Merge possible when rebase required

Summary

In GitLab v10.1.0 the merge methods semi-linear merge and ff-only were added to CE. These should disallow merging of merge requests which cannot be fast forwarded.

I have updated some of my projects to make use of the semi-linear merge method, but now merge requests can still be merged when it is not up to date with the target branch.

This does only happen when there are no conflicts in changed files though, if there are conflicts I do get the message I should first rebase my branch.

I have tested this on my CE installation of GitLab and on gitlab.com and found that this is a CE only issue.

Steps to reproduce

• Have a project in GitLab CE
• Make an edit to a file and create a MR for it
• Add another commit to the target branch of this MR (master) so our source branch is behind the target branch by some commit.
• Open the MR and see we can still merge it even though the target branch is ahead 1 commit.

Example Project

Cannot create an example project on gitlab.com as it runs GitLab EE which is not affected, only CE is.

What is the current bug behavior?

I can merge a MR with semi-linear or ff-only merge method enabled even though the target branch is ahead of the source branch.

What is the expected correct behavior?

I should see a message that tells me to rebase the source branch onto the target branch.

Relevant logs and/or screenshots

As you can see, my branch is 2 commits behind master, but I can still click merge and it will merge the MR.

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

System information
System:
Current User:   git
Using RVM:      no
Ruby Version:   2.3.5p376
Gem Version:    2.6.13
Bundler Version:1.13.7
Rake Version:   12.1.0
Redis Version:  3.2.5
Git Version:    2.13.6
Sidekiq Version:5.0.4
Go Version:     unknown
GitLab information
Version:        10.1.1
Revision:       cc27e5f
Directory:      /opt/gitlab/embedded/service/gitlab-rails
DB Adapter:     postgresql
URL:            https://$SERVER_FQDN
HTTP Clone URL: https://$SERVER_FQDN/some-group/some-project.git
SSH Clone URL:  git@$SERVER_FQDN:some-group/some-project.git
Using LDAP:     no
Using Omniauth: no
GitLab Shell
Version:        5.9.3
Repository storage paths:

• default: /var/opt/gitlab/git-data/repositories Hooks: /opt/gitlab/embedded/service/gitlab-shell/hooks Git: /opt/gitlab/embedded/bin/git

