Fix graphql resolver complexity

Since 1.9 graphql always sets complexity for resolvers, which
caused that our default resolver complexity proc wasn't used.

Unless complexity is set explicitly then graphql now uses complexity
`1`. To avoid this behavior, we use `complexity` method to set
complexity to `0`, then when instantiating a field we use our
default complexity proc if complexity is `0`.

app/graphql/resolvers/base_resolver.rb

@@ -28,6 +28,10 @@ module Resolvers
       end
     end
 
+    def self.complexity
+      0
+    end
+
     def self.resolver_complexity(args, child_complexity:)
       complexity = 1
       complexity += 1 if args[:sort]

app/graphql/types/base_field.rb

@@ -9,7 +9,7 @@ module Types
     def initialize(*args, **kwargs, &block)
       @calls_gitaly = !!kwargs.delete(:calls_gitaly)
       @constant_complexity = !!kwargs[:complexity]
-      kwargs[:complexity] ||= field_complexity(kwargs[:resolver_class])
+      kwargs[:complexity] = field_complexity(kwargs[:resolver_class], kwargs[:complexity])
       @feature_flag = kwargs[:feature_flag]
       kwargs = check_feature_flag(kwargs)
 
@@ -51,7 +51,9 @@ module Types
       args
     end
 
-    def field_complexity(resolver_class)
+    def field_complexity(resolver_class, current)
+      return current if current.present? && current > 0
+
       if resolver_class
         field_resolver_complexity
       else
@@ -66,22 +68,30 @@ module Types
       # proc because we set complexity depending on arguments and number of
       # items which can be loaded.
       proc do |ctx, args, child_complexity|
+        next base_complexity unless resolver_complexity_enabled?(ctx)
+
         # Resolvers may add extra complexity depending on used arguments
         complexity = child_complexity + self.resolver&.try(:resolver_complexity, args, child_complexity: child_complexity).to_i
         complexity += 1 if calls_gitaly?
-
-        field_defn = to_graphql
-
-        if field_defn.connection?
-          # Resolvers may add extra complexity depending on number of items being loaded.
-          page_size   = field_defn.connection_max_page_size || ctx.schema.default_max_page_size
-          limit_value = [args[:first], args[:last], page_size].compact.min
-          multiplier  = self.resolver&.try(:complexity_multiplier, args).to_f
-          complexity += complexity * limit_value * multiplier
-        end
+        complexity += complexity * connection_complexity_multiplier(ctx, args)
 
         complexity.to_i
       end
     end
+
+    def resolver_complexity_enabled?(ctx)
+      ctx.fetch(:graphql_resolver_complexity_flag) { |key| ctx[key] = Feature.enabled?(:graphql_resolver_complexity) }
+    end
+
+    def connection_complexity_multiplier(ctx, args)
+      # Resolvers may add extra complexity depending on number of items being loaded.
+      field_defn = to_graphql
+      return 0 unless field_defn.connection?
+
+      page_size   = field_defn.connection_max_page_size || ctx.schema.default_max_page_size
+      limit_value = [args[:first], args[:last], page_size].compact.min
+      multiplier  = self.resolver&.try(:complexity_multiplier, args).to_f
+      limit_value * multiplier
+    end
   end
 end

ee/app/graphql/resolvers/epic_resolver.rb

@@ -92,5 +92,10 @@ module Resolvers
 
       complexity
     end
+
+    # https://gitlab.com/gitlab-org/gitlab/issues/205312
+    def self.complexity_multiplier(args)
+      0.001
+    end
   end
 end

ee/app/graphql/types/design_management/design_collection_type.rb

@@ -17,7 +17,8 @@ module Types
             Types::DesignManagement::DesignType.connection_type,
             null: false,
             resolver: Resolvers::DesignManagement::DesignsResolver,
-            description: 'All designs for the design collection'
+            description: 'All designs for the design collection',
+            complexity: 5
 
       field :versions,
             Types::DesignManagement::VersionType.connection_type,

spec/graphql/types/base_field_spec.rb

@@ -46,7 +46,15 @@ describe Types::BaseField do
       expect(field.to_graphql.complexity).to eq 12
     end
 
-    context 'when field has a resolver proc' do
+    context 'when field has a resolver' do
+      context 'when a valid complexity is already set' do
+        let(:field) { described_class.new(name: 'test', type: GraphQL::STRING_TYPE.connection_type, resolver_class: resolver, complexity: 2, max_page_size: 100, null: true) }
+
+        it 'uses this complexity' do
+          expect(field.to_graphql.complexity).to eq 2
+        end
+      end
+
       context 'and is a connection' do
         let(:field) { described_class.new(name: 'test', type: GraphQL::STRING_TYPE.connection_type, resolver_class: resolver, max_page_size: 100, null: true) }
 
@@ -59,6 +67,17 @@ describe Types::BaseField do
           expect(field.to_graphql.complexity.call({}, { first: 1 }, 2)).to eq 2
           expect(field.to_graphql.complexity.call({}, { first: 1, foo: true }, 2)).to eq 4
         end
+
+        context 'when graphql_resolver_complexity is disabled' do
+          before do
+            stub_feature_flags(graphql_resolver_complexity: false)
+          end
+
+          it 'sets default field complexity' do
+            expect(field.to_graphql.complexity.call({}, {}, 2)).to eq 1
+            expect(field.to_graphql.complexity.call({}, { first: 50 }, 2)).to eq 1
+          end
+        end
       end
 
       context 'and is not a connection' do

spec/requests/api/graphql_spec.rb

@@ -152,4 +152,52 @@ describe 'GraphQL' do
       end
     end
   end
+
+  describe 'resolver complexity' do
+    let_it_be(:project) { create(:project, :public) }
+    let(:query) do
+      graphql_query_for(
+        'project',
+        { 'fullPath' => project.full_path },
+        query_graphql_field(resource, {}, 'edges { node { iid } }')
+      )
+    end
+
+    before do
+      stub_const('GitlabSchema::DEFAULT_MAX_COMPLEXITY', 6)
+      stub_feature_flags(graphql_resolver_complexity: true)
+    end
+
+    context 'when fetching single resource' do
+      let(:resource) { 'issues(first: 1)' }
+
+      it 'processes the query' do
+        post_graphql(query)
+
+        expect(graphql_errors).to be_nil
+      end
+    end
+
+    context 'when fetching too many resources' do
+      let(:resource) { 'issues(first: 100)' }
+
+      it 'returns an error' do
+        post_graphql(query)
+
+        expect_graphql_errors_to_include(/which exceeds max complexity/)
+      end
+
+      context 'when graphql_resolver_complexity is disabled' do
+        before do
+          stub_feature_flags(graphql_resolver_complexity: false)
+        end
+
+        it 'processes the query' do
+          post_graphql(query)
+
+          expect(graphql_errors).to be_nil
+        end
+      end
+    end
+  end
 end