Enables GraphQL batch requests

Enabling GraphQL batch requests allows for multiple queries
to be sent in 1 request reducing the amount of requests
we send to the server.

Responses come come back in the same order as the queries were
provided.

8 files changed, 181 insertions, 34 deletions

diff --git a/app/controllers/graphql_controller.rb b/app/controllers/graphql_controller.rb
index 7b5dc22815c..e8f38899647 100644
--- a/app/controllers/graphql_controller.rb
+++ b/app/controllers/graphql_controller.rb
@@ -16,13 +16,8 @@ class GraphqlController < ApplicationController
   before_action(only: [:execute]) { authenticate_sessionless_user!(:api) }
 
   def execute
-    variables = Gitlab::Graphql::Variables.new(params[:variables]).to_h
-    query = params[:query]
-    operation_name = params[:operationName]
-    context = {
-      current_user: current_user
-    }
-    result = GitlabSchema.execute(query, variables: variables, context: context, operation_name: operation_name)
+    result = multiplex? ? execute_multiplex : execute_query
+
     render json: result
   end
 
@@ -38,6 +33,43 @@ class GraphqlController < ApplicationController
 
   private
 
+  def execute_multiplex
+    GitlabSchema.multiplex(multiplex_queries, context: context)
+  end
+
+  def execute_query
+    variables = build_variables(params[:variables])
+    operation_name = params[:operationName]
+
+    GitlabSchema.execute(query, variables: variables, context: context, operation_name: operation_name)
+  end
+
+  def query
+    params[:query]
+  end
+
+  def multiplex_queries
+    params[:_json].map do |single_query_info|
+      {
+        query: single_query_info[:query],
+        variables: build_variables(single_query_info[:variables]),
+        operation_name: single_query_info[:operationName]
+      }
+    end
+  end
+
+  def context
+    @context ||= { current_user: current_user }
+  end
+
+  def build_variables(variable_info)
+    Gitlab::Graphql::Variables.new(variable_info).to_h
+  end
+
+  def multiplex?
+    params[:_json].present?
+  end
+
   def authorize_access_api!
     access_denied!("API not accessible for user.") unless can?(current_user, :access_api)
   end
diff --git a/app/graphql/gitlab_schema.rb b/app/graphql/gitlab_schema.rb
index 897e12c1b56..a63f45f231c 100644
--- a/app/graphql/gitlab_schema.rb
+++ b/app/graphql/gitlab_schema.rb
@@ -7,7 +7,7 @@ class GitlabSchema < GraphQL::Schema
   AUTHENTICATED_COMPLEXITY = 250
   ADMIN_COMPLEXITY         = 300
 
-  ANONYMOUS_MAX_DEPTH = 10
+  DEFAULT_MAX_DEPTH = 10
   AUTHENTICATED_MAX_DEPTH = 15
 
   use BatchLoader::GraphQL
@@ -23,10 +23,21 @@ class GitlabSchema < GraphQL::Schema
   default_max_page_size 100
 
   max_complexity DEFAULT_MAX_COMPLEXITY
+  max_depth DEFAULT_MAX_DEPTH
 
   mutation(Types::MutationType)
 
   class << self
+    def multiplex(queries, **kwargs)
+      kwargs[:max_complexity] ||= max_query_complexity(kwargs[:context])
+
+      queries.each do |query|
+        query[:max_depth] = max_query_depth(kwargs[:context])
+      end
+
+      super(queries, **kwargs)
+    end
+
     def execute(query_str = nil, **kwargs)
       kwargs[:max_complexity] ||= max_query_complexity(kwargs[:context])
       kwargs[:max_depth] ||= max_query_depth(kwargs[:context])
@@ -54,7 +65,7 @@ class GitlabSchema < GraphQL::Schema
       if current_user
         AUTHENTICATED_MAX_DEPTH
       else
-        ANONYMOUS_MAX_DEPTH
+        DEFAULT_MAX_DEPTH
       end
     end
   end
diff --git a/changelogs/unreleased/bvl-graphql-multiplex.yml b/changelogs/unreleased/bvl-graphql-multiplex.yml
new file mode 100644
index 00000000000..56d39e447a5
--- /dev/null
+++ b/changelogs/unreleased/bvl-graphql-multiplex.yml
@@ -0,0 +1,5 @@
+---
+title: Support multiplex GraphQL queries
+merge_request: 28273
+author:
+type: added
diff --git a/doc/api/graphql/index.md b/doc/api/graphql/index.md
index 10e1ef0e533..9195ba4cdf1 100644
--- a/doc/api/graphql/index.md
+++ b/doc/api/graphql/index.md
@@ -48,6 +48,14 @@ A first iteration of a GraphQL API includes the following queries
 1. `project` : Within a project it is also possible to fetch a `mergeRequest` by IID.
 1. `group` : Only basic group information is currently supported.
 
