Limit amount of JUnit tests returned

Currently, we do not cap amount of tests returned to frontend,
thus in some extreme cases we can see a MBs of data stored in Redis.

This adds an upper limit of 100 tests per-suite.

We will continue showing the total counters correctly,
but we will limit amount of tests that will be presented.

2 files changed, 31 insertions, 4 deletions

diff --git a/app/serializers/test_suite_comparer_entity.rb b/app/serializers/test_suite_comparer_entity.rb
index 9fa3a897ebe..d402a4d5718 100644
--- a/app/serializers/test_suite_comparer_entity.rb
+++ b/app/serializers/test_suite_comparer_entity.rb
@@ -1,6 +1,9 @@
 # frozen_string_literal: true
 
 class TestSuiteComparerEntity < Grape::Entity
+  DEFAULT_MAX_TESTS = 100
+  DEFAULT_MIN_TESTS = 10
+
   expose :name
   expose :total_status, as: :status
 
@@ -10,7 +13,27 @@ class TestSuiteComparerEntity < Grape::Entity
     expose :failed_count, as: :failed
   end
 
-  expose :new_failures, using: TestCaseEntity
-  expose :resolved_failures, using: TestCaseEntity
-  expose :existing_failures, using: TestCaseEntity
+  # rubocop: disable CodeReuse/ActiveRecord
+  expose :new_failures, using: TestCaseEntity do |suite|
+    suite.new_failures.take(max_tests)
+  end
+
+  expose :existing_failures, using: TestCaseEntity do |suite|
+    suite.existing_failures.take(
+      max_tests(suite.new_failures))
+  end
+
+  expose :resolved_failures, using: TestCaseEntity do |suite|
+    suite.resolved_failures.take(
+      max_tests(suite.new_failures, suite.existing_failures))
+  end
+
+  private
+
+  def max_tests(*used)
+    return Integer::MAX unless Feature.enabled?(:ci_limit_test_reports_size, default_enabled: true)
+
+    [DEFAULT_MAX_TESTS - used.map(&:count).sum, DEFAULT_MIN_TESTS].max
+  end
+  # rubocop: enable CodeReuse/ActiveRecord
 end
diff --git a/app/services/ci/compare_reports_base_service.rb b/app/services/ci/compare_reports_base_service.rb
index d5625857599..6c2d80d8f45 100644
--- a/app/services/ci/compare_reports_base_service.rb
+++ b/app/services/ci/compare_reports_base_service.rb
@@ -8,7 +8,7 @@ module Ci
         status: :parsed,
         key: key(base_pipeline, head_pipeline),
         data: serializer_class
-          .new(project: project)
+          .new(**serializer_params)
           .represent(comparer).as_json
       }
     rescue Gitlab::Ci::Parsers::ParserError => e
@@ -40,6 +40,10 @@ module Ci
       raise NotImplementedError
     end
 
+    def serializer_params
+      { project: project }
+    end
+
     def get_report(pipeline)
       raise NotImplementedError
     end