Add exclusive lease to mergeability check process

Concurrent calls to UserMergeToRef RPC updating a single ref
can lead to an opaque fail that is being rescued at Gitaly.

So this commit adds an exclusive lease to the mergeability
check process with the key as the current MR ID.

1 files changed, 3 insertions, 2 deletions

diff --git a/lib/gitlab/exclusive_lease_helpers.rb b/lib/gitlab/exclusive_lease_helpers.rb
index 7961d4bbd6e..61eb030563d 100644
--- a/lib/gitlab/exclusive_lease_helpers.rb
+++ b/lib/gitlab/exclusive_lease_helpers.rb
@@ -15,17 +15,18 @@ module Gitlab
       raise ArgumentError, 'Key needs to be specified' unless key
 
       lease = Gitlab::ExclusiveLease.new(key, timeout: ttl)
+      retried = false
 
       until uuid = lease.try_obtain
         # Keep trying until we obtain the lease. To prevent hammering Redis too
         # much we'll wait for a bit.
         sleep(sleep_sec)
-        break if (retries -= 1) < 0
+        (retries -= 1) < 0 ? break : retried ||= true
       end
 
       raise FailedToObtainLockError, 'Failed to obtain a lock' unless uuid
 
-      yield
+      yield(retried)
     ensure
       Gitlab::ExclusiveLease.cancel(key, uuid)
     end