Bad concurrency for prod deploys

[janbrasna]

With many PRs merged in succession and the time taken in CI before checking out and trying to push to gh-pages after building, if there are more jobs running at the same time, you obviously run into the issue:

[gh-pages d8c30f64] Deployed with mkdocs, version 1.1.2 from /home/circleci/.local/share/virtualenvs/code-6yRgnUSz/lib/python3.8/site-packages/mkdocs (Python 3.8)
 552 files changed, 859 insertions(+), 859 deletions(-)
To github.com:fastlane/docs.git
 ! [rejected]          gh-pages -> gh-pages (fetch first)
error: failed to push some refs to 'git@github.com:fastlane/docs.git'
hint: Updates were rejected because the remote contains work that you do
hint: not have locally. This is usually caused by another repository pushing
hint: to the same ref. You may want to first integrate the remote changes
hint: (e.g., 'git pull ...') before pushing again.
hint: See the 'Note about fast-forwards' in 'git push --help' for details.

Exited with code exit status 1

So you effectively end up not having the merged changeset published at that point. You can only hope the next push to master won't take too long to happen, to incorporate all the previous (failed) deploys to prod with it… 🤷

I'm not a CircleCI expert so take this with a pinch of salt, but… it seems the earliest commit "wins" here trying to deploy to prod, whereas normally you'd have the most recent cancelling the previous ones and eventually "winning" in the priority to deploy, not being blocked by the previous ones running concurrently to cause conflicts at the end.