When rebasing an MR that has approval rules, the "The source branch HEAD has recently changed" message is intermittently shown in error

Summary

When rebasing an MR that has approval rules, the page may display a "The source branch HEAD has recently changed..." when no changes have been made. Once the page is refreshed the user is able to merge as expected.

This does not appear to occur when no approvals are needed for the MR.

Steps to reproduce

  1. Set Merge method to Merge commit with semi-linear history or Fast-forward merge under Settings -> General -> Merge requests -> Merge method
  2. Require at least one approval for MRs
  3. Create a new branch and make a change
  4. Make a change on master
  5. Create an MR using the new branch
  6. Approve the MR
  7. Click Rebase
  8. Rebase will come back after a second or two, then a few seconds later it grays out again and "Checking ability to merge automatically" will display
  9. After some more time, the Merge button may become available, or the page may display "The source branch HEAD has recently changed. Please reload the page and review the changes before merging". No changes were made to the source branch after the creation of the MR.

What is the current bug behavior?

MR page requires a refresh after a rebase before a merge can be executed

What is the expected correct behavior?

The MR page should immediately allow merges post-rebase

Relevant logs and/or screenshots

No errors generated in the logs.

stuck-rebase

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info 
System information
System:         Ubuntu 18.04
Proxy:          no
Current User:   git
Using RVM:      no
Ruby Version:   2.6.3p62
Gem Version:    2.7.9
Bundler Version:1.17.3
Rake Version:   12.3.2
Redis Version:  3.2.12
Git Version:    2.22.0
Sidekiq Version:5.2.7
Go Version:     unknown


GitLab information
Version:        12.3.2-ee
Revision:       9ae54250d64
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     PostgreSQL
DB Version:     10.9
Elasticsearch:  no
Geo:            no
Using LDAP:     no
Using Omniauth: yes
Omniauth Providers:


GitLab Shell
Version:        10.0.0
Repository storage paths:
  • default: /var/opt/gitlab/git-data/repositories GitLab Shell path: /opt/gitlab/embedded/service/gitlab-shell Git: /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check 
Checking GitLab subtasks ...


Checking GitLab Shell ...


GitLab Shell: ... GitLab Shell version >= 10.0.0 ? ... OK (10.0.0)
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Redis available via internal API: OK


gitlab-shell self-check successful


Checking GitLab Shell ... Finished


Checking Gitaly ...


Gitaly: ... default ... OK


Checking Gitaly ... Finished


Checking Sidekiq ...


Sidekiq: ... Running? ... yes
Number of Sidekiq processes ... 1


Checking Sidekiq ... Finished


Checking Incoming Email ...


Incoming Email: ... Reply by email is disabled in config/gitlab.yml


Checking Incoming Email ... Finished


Checking LDAP ...


LDAP: ... LDAP is disabled in config/gitlab.yml


Checking LDAP ... Finished


Checking GitLab App ...


Git configured correctly? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config up to date? ... yes
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory exists? ... yes
Uploads directory has correct permissions? ... yes
Uploads directory tmp has correct permissions? ... skipped (no tmp uploads folder yet)
Init script exists? ... skipped (omnibus-gitlab has no init script)
Init script up-to-date? ... skipped (omnibus-gitlab has no init script)
Projects have namespace: ...
1/1 ... yes
1/2 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.5.3 ? ... yes (2.6.3)
Git version >= 2.22.0 ? ... yes (2.22.0)
Git user has default SSH configuration? ... yes
Active users: ... 2
Is authorized keys file accessible? ... yes
Elasticsearch version 5.6 - 6.x? ... skipped (elasticsearch is disabled)


Checking GitLab App ... Finished


Checking GitLab subtasks ... Finished

Possible fixes

On my own instance the problem is quite intermittent, but it happens consistently on a customer's install.

/cc @oswaldo @nick.thomas