+### Multiplex queries
+
+GitLab supports batching queries into a single request using
+[apollo-link-batch-http](https://www.apollographql.com/docs/link/links/batch-http). More
+info about multiplexed queries is also available for
+[graphql-ruby](https://graphql-ruby.org/queries/multiplex.html) the
+library GitLab uses on the backend.
+
 ## GraphiQL
 
 The API can be explored by using the GraphiQL IDE, it is available on your
diff --git a/spec/graphql/gitlab_schema_spec.rb b/spec/graphql/gitlab_schema_spec.rb
index c138c87c4ac..e9149f4250f 100644
--- a/spec/graphql/gitlab_schema_spec.rb
+++ b/spec/graphql/gitlab_schema_spec.rb
@@ -56,10 +56,10 @@ describe GitlabSchema do
           described_class.execute('query', context: {})
         end
 
-        it 'returns ANONYMOUS_MAX_DEPTH' do
+        it 'returns DEFAULT_MAX_DEPTH' do
           expect(GraphQL::Schema)
             .to receive(:execute)
-            .with('query', hash_including(max_depth: GitlabSchema::ANONYMOUS_MAX_DEPTH))
+            .with('query', hash_including(max_depth: GitlabSchema::DEFAULT_MAX_DEPTH))
 
           described_class.execute('query', context: {})
         end
diff --git a/spec/requests/api/graphql/gitlab_schema_spec.rb b/spec/requests/api/graphql/gitlab_schema_spec.rb
index dd518274f82..a724c5c3f1c 100644
--- a/spec/requests/api/graphql/gitlab_schema_spec.rb
+++ b/spec/requests/api/graphql/gitlab_schema_spec.rb
@@ -3,41 +3,82 @@ require 'spec_helper'
 describe 'GitlabSchema configurations' do
   include GraphqlHelpers
 
-  let(:project) { create(:project, :repository) }
-  let(:query) { graphql_query_for('project', { 'fullPath' => project.full_path }, %w(id name description)) }
-  let(:current_user) { create(:user) }
+  let(:project) { create(:project) }
 
-  describe '#max_complexity' do
-    context 'when complexity is too high' do
-      it 'shows an error' do
-        allow(GitlabSchema).to receive(:max_query_complexity).and_return 1
+  shared_examples 'imposing query limits' do
+    describe '#max_complexity' do
+      context 'when complexity is too high' do
+        it 'shows an error' do
+          allow(GitlabSchema).to receive(:max_query_complexity).and_return 1
 
-        post_graphql(query, current_user: nil)
+          subject
 
-        expect(graphql_errors.first['message']).to include('which exceeds max complexity of 1')
+          expect(graphql_errors.flatten.first['message']).to include('which exceeds max complexity of 1')
+        end
       end
     end
-  end
 
-  describe '#max_depth' do
-    context 'when query depth is too high' do
-      it 'shows error' do
-        errors = [{ "message" => "Query has depth of 2, which exceeds max depth of 1" }]
-        allow(GitlabSchema).to receive(:max_query_depth).and_return 1
+    describe '#max_depth' do
+      context 'when query depth is too high' do
+        it 'shows error' do
+          errors = { "message" => "Query has depth of 2, which exceeds max depth of 1" }
+          allow(GitlabSchema).to receive(:max_query_depth).and_return 1
 
-        post_graphql(query)
+          subject
 
-        expect(graphql_errors).to eq(errors)
+          expect(graphql_errors.flatten).to include(errors)
+        end
       end
+
+      context 'when query depth is within range' do
+        it 'has no error' do
+          allow(GitlabSchema).to receive(:max_query_depth).and_return 5
+
+          subject
+
+          expect(Array.wrap(graphql_errors).compact).to be_empty
+        end
+      end
+    end
+  end
+
+  context 'regular queries' do
+    subject do
+      query = graphql_query_for('project', { 'fullPath' => project.full_path }, %w(id name description))
+      post_graphql(query)
     end
 
