Git apply no output / no effect – troubleshooting Drupal patches

Be aware that for Drupal, when testing patches from the core issues queue, you can only use the git apply command on the main repository:

https://git.drupal.org/project/drupal.git (browse code)

(i.e. choose 8.2.x or 8.3.x according to issue )

and not on drupal-composer/drupal-project (e.g. a DrupalVM install)

This is logical, the commit IDs in the .patch file simply can’t be found in that repo, so git skips them.  Instead you should use:

patch -p1 < example.patch

…as described here. (You still use -R to reverse it.)

What’s less helpful is git apply will give you no warning there’s a problem – you’ll run the command, see [ok] but no other output, as though it had worked.

Likewise using any of these switches won’t print anything to the screen:

--verbose
--summary
--check

Annoying, the instructions for  --check imply it might tell you:

Instead of applying the patch, see if the patch is applicable to the current working tree and/or the index file and detects errors. Turns off “apply”.

(See also)

Suggested Checklist

• create a new branch for the patch you’re testing
• run git diff to check the files have actually been altered
• run drush cr too to reset cache/UI etc. before testing