1 files changed, 82 insertions, 0 deletions
diff --git a/lib/gitlab/git/checksum.rb b/lib/gitlab/git/checksum.rb
new file mode 100644
index 00000000000..3ef0f0a8854
--- /dev/null
+++ b/lib/gitlab/git/checksum.rb
@@ -0,0 +1,82 @@
+module Gitlab
+  module Git
+    class Checksum
+      include Gitlab::Git::Popen
+
+      EMPTY_REPOSITORY_CHECKSUM = '0000000000000000000000000000000000000000'.freeze
+
+      Failure = Class.new(StandardError)
+
+      attr_reader :path, :relative_path, :storage, :storage_path
+
+      def initialize(storage, relative_path)
+        @storage       = storage
+        @storage_path  = Gitlab.config.repositories.storages[storage].legacy_disk_path
+        @relative_path = "#{relative_path}.git"
+        @path          = File.join(storage_path, @relative_path)
+      end
+
+      def calculate
+        unless repository_exists?
+          failure!(Gitlab::Git::Repository::NoRepository, 'No repository for such path')
+        end
+
+        calculate_checksum_by_shelling_out
+      end
+
+      private
+
+      def repository_exists?
+        raw_repository.exists?
+      end
+
+      def calculate_checksum_by_shelling_out
+        args = %W(--git-dir=#{path} show-ref --heads --tags)
+        output, status = run_git(args)
+
+        if status&.zero?
+          refs = output.split("\n")
+
+          result = refs.inject(nil) do |checksum, ref|
+            value = Digest::SHA1.hexdigest(ref).hex
+
+            if checksum.nil?
+              value
+            else
+              checksum ^ value
+            end
+          end
+
+          result.to_s(16)
+        else
+          # Empty repositories return with a non-zero status and an empty output.
+          if output&.empty?
+            EMPTY_REPOSITORY_CHECKSUM
+          else
+            failure!(Gitlab::Git::Checksum::Failure, output)
+          end
+        end
+      end
+
+      def failure!(klass, message)
+        Gitlab::GitLogger.error("'git show-ref --heads --tags' in #{path}: #{message}")
+
+        raise klass.new("Could not calculate the checksum for #{path}: #{message}")
+      end
+
+      def circuit_breaker
+        @circuit_breaker ||= Gitlab::Git::Storage::CircuitBreaker.for_storage(storage)
+      end
+
+      def raw_repository
+        Gitlab::Git::Repository.new(storage, relative_path, nil)
+      end
+
+      def run_git(args)
+        circuit_breaker.perform do
+          popen([Gitlab.config.git.bin_path, *args], path)
+        end
+      end
+    end
+  end
+end