Added checks for migration downtimemigration-downtime-tags

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]

4 files changed, 140 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
diff --git a/lib/gitlab/downtime_check/message.rb b/lib/gitlab/downtime_check/message.rb
new file mode 100644
index 00000000000..4446e921e0d
--- /dev/null
+++ b/lib/gitlab/downtime_check/message.rb
@@ -0,0 +1,28 @@
+module Gitlab
+  class DowntimeCheck
+    class Message
+      attr_reader :path, :offline, :reason
+
+      OFFLINE = "\e[32moffline\e[0m"
+      ONLINE = "\e[31monline\e[0m"
+
+      # path - The file path of the migration.
+      # offline - When set to `true` the migration will require downtime.
+      # reason - The reason as to why the migration requires downtime.
+      def initialize(path, offline = false, reason = nil)
+        @path = path
+        @offline = offline
+        @reason = reason
+      end
+
+      def to_s
+        label = offline ? OFFLINE : ONLINE
+
+        message = "[#{label}]: #{path}"
+        message += ": #{reason}" if reason
+
+        message
+      end
+    end
+  end
+end
diff --git a/lib/tasks/downtime_check.rake b/lib/tasks/downtime_check.rake
new file mode 100644
index 00000000000..30a2e9be5ce
--- /dev/null
+++ b/lib/tasks/downtime_check.rake
@@ -0,0 +1,26 @@
+desc 'Checks if migrations in a branch require downtime'
+task downtime_check: :environment do
+  # First we'll want to make sure we're comparing with the right upstream
+  # repository/branch.
+  current_branch = `git rev-parse --abbrev-ref HEAD`.strip
+
+  # Either the developer ran this task directly on the master branch, or they're
+  # making changes directly on the master branch.
+  if current_branch == 'master'
+    if defined?(Gitlab::License)
+      repo = 'gitlab-ee'
+    else
+      repo = 'gitlab-ce'
+    end
+
+    `git fetch https://gitlab.com/gitlab-org/#{repo}.git --depth 1`
+
+    compare_with = 'FETCH_HEAD'
+  # The developer is working on a different branch, in this case we can just
+  # compare with the master branch.
+  else
+    compare_with = 'master'
+  end
+
+  Rake::Task['gitlab:db:downtime_check'].invoke(compare_with)
+end
diff --git a/lib/tasks/gitlab/db.rake b/lib/tasks/gitlab/db.rake
index 7230b9485be..0ec19e1a625 100644
--- a/lib/tasks/gitlab/db.rake
+++ b/lib/tasks/gitlab/db.rake
@@ -46,5 +46,20 @@ namespace :gitlab do
         Rake::Task['db:seed_fu'].invoke
       end
     end
+
+    desc 'Checks if migrations require downtime or not'
+    task :downtime_check, [:ref] => :environment do |_, args|
+      abort 'You must specify a Git reference to compare with' unless args[:ref]
+
+      require 'shellwords'
+
+      ref = Shellwords.escape(args[:ref])
+
+      migrations = `git diff #{ref}.. --name-only -- db/migrate`.lines.
+        map { |file| Rails.root.join(file.strip).to_s }.
+        select { |file| File.file?(file) }
+
+      Gitlab::DowntimeCheck.new.check_and_print(migrations)
+    end
   end
 end