woocommerce product password protect functions.php

if ( !post_password_required() ) {
    // Display your product
} else {
    echo get_the_password_form();
}

// via https://wordpress.org/support/topic/protect-a-product-by-password

function password_visibility(){
  $post = get_post();
  if(!is_admin() && !empty($post->post_password)):

    require_once ABSPATH . WPINC . '/class-phpass.php';
    $hasher = new PasswordHash( 8, true );

    $expire = apply_filters( 'post_password_expires', time() + 10 * DAY_IN_SECONDS );
    $secure = ( 'https' === parse_url( home_url(), PHP_URL_SCHEME ) );

    setcookie( 'wp-postpass_' . COOKIEHASH, $hasher->HashPassword( wp_unslash( $post->post_password ) ), $expire, COOKIEPATH, COOKIE_DOMAIN, $secure );

    wp_safe_redirect( wp_get_referer() );

  endif;
}

add_action( 'pre_get_posts' , 'password_visibility' );

// via https://wordpress.org/support/topic/bipass-password-protected-posts-or-pages-or-woocommerce-products