Add `ci_dag_limit_needs` feature flag

This makes to limit `needs:` to 5 by default.
Allow to increase the limit to 50 with disable of FF.

lib/gitlab/ci/pipeline/seed/build.rb

@@ -9,6 +9,10 @@ module Gitlab
 
           delegate :dig, to: :@attributes
 
+          # When the `ci_dag_limit_needs` is enabled it uses the lower limit
+          LOW_NEEDS_LIMIT = 5
+          HARD_NEEDS_LIMIT = 50
+
           def initialize(pipeline, attributes, previous_stages)
             @pipeline = pipeline
             @attributes = attributes
@@ -77,9 +81,15 @@ module Gitlab
           end
 
           def needs_errors
-            return unless Feature.enabled?(:ci_dag_support, @pipeline.project)
             return if @needs_attributes.nil?
 
+            if @needs_attributes.size > max_needs_allowed
+              return [
+                "#{name}: one job can only need #{max_needs_allowed} others, but you have listed #{@needs_attributes.size}. " \
+                  "See needs keyword documentation for more details"
+              ]
+            end
+
             @needs_attributes.flat_map do |need|
               result = @previous_stages.any? do |stage|
                 stage.seeds_names.include?(need[:name])
@@ -88,6 +98,14 @@ module Gitlab
               "#{name}: needs '#{need[:name]}'" unless result
             end.compact
           end
+
+          def max_needs_allowed
+            if Feature.enabled?(:ci_dag_limit_needs, @project, default_enabled: true)
+              LOW_NEEDS_LIMIT
+            else
+              HARD_NEEDS_LIMIT
+            end
+          end
         end
       end
     end

spec/lib/gitlab/ci/pipeline/seed/build_spec.rb

@@ -386,12 +386,16 @@ describe Gitlab::Ci::Pipeline::Seed::Build do
   describe 'applying needs: dependency' do
     subject { seed_build }
 
+    let(:needs_count) { 1 }
+
+    let(:needs_attributes) do
+      Array.new(needs_count, name: 'build')
+    end
+
     let(:attributes) do
       {
         name: 'rspec',
-        needs_attributes: [{
-          name: 'build'
-        }]
+        needs_attributes: needs_attributes
       }
     end
 
@@ -429,5 +433,31 @@ describe Gitlab::Ci::Pipeline::Seed::Build do
         expect(subject.errors).to be_empty
       end
     end
+
+    context 'when lower limit of needs is reached' do
+      before do
+        stub_feature_flags(ci_dag_limit_needs: true)
+      end
+
+      let(:needs_count) { described_class::LOW_NEEDS_LIMIT + 1 }
+
+      it "returns an error" do
+        expect(subject.errors).to contain_exactly(
+          "rspec: one job can only need 5 others, but you have listed 6. See needs keyword documentation for more details")
+      end
+    end
+
+    context 'when upper limit of needs is reached' do
+      before do
+        stub_feature_flags(ci_dag_limit_needs: false)
+      end
+
+      let(:needs_count) { described_class::HARD_NEEDS_LIMIT + 1 }
+
+      it "returns an error" do
+        expect(subject.errors).to contain_exactly(
+          "rspec: one job can only need 50 others, but you have listed 51. See needs keyword documentation for more details")
+      end
+    end
   end
 end