Merge in v12.3.0 corrupts source branch

Summary

When resolving conflicts in a merge request using the Gitlab web UI, all of the files and changes from the destination (target) branch are merged into the source prior to merging the source branch into the destination branch. There are cases where we do not want the source branch altered but only want the changes merged into the destination branch. This has caused major problems for us as we had to back out of the corruption of our source branches. Doing a merge with the git command line works fine. Gitlab v12.3.0 running on Red Hat EL 7.7, Omnibus installation

Steps to reproduce

  1. Create two branches within a project and make unique changes in each branch.
  2. Create a merge request for two non-master branches, each with changes.
  3. Navigate to "Merge Requests" and click on the "Resolve conflicts" button.
  4. The Merge Conflicts page presents the options to "use ours" or "use theirs" for one or more file conflicts. The commit message text says "Merge branch 'dest' into 'src'" regardless of which option is selected. The destination branch should not be merged into the source branch but there does not appear to be any way to resolve the conflict in the UI without this happening.
  5. Now that the merge conflicts are resolved, the source branch is updated with any files or changes found in the destination branch.
  6. Navigating back to the merge request page and hitting the "Merge" button updates the destination branch with the results of the merge. Now both branches are identical.

Example Project

https://gitlab.com/MarkTuck/test_proj

What is the current bug behavior?

The source branch is updated with changes and additions from the destination branch.

What is the expected correct behavior?

The source branch should be unaltered. Changes should only be occurring in the merge target branch.

Relevant logs and/or screenshots

N/A

Output of checks

This bug happens on GitLab.com as well as on our local installation of gitlab-ce

Results of GitLab environment info

Expand for output related to GitLab environment info 
System information
System:
Current User:	git
Using RVM:	no
Ruby Version:	2.6.3p62
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:	go1.11.5 linux/amd64


GitLab information
Version:	12.3.0
Revision:	7099ecf77cb
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	PostgreSQL
DB Version:	10.9
URL:		https://gitlab.library.dartmouth.edu
HTTP Clone URL:	https://gitlab.library.dartmouth.edu/some-group/some-project.git
SSH Clone URL:	git@gitlab.library.dartmouth.edu:some-group/some-project.git
Using LDAP:	no
Using Omniauth:	yes
Omniauth Providers: cas3


GitLab Shell
Version:	10.0.0
Repository storage paths:
  • default: /data/gitlab-data/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: ... Checking Reply by email ...


IMAP server credentials are correct? ... yes
Init.d configured correctly? ... skipped
MailRoom running? ... skipped


Checking Reply by email ... Finished


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? ... yes
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: ...
40/2 ... yes
40/3 ... yes
12/5 ... yes
3/7 ... yes
40/8 ... yes
40/9 ... yes
40/10 ... yes
3/11 ... yes
3/12 ... yes
3/13 ... yes
2/15 ... yes
3/18 ... yes
23/19 ... yes
38/20 ... yes
22/21 ... yes
3/22 ... yes
3/23 ... yes
22/24 ... yes
3/46 ... yes
3/47 ... yes
22/49 ... yes
22/50 ... yes
3/51 ... yes
22/52 ... yes
39/73 ... yes
39/74 ... yes
39/75 ... yes
39/76 ... yes
38/77 ... yes
39/78 ... yes
39/79 ... yes
39/80 ... yes
39/81 ... yes
39/82 ... yes
39/83 ... yes
39/84 ... yes
38/85 ... yes
3/86 ... yes
39/87 ... yes
38/88 ... yes
3/89 ... yes
39/90 ... yes
38/91 ... yes
38/92 ... yes
22/93 ... yes
3/94 ... yes
40/95 ... yes
3/96 ... yes
32/105 ... yes
32/106 ... yes
32/107 ... yes
32/108 ... yes
38/110 ... yes
38/111 ... yes
38/114 ... yes
3/115 ... yes
3/116 ... yes
12/117 ... yes
23/118 ... yes
23/119 ... yes
23/120 ... yes
23/121 ... yes
23/122 ... yes
23/123 ... yes
23/124 ... yes
23/125 ... yes
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3/140 ... yes
23/141 ... yes
34/142 ... yes
35/145 ... yes
35/146 ... yes
35/147 ... yes
35/148 ... yes
35/149 ... yes
35/150 ... yes
35/151 ... yes
35/152 ... yes
23/153 ... yes
39/154 ... yes
39/155 ... yes
38/156 ... yes
3/157 ... yes
37/158 ... yes
38/159 ... yes
38/160 ... yes
38/161 ... yes
38/162 ... yes
3/167 ... yes
38/168 ... yes
39/169 ... yes
37/170 ... yes
37/171 ... yes
37/172 ... yes
38/173 ... yes
37/174 ... yes
3/175 ... yes
37/176 ... yes
37/177 ... yes
3/178 ... yes
37/179 ... yes
14/181 ... yes
3/182 ... yes
3/183 ... yes
3/184 ... yes
37/185 ... yes
38/186 ... yes
38/187 ... yes
3/188 ... yes
3/189 ... yes
43/225 ... yes
43/226 ... yes
37/334 ... yes
40/335 ... yes
42/336 ... yes
41/337 ... yes
41/338 ... yes
42/339 ... yes
42/340 ... yes
42/341 ... yes
42/342 ... yes
42/343 ... yes
42/344 ... yes
42/345 ... yes
42/346 ... yes
42/347 ... yes
42/348 ... yes
42/349 ... yes
42/350 ... yes
42/352 ... yes
42/353 ... yes
42/354 ... yes
42/355 ... yes
42/356 ... yes
42/357 ... yes
42/358 ... yes
42/359 ... yes
42/360 ... yes
41/361 ... yes
14/362 ... yes
37/372 ... yes
37/374 ... 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: ... 22
Is authorized keys file accessible? ... yes


Checking GitLab App ... Finished


Checking GitLab subtasks ... Finished

Possible fixes

The only workaround is to use git merge on the command line.