Expose GraphQL query complexity score

Allows the complexity score of a GraphQL query to be queryable by
clients:

    {
      queryComplexity {
        limit
        score
      }
    }

Changelog: added

app/graphql/types/query_complexity_type.rb

+# frozen_string_literal: true
+
+module Types
+  # rubocop: disable Graphql/AuthorizeTypes
+  class QueryComplexityType < ::Types::BaseObject
+    ANALYZER = GraphQL::Analysis::QueryComplexity.new { |_query, complexity| complexity }
+
+    graphql_name 'QueryComplexity'
+
+    alias_method :query, :object
+
+    field :limit, GraphQL::INT_TYPE,
+          null: true,
+          method: :max_complexity,
+          see: {
+            'GitLab documentation on this limit' =>
+              'https://docs.gitlab.com/ee/api/graphql/index.html#max-query-complexity'
+          },
+          description: 'GraphQL query complexity limit.'
+
+    field :score, GraphQL::INT_TYPE,
+          null: true,
+          description: 'GraphQL query complexity score.'
+
+    def score
+      ::GraphQL::Analysis.analyze_query(query, [ANALYZER]).first
+    end
+  end
+  # rubocop: enable Graphql/AuthorizeTypes
+end

app/graphql/types/query_type.rb

@@ -36,6 +36,10 @@ module Types
           resolver: Resolvers::MetadataResolver,
           description: 'Metadata about GitLab.'
 
+    field :query_complexity, Types::QueryComplexityType,
+          null: true,
+          description: 'Information about the complexity of the GraphQL query.'
+
     field :snippets,
           Types::SnippetType.connection_type,
           null: true,
@@ -170,6 +174,10 @@ module Types
     def application_settings
       Gitlab::CurrentSettings.current_application_settings
     end
+
+    def query_complexity
+      context.query
+    end
   end
 end
 

doc/api/graphql/getting_started.md

@@ -298,6 +298,24 @@ query IssueTypes {
 More about introspection:
 [GraphQL documentation](https://graphql.org/learn/introspection/)
 
+### Query complexity
+
+The calculated [complexity score and limit](index.md#max-query-complexity) for a query can be revealed to clients by
+querying for `queryComplexity`.
+
+```graphql
+query {
+  queryComplexity {
+    score
+    limit
+  }
+
+  project(fullPath: "gitlab-org/graphql-sandbox") {
+    name
+  }
+}
+```
+
 ## Sorting
 
 Some of the GitLab GraphQL endpoints allow you to specify how to sort a

doc/api/graphql/index.md

@@ -166,7 +166,7 @@ The complexity of a single query is limited to a maximum of:
 - `200` for unauthenticated requests.
 - `250` for authenticated requests.
 
-There is no way to discover the complexity of a query except by exceeding the limit.
+The complexity score of a query and limit for the request [can be queried for](getting_started.md#query-complexity).
 
 If a query exceeds the complexity limit an error message response will
 be returned.

doc/api/graphql/reference/index.md

@@ -286,6 +286,12 @@ four standard [pagination arguments](#connection-pagination-arguments):
 | <a id="queryprojectssort"></a>`sort` | [`String`](#string) | Sort order of results. |
 | <a id="queryprojectstopics"></a>`topics` | [`[String!]`](#string) | Filters projects by topics. |
 
+### `Query.queryComplexity`
+
+Information about the complexity of the GraphQL query.
+
+Returns [`QueryComplexity`](#querycomplexity).
+
 ### `Query.runner`
 
 Find a runner. Available only when feature flag `runner_graphql_query` is enabled.
@@ -12335,6 +12341,15 @@ Pypi metadata.
 | <a id="pypimetadataid"></a>`id` | [`PackagesPypiMetadatumID!`](#packagespypimetadatumid) | ID of the metadatum. |
 | <a id="pypimetadatarequiredpython"></a>`requiredPython` | [`String`](#string) | Required Python version of the Pypi package. |
 
+### `QueryComplexity`
+
+#### Fields
+
+| Name | Type | Description |
+| ---- | ---- | ----------- |
+| <a id="querycomplexitylimit"></a>`limit` | [`Int`](#int) | GraphQL query complexity limit. See [GitLab documentation on this limit](https://docs.gitlab.com/ee/api/graphql/index.html#max-query-complexity). |
+| <a id="querycomplexityscore"></a>`score` | [`Int`](#int) | GraphQL query complexity score. |
+
 ### `RecentFailures`
 
 Recent failure history of a test case.

doc/development/api_graphql_styleguide.md

@@ -68,9 +68,7 @@ Complexity is explained [on our client-facing API page](../api/graphql/index.md#
 Fields default to adding `1` to a query's complexity score, but developers can
 [specify a custom complexity](#field-complexity) when defining a field.
 
-To estimate the complexity of a query, you can run the
-[`gitlab:graphql:analyze`](rake_tasks.md#analyze-graphql-queries)
-Rake task.
+The complexity score of a query [can itself be queried for](../api/graphql/getting_started.md#query-complexity).
 
 ### Request timeout
 

spec/graphql/types/query_complexity_type_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe GitlabSchema.types['QueryComplexity'] do
+  include GraphqlHelpers
+
+  specify do
+    expect(described_class).to have_graphql_fields(:limit, :score).only
+  end
+
+  it 'works when executed' do
+    query = <<-GQL
+      query {
+        queryComplexity {
+          score
+          limit
+        }
+
+        currentUser {
+          name
+        }
+      }
+    GQL
+
+    query_result = run_with_clean_state(query).to_h
+
+    data = graphql_dig_at(query_result, :data, :queryComplexity)
+
+    expect(data).to include(
+      'score' => be > 0,
+      'limit' => GitlabSchema::DEFAULT_MAX_COMPLEXITY
+    )
+  end
+end