Fixed #2867: Cache clear errors on ACSF deploys.

[danepowell]

Fixes #2867

Drush is about to introduce an environment variable that allows you to specify the cache directory: drush-ops/drush#3597

We can use this to set the cache directory to local ephemeral (block) storage rather than Gluster in order to avoid race conditions. I've verified that this fixes #2867 when using the patched version of Drush.

Ideally we would shoehorn that Drush PR into BLT somehow, but I'm not sure how to do it since we don't patch anything else in our composer.json. I suppose people who really want the patch before the next stable Drush release can apply it themselves.

[weitzman]

If you guys could also poke the Acquia Cloud to set this globally that would be much appreciated. I cant think of a downside.

[danepowell]

I've tweaked the patch to isolate caches per site, so it's pretty much "impossible" (at the risk of jinxing myself...) to run into race conditions. A few of us have tested this patch, it should be good to merge.

We're also working with Acquia Cloud to build this into the platform somehow, so that non-BLT customers can take advantage of safe caching.

[bkosborne]

@danepowell Do you know if anything ever came of this as a platform-wide enhancement in Acquia Cloud?

[danepowell]

It does look like some changes related to DRUSH_PATHS_CACHE_DIRECTORY were added to ACSF some time in 2020. I'm sorry, I can't tell you much more than that (3 years ago is ancient history to me! 😄 )