GraphQL - Add extra complexity for resolvers

If a field is a resolver, its complexity is automatically
increased. By default we add extra points for sort and search
arguments (which will be common for various resolvers).

For specific resolvers we add field-specific complexity, e.g.
for Issues complexity is increased if we filter issues by `labelName`
(because then SQL query is more complex). We may want to tune these
values in future depending on real-life results.

Complexity is also dependent on the number of loaded nodes, but only
if we don't search by specific ID(s). Also added complexity is limited
(by default only twice more than child complexity) - the reason is
that although it's more complex to process more items, the complexity
increase is not linear (there is not so much difference between loading
10, 20 or 100 records from DB).

6 files changed, 76 insertions, 4 deletions

diff --git a/app/graphql/resolvers/base_resolver.rb b/app/graphql/resolvers/base_resolver.rb
index 063def75d38..31850c2cadb 100644
--- a/app/graphql/resolvers/base_resolver.rb
+++ b/app/graphql/resolvers/base_resolver.rb
@@ -9,5 +9,24 @@ module Resolvers
         end
       end
     end
+
+    def self.resolver_complexity(args)
+      complexity = 1
+      complexity += 1 if args[:sort]
+      complexity += 5 if args[:search]
+
+      complexity
+    end
+
+    def self.complexity_multiplier(args)
+      # When fetching many items, additional complexity is added to the field
+      # depending on how many items is fetched. For each item we add 1% of the
+      # original complexity - this means that loading 100 items (our default
+      # maxp_age_size limit) doubles the original complexity.
+      #
+      # Complexity is not increased when searching by specific ID(s), because
+      # complexity difference is minimal in this case.
+      [args[:iid], args[:iids]].any? ? 0 : 0.01
+    end
   end
 end
diff --git a/app/graphql/resolvers/concerns/resolves_pipelines.rb b/app/graphql/resolvers/concerns/resolves_pipelines.rb
index 8fd26d85994..a166211fc18 100644
--- a/app/graphql/resolvers/concerns/resolves_pipelines.rb
+++ b/app/graphql/resolvers/concerns/resolves_pipelines.rb
@@ -19,6 +19,16 @@ module ResolvesPipelines
              description: "Filter pipelines by the sha of the commit they are run for"
   end
 
+  class_methods do
+    def resolver_complexity(args)
+      complexity = super
+      complexity += 2 if args[:sha]
+      complexity += 2 if args[:ref]
+
+      complexity
+    end
+  end
+
   def resolve_pipelines(project, params = {})
     PipelinesFinder.new(project, context[:current_user], params).execute
   end
diff --git a/app/graphql/resolvers/issues_resolver.rb b/app/graphql/resolvers/issues_resolver.rb
index 54d32a688bf..1c3c24ad6dc 100644
--- a/app/graphql/resolvers/issues_resolver.rb
+++ b/app/graphql/resolvers/issues_resolver.rb
@@ -57,5 +57,12 @@ module Resolvers
 
       IssuesFinder.new(context[:current_user], args).execute
     end
+
+    def self.resolver_complexity(args)
+      complexity = super
+      complexity += 2 if args[:labelName]
+
+      complexity
+    end
   end
 end
diff --git a/app/graphql/resolvers/project_resolver.rb b/app/graphql/resolvers/project_resolver.rb
index ac7c9b0ce2e..2132447da5e 100644
--- a/app/graphql/resolvers/project_resolver.rb
+++ b/app/graphql/resolvers/project_resolver.rb
@@ -9,5 +9,9 @@ module Resolvers
     def resolve(full_path:)
       model_by_full_path(Project, full_path)
     end
+
+    def self.complexity_multiplier(args)
+      0
+    end
   end
 end
diff --git a/app/graphql/types/base_field.rb b/app/graphql/types/base_field.rb
index 8c8b8a82d3e..15331129134 100644
--- a/app/graphql/types/base_field.rb
+++ b/app/graphql/types/base_field.rb
@@ -7,10 +7,40 @@ module Types
     DEFAULT_COMPLEXITY = 1
 
     def initialize(*args, **kwargs, &block)
-      # complexity is already defaulted to 1, but let's make it explicit
-      kwargs[:complexity] ||= DEFAULT_COMPLEXITY
+      kwargs[:complexity] ||= field_complexity(kwargs[:resolver_class])
 
       super(*args, **kwargs, &block)
     end
+
+    private
+
+    def field_complexity(resolver_class)
+      if resolver_class
+        field_resolver_complexity
+      else
+        DEFAULT_COMPLEXITY
+      end
+    end
+
+    def field_resolver_complexity
+      # Complexity can be either integer or proc. If proc is used then it's
+      # called when computing a query complexity and context and query
+      # arguments are available for computing complexity.  For resolvers we use
+      # proc because we set complexity depending on arguments and number of
+      # items which can be loaded.
+      proc do |ctx, args, child_complexity|
+        page_size = @max_page_size || ctx.schema.default_max_page_size
+        limit_value = [args[:first], args[:last], page_size].compact.min
+
+        # Resolvers may add extra complexity depending on used arguments
+        complexity = child_complexity + self.resolver&.try(:resolver_complexity, args).to_i
+
+        # Resolvers may add extra complexity depending on number of items being loaded.
+        multiplier = self.resolver&.try(:complexity_multiplier, args).to_f
+        complexity += complexity * limit_value * multiplier
+
+        complexity.to_i
+      end
+    end
   end
 end
diff --git a/app/graphql/types/issue_type.rb b/app/graphql/types/issue_type.rb
index adb137dfee3..b21a226d07f 100644
--- a/app/graphql/types/issue_type.rb
+++ b/app/graphql/types/issue_type.rb
@@ -19,9 +19,11 @@ module Types
           null: false,
           resolve: -> (obj, _args, _ctx) { Gitlab::Graphql::Loaders::BatchModelLoader.new(User, obj.author_id).find }
 
-    field :assignees, Types::UserType.connection_type, null: true
+    # Remove complexity when BatchLoader is used
+    field :assignees, Types::UserType.connection_type, null: true, complexity: 5
 
-    field :labels, Types::LabelType.connection_type, null: true
+    # Remove complexity when BatchLoader is used
+    field :labels, Types::LabelType.connection_type, null: true, complexity: 5
     field :milestone, Types::MilestoneType,
           null: true,
           resolve: -> (obj, _args, _ctx) { Gitlab::Graphql::Loaders::BatchModelLoader.new(Milestone, obj.milestone_id).find }