[SDTEST-878] Optimise LocalRepository.relative_to_root helper to make test impact analysis faster

lib/datadog/ci/git/local_repository.rb

@@ -30,16 +30,44 @@ def self.root
           @root = git_root || Dir.pwd
         end
 
+        # ATTENTION: this function is running in a hot path
+        # and should be optimized for performance
         def self.relative_to_root(path)
           return "" if path.nil?
 
           root_path = root
           return path if root_path.nil?
 
-          path = Pathname.new(File.expand_path(path))
-          root_path = Pathname.new(root_path)
+          if File.absolute_path?(path)
+            # prefix_index is where the root path ends in the given path
+            prefix_index = root_path.size
+
+            # impossible case - absolute paths are returned from code coverage tool that always checks
+            # that root is a prefix of the path
+            return "" if path.size < prefix_index
+
+            prefix_index += 1 if path[prefix_index] == File::SEPARATOR
+            res = path[prefix_index..]
+          else
+            # prefix_to_root is a difference between the root path and the given path
+            if @prefix_to_root == ""
+              return path
+            elsif @prefix_to_root
+              return File.join(@prefix_to_root, path)
+            end
+
+            pathname = Pathname.new(File.expand_path(path))
+            root_path = Pathname.new(root_path)
+
+            # relative_path_from is an expensive function
+            res = pathname.relative_path_from(root_path).to_s
+
+            unless defined?(@prefix_to_root)
+              @prefix_to_root = res&.gsub(path, "") if res.end_with?(path)
+            end
+          end
 
-          path.relative_path_from(root_path).to_s
+          res || ""
         end
 
         def self.repository_name

lib/datadog/ci/transport/http.rb

@@ -149,6 +149,10 @@ def code
             nil
           end
 
+          def response_size
+            0
+          end
+
           def inspect
             "ErrorResponse error:#{error}"
           end

spec/datadog/ci/test_optimisation/coverage/event_spec.rb

@@ -86,7 +86,7 @@
     context "when file paths are absolute" do
       let(:coverage) do
         {
-          absolute_path("./project/file.rb") => true
+          absolute_path("project/file.rb") => true
         }
       end
 
@@ -104,6 +104,45 @@
       end
     end
 
+    context "when file paths are relative" do
+      let(:current_folder) { File.basename(Dir.pwd) }
+
+      before do
+        if Datadog::CI::Git::LocalRepository.instance_variable_defined?(:@prefix_to_root)
+          Datadog::CI::Git::LocalRepository.remove_instance_variable(:@prefix_to_root)
+        end
+
+        new_root = Dir.pwd.gsub("/#{current_folder}", "")
+        new_root = "/" if new_root.empty?
+        allow(Datadog::CI::Git::LocalRepository).to receive(:root).and_return(new_root)
+      end
+
+      after do
+        Datadog::CI::Git::LocalRepository.remove_instance_variable(:@prefix_to_root)
+      end
+
+      let(:coverage) do
+        {
+          "project/file.rb" => true,
+          "project/file2.rb" => true
+        }
+      end
+
+      it "converts all file paths to relative to the git root" do
+        expect(msgpack_json).to eq(
+          {
+            "test_session_id" => 3,
+            "test_suite_id" => 2,
+            "span_id" => 1,
+            "files" => [
+              {"filename" => "#{current_folder}/project/file.rb"},
+              {"filename" => "#{current_folder}/project/file2.rb"}
+            ]
+          }
+        )
+      end
+    end
+
     context "coverage in lines format" do
       let(:coverage) { {"file.rb" => {1 => true, 2 => true, 3 => true}} }