Race condition when retrying builds

Summary

Seen on https://gitlab.com/gitlab-org/gitlab-ce/pipelines/5475323 (rspec 18 20 job). If two people retry a failed build simultaneously, two builds are created and both run. This suggests a race condition retrying builds.

Steps to reproduce

Open a failed build in two browser tabs. Click retry in both

Expected behavior

Only one should create a new build. The other should fail.

Actual behavior

Two builds are created

Possible fixes

Wrap build creation for retry in a transaction and abort if another retry build for the same job has been created.