedir'] ) . $uploads['subdir'];
        }

        $uploads['error'] = sprintf(
          /* translators: %s: directory path */
          __( 'Unable to create directory %s. Is its parent directory writable by the server?' ),
          esc_html( $error_path )
        );
      }

      $tested_paths[ $path ] = $uploads['error'];
    }
  }

  return $uploads;
}

wp_upload_dir-wp.php

/**
 * Get an array containing the current upload directory's path and url.
 *
 * Checks the 'upload_path' option, which should be from the web root folder,
 * and if it isn't empty it will be used. If it is empty, then the path will be
 * 'WP_CONTENT_DIR/uploads'. If the 'UPLOADS' constant is defined, then it will
 * override the 'upload_path' option and 'WP_CONTENT_DIR/uploads' path.
 *
 * The upload URL path is set either by the 'upload_url_path' option or by using
 * the 'WP_CONTENT_URL' constant and appending '/uploads' to the path.
 *
 * If the 'uploads_use_yearmonth_folders' is set to true (checkbox if checked in
 * the administration settings panel), then the time will be used. The format
 * will be year first and then month.
 *
 * If the path couldn't be created, then an error will be returned with the key
 * 'error' containing the error message. The error suggests that the parent
 * directory is not writable by the server.
 *
 * On success, the returned array will have many indices:
 * 'path' - base directory and sub directory or full path to upload directory.
 * 'url' - base url and sub directory or absolute URL to upload directory.
 * 'subdir' - sub directory if uploads use year/month folders option is on.
 * 'basedir' - path without subdir.
 * 'baseurl' - URL path without subdir.
 * 'error' - false or error message.
 *
 * @since 2.0.0
 * @uses _wp_upload_dir()
 *
 * @staticvar array $cache
 * @staticvar array $tested_paths
 *
 * @param string $time Optional. Time formatted in 'yyyy/mm'. Default null.
 * @param bool   $create_dir Optional. Whether to check and create the uploads directory.
 *                           Default true for backward compatibility.
 * @param bool   $refresh_cache Optional. Whether to refresh the cache. Default false.
 * @return array See above for description.
 */
function wp_upload_dir( $time = null, $create_dir = true, $refresh_cache = false ) {
  static $cache = array(), $tested_paths = array();

  $key = sprintf( '%d-%s', get_current_blog_id(), (string) $time );

  if ( $refresh_cache || empty( $cache[ $key ] ) ) {
    $cache[ $key ] = _wp_upload_dir( $time );
  }

  /**
   * Filters the uploads directory data.
   *
   * @since 2.0.0
   *
   * @param array $uploads Array of upload directory data with keys of 'path',
   *                       'url', 'subdir, 'basedir', and 'error'.
   */
  $uploads = apply_filters( 'upload_dir', $cache[ $key ] );

  if ( $create_dir ) {
    $path = $uploads['path'];

    if ( array_key_exists( $path, $tested_paths ) ) {
      $uploads['error'] = $tested_paths[ $path ];
    } else {
      if ( ! wp_mkdir_p( $path ) ) {
        if ( 0 === strpos( $uploads['basedir'], ABSPATH ) ) {
          $error_path = str_replace( ABSPATH, '', $uploads['basedir'] ) . $uploads['subdir'];
        } else {
          $error_path = basename( $uploads['basedir'] ) . $uploads['subdir'];
        }

        $uploads['error'] = sprintf(
          /* translators: %s: directory path */
          __( 'Unable to create directory %s. Is its parent directory writable by the server?' ),
          esc_html( $error_path )
        );
      }

      $tested_paths[ $path ] = $uploads['error'];
    }
  }

  return $uploads;
}