-    context 'when query depth is within range' do
-      it 'has no error' do
-        allow(GitlabSchema).to receive(:max_query_depth).and_return 5
+    it_behaves_like 'imposing query limits'
+  end
+
+  context 'multiplexed queries' do
+    subject do
+      queries = [
+        { query: graphql_query_for('project', { 'fullPath' => project.full_path }, %w(id name description)) },
+        { query: graphql_query_for('echo', { 'text' => "$test" }, []), variables: { "test" => "Hello world" } }
+      ]
+
+      post_multiplex(queries)
+    end
+
+    it_behaves_like 'imposing query limits' do
+      it "fails all queries when only one of the queries is too complex" do
+        # The `project` query above has a complexity of 5
+        allow(GitlabSchema).to receive(:max_query_complexity).and_return 4
+
+        subject
 
-        post_graphql(query)
+        # Expect a response for each query, even though it will be empty
+        expect(json_response.size).to eq(2)
+        json_response.each do |single_query_response|
+          expect(single_query_response).not_to have_key('data')
+        end
 
-        expect(graphql_errors).to be_nil
+        # Expect errors for each query
+        expect(graphql_errors.size).to eq(2)
+        graphql_errors.each do |single_query_errors|
+          expect(single_query_errors.first['message']).to include('which exceeds max complexity of 4')
+        end
       end
     end
   end
diff --git a/spec/requests/api/graphql/multiplexed_queries_spec.rb b/spec/requests/api/graphql/multiplexed_queries_spec.rb
new file mode 100644
index 00000000000..844fd979285
--- /dev/null
+++ b/spec/requests/api/graphql/multiplexed_queries_spec.rb
@@ -0,0 +1,39 @@
+# frozen_string_literal: true
+require 'spec_helper'
+
+describe 'Multiplexed queries' do
+  include GraphqlHelpers
+
+  it 'returns responses for multiple queries' do
+    queries = [
+      { query: 'query($text: String) { echo(text: $text) }',
+        variables: { 'text' => 'Hello' } },
+      { query: 'query($text: String) { echo(text: $text) }',
+        variables: { 'text' => 'World' } }
+    ]
+
+    post_multiplex(queries)
+
+    first_response = json_response.first['data']['echo']
+    second_response = json_response.last['data']['echo']
+
+    expect(first_response).to eq('nil says: Hello')
+    expect(second_response).to eq('nil says: World')
+  end
+
+  it 'returns error and data combinations' do
+    queries = [
+      { query: 'query($text: String) { broken query }' },
+      { query: 'query working($text: String) { echo(text: $text) }',
+        variables: { 'text' => 'World' } }
+    ]
+
+    post_multiplex(queries)
+
+    first_response = json_response.first['errors']
+    second_response = json_response.last['data']['echo']
+
+    expect(first_response).not_to be_empty
+    expect(second_response).to eq('nil says: World')
+  end
+end
diff --git a/spec/support/helpers/graphql_helpers.rb b/spec/support/helpers/graphql_helpers.rb
index 44ed9da25fc..e95c7f2a6d6 100644
--- a/spec/support/helpers/graphql_helpers.rb
+++ b/spec/support/helpers/graphql_helpers.rb
@@ -134,6 +134,10 @@ module GraphqlHelpers
     end.join(", ")
   end
 
+  def post_multiplex(queries, current_user: nil, headers: {})
+    post api('/', current_user, version: 'graphql'), params: { _json: queries }, headers: headers
+  end
+
   def post_graphql(query, current_user: nil, variables: nil, headers: {})
     post api('/', current_user, version: 'graphql'), params: { query: query, variables: variables }, headers: headers
   end
@@ -147,7 +151,14 @@ module GraphqlHelpers
   end
 
   def graphql_errors
-    json_response['errors']
+    case json_response
+    when Hash # regular query
+      json_response['errors']
+    when Array # multiplexed queries
+      json_response.map { |response| response['errors'] }
+    else
+      raise "Unkown GraphQL response type #{json_response.class}"
+    end
   end
 
   def graphql_mutation_response(mutation_name)