Results of GitLab application Check

Expand for output related to the GitLab application check

Checking GitLab Shell ...
GitLab Shell version >= 5.9.3 ? ... OK (5.9.3)
Repo base directory exists?
default... yes
Repo storage directories are symlinks?
default... no
Repo paths owned by git:root, or git:git?
default... yes
Repo paths access is drwxrws---?
default... yes
hooks directories in repos are links: ...
8/2 ... ok
8/3 ... ok
8/4 ... ok
8/9 ... ok
8/10 ... ok
4/11 ... ok
4/12 ... ok
4/13 ... ok
8/14 ... ok
4/15 ... ok
8/21 ... ok
8/22 ... ok
13/27 ... ok
9/32 ... ok
4/33 ... ok
8/34 ... ok
9/35 ... ok
13/36 ... ok
13/37 ... ok
4/38 ... ok
9/39 ... ok
9/40 ... ok
9/41 ... ok
13/42 ... ok
8/43 ... ok
13/44 ... ok
8/46 ... ok
8/47 ... ok
13/48 ... ok
8/49 ... ok
4/50 ... ok
8/54 ... ok
9/55 ... ok
9/56 ... ok
9/63 ... ok
8/65 ... ok
53/66 ... ok
8/67 ... ok
9/68 ... ok
8/69 ... ok
8/74 ... ok
8/75 ... ok
53/77 ... ok
8/78 ... ok
8/81 ... ok
8/82 ... ok
8/83 ... ok
8/84 ... ok
4/85 ... ok
8/87 ... ok
8/88 ... ok
4/89 ... ok
4/90 ... ok
4/92 ... ok
8/94 ... ok
8/95 ... ok
55/96 ... ok
5/99 ... ok
8/101 ... ok
8/102 ... ok
8/104 ... ok
8/108 ... ok
5/109 ... ok
5/110 ... ok
8/111 ... ok
4/112 ... ok
5/114 ... ok
4/115 ... ok
4/116 ... ok
4/117 ... ok
4/118 ... ok
5/120 ... ok
5/123 ... ok
5/124 ... ok
4/125 ... ok
5/126 ... ok
8/127 ... ok
4/128 ... ok
4/129 ... ok
4/132 ... ok
5/133 ... ok
5/135 ... ok
4/136 ... ok
4/138 ... ok
8/139 ... ok
4/140 ... ok
8/141 ... ok
5/142 ... ok
4/143 ... ok
5/145 ... ok
4/146 ... ok
5/147 ... ok
5/149 ... ok
58/150 ... ok
5/151 ... ok
4/152 ... ok
4/153 ... ok
4/154 ... ok
8/155 ... ok
5/157 ... ok
4/158 ... ok
4/159 ... ok
8/160 ... ok
4/161 ... ok
58/164 ... ok
8/165 ... ok
58/166 ... ok
4/167 ... ok
58/168 ... ok
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Redis available via internal API: OK
Access to /var/opt/gitlab/.ssh/authorized_keys: OK
gitlab-shell self-check successful
Checking GitLab Shell ... Finished
Checking Sidekiq ...
Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Reply by email is disabled in config/gitlab.yml
Checking LDAP ...
LDAP is disabled in config/gitlab.yml
Checking LDAP ... Finished
Checking GitLab ...
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: ...
8/2 ... yes
8/3 ... yes
8/4 ... yes
8/9 ... yes
8/10 ... yes
4/11 ... yes
4/12 ... yes
4/13 ... yes
8/14 ... yes
4/15 ... yes
8/21 ... yes
8/22 ... yes
13/27 ... yes
9/32 ... yes
4/33 ... yes
8/34 ... yes
9/35 ... yes
13/36 ... yes
13/37 ... yes
4/38 ... yes
9/39 ... yes
9/40 ... yes
9/41 ... yes
13/42 ... yes
8/43 ... yes
13/44 ... yes
8/46 ... yes
8/47 ... yes
13/48 ... yes
8/49 ... yes
4/50 ... yes
8/54 ... yes
9/55 ... yes
9/56 ... yes
9/63 ... yes
8/65 ... yes
53/66 ... yes
8/67 ... yes
9/68 ... yes
8/69 ... yes
8/74 ... yes
8/75 ... yes
53/77 ... yes
8/78 ... yes
8/81 ... yes
8/82 ... yes
8/83 ... yes
8/84 ... yes
4/85 ... yes
8/87 ... yes
8/88 ... yes
4/89 ... yes
4/90 ... yes
4/92 ... yes
8/94 ... yes
8/95 ... yes
55/96 ... yes
5/99 ... yes
8/101 ... yes
8/102 ... yes
8/104 ... yes
8/108 ... yes
5/109 ... yes
5/110 ... yes
8/111 ... yes
4/112 ... yes
5/114 ... yes
4/115 ... yes
4/116 ... yes
4/117 ... yes
4/118 ... yes
5/120 ... yes
5/123 ... yes
5/124 ... yes
4/125 ... yes
5/126 ... yes
8/127 ... yes
4/128 ... yes
4/129 ... yes
4/132 ... yes
5/133 ... yes
5/135 ... yes
4/136 ... yes
4/138 ... yes
8/139 ... yes
4/140 ... yes
8/141 ... yes
5/142 ... yes
4/143 ... yes
5/145 ... yes
4/146 ... yes
5/147 ... yes
5/149 ... yes
58/150 ... yes
5/151 ... yes
4/152 ... yes
4/153 ... yes
4/154 ... yes
8/155 ... yes
5/157 ... yes
4/158 ... yes
4/159 ... yes
8/160 ... yes
4/161 ... yes
58/164 ... yes
8/165 ... yes
58/166 ... yes
4/167 ... yes
58/168 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.3.3 ? ... yes (2.3.5)
Git version >= 2.7.3 ? ... yes (2.13.6)
Git user has default SSH configuration? ... yes
Active users: ... 5
Checking GitLab ... Finished