Multiple concurrent API requests against repository.

Summary

When using multiple concurrent accesses to the files repository through the API, the "first" request goes ahead, while the followings get a 400 BadRequest response.

Steps to reproduce

Just run 2 concurrent curl in parallel

curl -v https://HOST/api/v4/projects/:id/repository/files/filename -H "Private-token: XXXXXXXXXXXXXXXXXXXX" --data "branch=master&content=thecontent1&commit_message=msg1" -X PUT & \
curl -v https://HOST/api/v4/projects/:id/repository/files/filename -H "Private-token: XXXXXXXXXXXXXXXXXXXX" --data "branch=master&content=thecontent2&commit_message=msg2" -X PUT

What is the current bug behavior?

There is a race condition between the two of them: one gets 200 OK, the other gets 400 BadRequest. Note that they are formally the very same request.

What is the expected correct behavior?

Both of them should get 200 OK. No matter of the content pushed. It is expected a race condition over the content (the last scheduled overwrites the previous, but both of them should pass and appear in the commit log).

Relevant logs and/or screenshots

The curl outputs are

{"file_path":"filename","branch":"master"}
{"message":"Could not update branch master. Please refresh and try again."}

Output of checks

Happens on gitlab-ce.

Results of GitLab environment info

System information
System:		Ubuntu 16.04
Current User:	git
Using RVM:	no
Ruby Version:	2.3.3p222
Gem Version:	2.6.6
Bundler Version:1.13.7
Rake Version:	10.5.0
Redis Version:	3.2.5
Git Version:	2.11.1
Sidekiq Version:4.2.7
GitLab information
Version:	9.1.2
Revision:	df1403f
Directory:	/opt/gitlab/embedded/service/gitlab-rails
DB Adapter:	postgresql
URL:		https://tip.telecomitalia.local
HTTP Clone URL:	https://tip.telecomitalia.local/some-group/some-project.git
SSH Clone URL:	git@tip.telecomitalia.local:some-group/some-project.git
Using LDAP:	yes
Using Omniauth:	no
GitLab Shell
Version:	5.0.2
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

Checking GitLab Shell ...
GitLab Shell version >= 5.0.2 ? ... OK (5.0.2)
Repo base directory exists?
default... yes
Repo storage directories are symlinks?
default... no
Repo paths owned by git:git?
default... yes
Repo paths access is drwxrws---?
default... yes
hooks directories in repos are links: ...
3/1 ... ok
3/2 ... repository is empty
3/3 ... ok
3/5 ... repository is empty
3/8 ... ok
3/9 ... ok
25/10 ... ok
3/11 ... ok
Running /opt/gitlab/embedded/service/gitlab-shell/bin/check
Check GitLab API access: OK
Access to /var/opt/gitlab/.ssh/authorized_keys: OK
Send ping to redis server: OK
gitlab-shell self-check successful
Checking GitLab Shell ... Finished
Checking Sidekiq ...
Running? ... yes
Number of Sidekiq processes ... 1
Checking Sidekiq ... Finished
Checking Reply by email ...
Reply by email is disabled in config/gitlab.yml
Checking Reply by email ... Finished
Checking LDAP ...
Server: ldapmain
LDAP authentication... Anonymous. No bind_dn or password configured
LDAP users with access to your GitLab server (only showing the first 100 results)
[CUT]
Checking LDAP ... Finished
Checking GitLab ...
Git configured with autocrlf=input? ... yes
Database config exists? ... yes
All migrations up? ... yes
Database contains orphaned GroupMembers? ... no
GitLab config exists? ... yes
GitLab config outdated? ... no
Log directory writable? ... yes
Tmp directory writable? ... yes
Uploads directory setup correctly? ... 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: ...
3/1 ... yes
3/2 ... yes
3/3 ... yes
3/5 ... yes
3/8 ... yes
3/9 ... yes
25/10 ... yes
3/11 ... yes
Redis version >= 2.8.0? ... yes
Ruby version >= 2.1.0 ? ... yes (2.3.3)
Your git bin path is "/opt/gitlab/embedded/bin/git"
Git version >= 2.7.3 ? ... yes (2.11.1)
Active users: 21
Checking GitLab ... Finished

Possible fixes

Not yet.