Added checks for migration downtime

These new checks can be used to check if migrations require downtime or
not (as tagged by their authors). In CI this compares the current branch
with master so migrations added by merge requests are automatically
verified.

To check the migrations added since a Git reference simply run:

    bundle exec rake gitlab:db:downtime_check[GIT_REF]

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