Squashing MRs takes too long
Summary
After deployment of GitLab EE 10.4.0-rc2, merge requests that have the "squash commits" feature enabled take extremely long when merging.
Steps to reproduce
- Enable "squash commits" on a multi-commit MR.
- Merge.
Example Project
https://gitlab.com/petrboros/retino
What is the current bug behavior?
It takes very long to merge squashed MRs. This means order of magnitude slower than before the release. Sometimes it fails completely - after disabling squashing, merging works fine. Sometimes the MR frontend gets stuck in "merging", but in fact the MR has already been merged and we have to reload the page to find out.
It seems that the heavier the changes are, the longer the squashing/merging takes, but we don't have any detailed data on this and it is also intuitive.
What is the expected correct behavior?
Merging MRs should be fast (as it was before).
Output of checks
This bug happens on GitLab.com