1 files changed, 83 insertions, 0 deletions
diff --git a/lib/gitlab/ci/trace/checksum.rb b/lib/gitlab/ci/trace/checksum.rb
new file mode 100644
index 00000000000..62532ef1cd2
--- /dev/null
+++ b/lib/gitlab/ci/trace/checksum.rb
@@ -0,0 +1,83 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Ci
+    class Trace
+      ##
+      # Trace::Checksum class is responsible for calculating a CRC32 checksum
+      # of an entire build trace using partial build trace chunks stored in a
+      # database.
+      #
+      # CRC32 checksum can be easily calculated by combining partial checksums
+      # in a right order.
+      #
+      # Then we compare CRC32 checksum provided by a GitLab Runner and expect
+      # it to be the same as the CRC32 checksum derived from partial chunks.
+      #
+      class Checksum
+        include Gitlab::Utils::StrongMemoize
+
+        attr_reader :build
+
+        def initialize(build)
+          @build = build
+        end
+
+        def valid?
+          return false unless state_crc32.present?
+
+          state_crc32 == chunks_crc32
+        end
+
+        def state_crc32
+          strong_memoize(:state_crc32) { build.pending_state&.crc32 }
+        end
+
+        def chunks_crc32
+          strong_memoize(:chunks_crc32) do
+            trace_chunks.reduce(0) do |crc32, chunk|
+              Zlib.crc32_combine(crc32, chunk.crc32, chunk_size(chunk))
+            end
+          end
+        end
+
+        def last_chunk
+          strong_memoize(:last_chunk) { trace_chunks.max }
+        end
+
+        ##
+        # Trace chunks will be persisted in a database if an object store is
+        # not configured - in that case we do not want to load entire raw data
+        # of all the chunks into memory.
+        #
+        # We ignore `raw_data` attribute instead, and rely on internal build
+        # trace chunk database adapter to handle
+        # `ActiveModel::MissingAttributeError` exception.
+        #
+        # Alternative solution would be separating chunk data from chunk
+        # metadata on the database level too.
+        #
+        def trace_chunks
+          strong_memoize(:trace_chunks) do
+            build.trace_chunks.persisted
+              .select(::Ci::BuildTraceChunk.metadata_attributes)
+          end
+        end
+
+        def chunks_count
+          trace_chunks.to_a.size
+        end
+
+        private
+
+        def chunk_size(chunk)
+          if chunk == last_chunk
+            chunk.size
+          else
+            ::Ci::BuildTraceChunk::CHUNK_SIZE
+          end
+        end
+      end
+    end
+  end
+end