1 files changed, 71 insertions, 0 deletions
diff --git a/lib/gitlab/downtime_check.rb b/lib/gitlab/downtime_check.rb
new file mode 100644
index 00000000000..ab9537ed7d7
--- /dev/null
+++ b/lib/gitlab/downtime_check.rb
@@ -0,0 +1,71 @@
+module Gitlab
+  # Checks if a set of migrations requires downtime or not.
+  class DowntimeCheck
+    # The constant containing the boolean that indicates if downtime is needed
+    # or not.
+    DOWNTIME_CONST = :DOWNTIME
+
+    # The constant that specifies the reason for the migration requiring
+    # downtime.
+    DOWNTIME_REASON_CONST = :DOWNTIME_REASON
+
+    # Checks the given migration paths and returns an Array of
+    # `Gitlab::DowntimeCheck::Message` instances.
+    #
+    # migrations - The migration file paths to check.
+    def check(migrations)
+      migrations.map do |path|
+        require(path)
+
+        migration_class = class_for_migration_file(path)
+
+        unless migration_class.const_defined?(DOWNTIME_CONST)
+          raise "The migration in #{path} does not specify if it requires " \
+            "downtime or not"
+        end
+
+        if online?(migration_class)
+          Message.new(path)
+        else
+          reason = downtime_reason(migration_class)
+
+          unless reason
+            raise "The migration in #{path} requires downtime but no reason " \
+              "was given"
+          end
+
+          Message.new(path, true, reason)
+        end
+      end
+    end
+
+    # Checks the given migrations and prints the results to STDOUT/STDERR.
+    #
+    # migrations - The migration file paths to check.
+    def check_and_print(migrations)
+      check(migrations).each do |message|
+        puts message.to_s # rubocop: disable Rails/Output
+      end
+    end
+
+    # Returns the class for the given migration file path.
+    def class_for_migration_file(path)
+      File.basename(path, File.extname(path)).split('_', 2).last.camelize.
+        constantize
+    end
+
+    # Returns true if the given migration can be performed without downtime.
+    def online?(migration)
+      migration.const_get(DOWNTIME_CONST) == false
+    end
+
+    # Returns the downtime reason, or nil if none was defined.
+    def downtime_reason(migration)
+      if migration.const_defined?(DOWNTIME_REASON_CONST)
+        migration.const_get(DOWNTIME_REASON_CONST)
+      else
+        nil
+      end
+    end
+  end
+end