GraphQL: Add MRSetSubscription mutation

Adds a mutation for the current user to subscribe/unsubscribe from a
Merge Request.

app/graphql/mutations/merge_requests/set_subscription.rb

+# frozen_string_literal: true
+
+module Mutations
+  module MergeRequests
+    class SetSubscription < Base
+      graphql_name 'MergeRequestSetSubscription'
+
+      argument :subscribed_state,
+               GraphQL::BOOLEAN_TYPE,
+               required: true,
+               description: 'The desired state of the subscription'
+
+      def resolve(project_path:, iid:, subscribed_state:)
+        merge_request = authorized_find!(project_path: project_path, iid: iid)
+        project = merge_request.project
+
+        merge_request.set_subscription(current_user, subscribed_state, project)
+
+        {
+          merge_request: merge_request,
+          errors: merge_request.errors.full_messages
+        }
+      end
+    end
+  end
+end

app/graphql/types/mutation_type.rb

@@ -12,6 +12,7 @@ module Types
     mount_mutation Mutations::MergeRequests::SetLabels
     mount_mutation Mutations::MergeRequests::SetLocked
     mount_mutation Mutations::MergeRequests::SetMilestone
+    mount_mutation Mutations::MergeRequests::SetSubscription
     mount_mutation Mutations::MergeRequests::SetWip, calls_gitaly: true
     mount_mutation Mutations::MergeRequests::SetAssignees
     mount_mutation Mutations::Notes::Create::Note, calls_gitaly: true

changelogs/unreleased/31919-graphql-MR-label-mutation.yml

+---
+title: 'GraphQL: Create MR mutations needed for the sidebar'
+merge_request: 19913
+author:
+type: added

spec/graphql/mutations/merge_requests/set_subscription_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Mutations::MergeRequests::SetSubscription do
+  let(:merge_request) { create(:merge_request) }
+  let(:project) { merge_request.project }
+  let(:user) { create(:user) }
+  subject(:mutation) { described_class.new(object: nil, context: { current_user: user }) }
+
+  describe '#resolve' do
+    let(:subscribe) { true }
+    let(:mutated_merge_request) { subject[:merge_request] }
+    subject { mutation.resolve(project_path: merge_request.project.full_path, iid: merge_request.iid, subscribed_state: subscribe) }
+
+    it 'raises an error if the resource is not accessible to the user' do
+      expect { subject }.to raise_error(Gitlab::Graphql::Errors::ResourceNotAvailable)
+    end
+
+    context 'when the user can update the merge request' do
+      before do
+        merge_request.project.add_developer(user)
+      end
+
+      it 'returns the merge request as discussion locked' do
+        expect(mutated_merge_request).to eq(merge_request)
+        expect(mutated_merge_request.subscribed?(user, project)).to eq(true)
+        expect(subject[:errors]).to be_empty
+      end
+
+      context 'when passing subscribe as false' do
+        let(:subscribe) { false }
+
+        it 'unsubscribes from the discussion' do
+          merge_request.subscribe(user, project)
+
+          expect(mutated_merge_request.subscribed?(user, project)).to eq(false)
+        end
+      end
+    end
+  end
+end

spec/requests/api/graphql/mutations/merge_requests/set_subscription_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe 'Setting subscribed status of a merge request' do
+  include GraphqlHelpers
+
+  let(:current_user) { create(:user) }
+  let(:merge_request) { create(:merge_request) }
+  let(:project) { merge_request.project }
+  let(:input) { { subscribed_state: true } }
+
+  let(:mutation) do
+    variables = {
+      project_path: project.full_path,
+      iid: merge_request.iid.to_s
+    }
+    graphql_mutation(:merge_request_set_subscription, variables.merge(input),
+                     <<-QL.strip_heredoc
+                       clientMutationId
+                       errors
+                       mergeRequest {
+                         id
+                         subscribed
+                       }
+    QL
+    )
+  end
+
+  def mutation_response
+    graphql_mutation_response(:merge_request_set_subscription)['mergeRequest']['subscribed']
+  end
+
+  before do
+    project.add_developer(current_user)
+  end
+
+  it 'returns an error if the user is not allowed to update the merge request' do
+    post_graphql_mutation(mutation, current_user: create(:user))
+
+    expect(graphql_errors).not_to be_empty
+  end
+
+  it 'marks the merge request as WIP' do
+    post_graphql_mutation(mutation, current_user: current_user)
+
+    expect(response).to have_gitlab_http_status(:success)
+    expect(mutation_response).to eq(true)
+  end
+
+  context 'when passing subscribe false as input' do
+    let(:input) { { subscribed_state: false } }
+
+    it 'unmarks the merge request as subscribed' do
+      merge_request.subscribe(current_user, project)
+
+      post_graphql_mutation(mutation, current_user: current_user)
+
+      expect(response).to have_gitlab_http_status(:success)
+      expect(mutation_response).to eq(false)
+    end
+  end
+end