"Rebase onto master" silently fails.

Summary

Running on gitlab.com so it might be specific to that environment

When the project is set to "Fast-forward merge" for merge requests, you get the "Rebase onto master" button when the branch is behind master. Sometimes clicking nothing happens with no error.

Steps to reproduce

Haven't been able to reproduce this on demand, seems to happen randomly

• Have a project with "Fast-forward merge" for merge requests.
• Create a merge request on a branch.
• Have merge request go behind master
• Click "Rebase onto master" button

What is the current bug behavior?

• When you click the button it changes to the "Rebase in progress…" message.
• JS polling correctly shows that a rebase is in progress in the JSON message.
• It sits for maybe 5 minutes?
• It stops and button goes back to "Rebase onto master" with nothing happening and no error message.

This has happened in multiple different repositories and multiple merge requests, but doesn't always happen.

What is the expected correct behavior?

Either show a message that it cannot rebase automatically (This does happen for rebase conflicts). Or successfully rebase, or at least show errors of why it failed.

I go ahead and manually rebase in these cases, and the rebase does succeed automatically.

This doesn't happen every time, but I'd say about 20% of the merge requests where we need to click "Rebase onto master" before merging it does. Clicking multiple times doesn't work either (Although I think it did once on the second try a couple months ago)

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com

Possible fixes

Not sure.