Drush9 class mismatch with drupal

[weitzman]

I'm seeing another case where a global Drush gets an error, seemingly due to a class mismatch with Drupal core.

In your global Drush, composer update (if needed) to get to symfony/yaml 3.4. Then run drush cr using the global Drush and a naked Drupal 8 (no site local Drush). I'm seeing [error] Unable to parse at line 130 (near "{}"). which is a parse error here. Note the unusual indendentation+braces syntax in that line.

If you aren't getting the error, copy the whole default.services.yml to services.yml and rerun drush cr.

The error goes away if I downgrade to symfony/yaml:3.2 in my global Drush.

[weitzman]

Also, make sure you dont have PECL yaml as that works around symfony/yaml AFAIK

[greg-1-anderson]

Yeah, symfony/yaml is one of the packages that Drush needs early on. This leaves us vulnerable to class mismatch errors in symfony/yaml if the global Drush uses a different symfony/yaml than the global Drupal. I'm not sure if the error you are seeing is being caused by a transient error in symfony/yaml or a class mismatch error.

It seems that perhaps the safest thing to do would be to use a different yaml parser during the Drush preflight. However, this decision would impact consolidation/config, yaml/expander, etc., so I don't know that it is a practical measure we could contemplate.

[weitzman]

This is indeed a problem - https://www.drupal.org/project/drupal/issues/2927746#comment-12390037. I wonder if we should pin our symfony/yaml so it can't exceed 3.3 (for now). Not sure if we use a Conflicts statement or just a constraint for that.

[greg-1-anderson]

The short-term solution is to add another require statement for symfony/yaml in the require-dev section. This will pin the version of symfony/yaml for the global Drush, but will allow site-local Drush to float to whatever version of the package is being used by Drupal.

This is a fine short-term solution. We are going to run up on the reefs when Drupal core starts requiring Symfony 3.4, as we at that time will not be able to produce a global Drush that works with both the newer and the older Drupals. I thought we'd have a long run before we hit this problem, as I expected it wouldn't happen until Symfony 4, which Drupal won't be able to adopt for a long time due to its minimum PHP requirement of 7.1. The fact that symfony/yaml 3.4 breaks global Drush is a problem, since Drupal 8.5 is already working on adopting that version.

Perhaps what we should do is make consolidation/config use its own private yaml parser, and copy @grasmash's yaml-expander inside of it so that we don't have to worry about additional external dependencies being affected by the need for a custom yaml parser. We'd use symfony/yaml for everything else, and our custom parser only for consolidation/config (Drush configuration and alias files). Our custom yaml parser could be a re-namespaced copy of symfony/yaml, or something else.

[weitzman]

This was fixed by Drush adding a copy of the Parser from symfony/yaml. Also required changes in config and grasmash projects. See #3269