**Prevention of running 2 simultaneous updates**
Instead of using `RemoteMirror#update_status` and raise an error if
it's already running to prevent the same mirror being updated at the
same time we now use `Gitlab::ExclusiveLease` for that.
When we fail to obtain a lease in 3 tries, 30 seconds apart, we bail
and reschedule. We'll reschedule faster for the protected branches.
If the mirror already ran since it was scheduled, the job will be
skipped.
**Error handling: Remote side**
When an update fails because of a `Gitlab::Git::CommandError`, we
won't track this error in sentry, this could be on the remote side:
for example when branches have diverged.
In this case, we'll try 3 times scheduled 1 or 5 minutes apart.
In between, the mirror is marked as "to_retry", the error would be
visible to the user when they visit the settings page.
After 3 tries we'll mark the mirror as failed and notify the user.
We won't track this error in sentry, as it's not likely we can help
it.
The next event that would trigger a new refresh.
**Error handling: our side**
If an unexpected error occurs, we mark the mirror as failed, but we'd
still retry the job based on the regular sidekiq retries with
backoff. Same as we used to
The error would be reported in sentry, since its likely we need to do
something about it.
Retries in Sidekiq and in the remote mirror scheduler can cause repeated
attempts in quick succession if the sync fails. Each failure will then
send an e-mail to all project maintainers, which can spam users
unnecessarily.
Modify the logic to send one notification the first time the mirror
fails by setting `error_notification_sent` to `true` and reset the
flag after a successful sync.
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56222
Ash McKenzie