2 files changed, 42 insertions, 1 deletions
diff --git a/lib/gitlab/exclusive_lease.rb b/lib/gitlab/exclusive_lease.rb
new file mode 100644
index 00000000000..2ef50286b1d
--- /dev/null
+++ b/lib/gitlab/exclusive_lease.rb
@@ -0,0 +1,41 @@
+module Gitlab
+  # This class implements an 'exclusive lease'. We call it a 'lease'
+  # because it has a set expiry time. We call it 'exclusive' because only
+  # one caller may obtain a lease for a given key at a time. The
+  # implementation is intended to work across GitLab processes and across
+  # servers. It is a 'cheap' alternative to using SQL queries and updates:
+  # you do not need to change the SQL schema to start using
+  # ExclusiveLease.
+  #
+  # It is important to choose the timeout wisely. If the timeout is very
+  # high (1 hour) then the throughput of your operation gets very low (at
+  # most once an hour). If the timeout is lower than how long your
+  # operation may take then you cannot count on exclusivity. For example,
+  # if the timeout is 10 seconds and you do an operation which may take 20
+  # seconds then two overlapping operations may hold a lease for the same
+  # key at the same time.
+  #
+  class ExclusiveLease
+    def initialize(key, timeout:)
+      @key, @timeout = key, timeout
+    end
+
+    # Try to obtain the lease. Return true on success,
+    # false if the lease is already taken.
+    def try_obtain
+      # Performing a single SET is atomic
+      !!redis.set(redis_key, '1', nx: true, ex: @timeout)
+    end
+
+    private
+
+    def redis
+      # Maybe someday we want to use a connection pool...
+      @redis ||= Redis.new(url: Gitlab::RedisConfig.url)
+    end
+
+    def redis_key
+      "gitlab:exclusive_lease:#{@key}"
+    end
+  end
+end
diff --git a/lib/gitlab/user_access.rb b/lib/gitlab/user_access.rb
index 4885baf9526..d1b42c1f9b9 100644
--- a/lib/gitlab/user_access.rb
+++ b/lib/gitlab/user_access.rb
@@ -3,7 +3,7 @@ module Gitlab
     def self.allowed?(user)
       return false if user.blocked?
 
-      if user.requires_ldap_check?
+      if user.requires_ldap_check? && user.try_obtain_ldap_lease
         return false unless Gitlab::LDAP::Access.allowed?(user)
       end