Add link_type filter argument to issue_links resolver

This will allow client applications to filter the issue_links of
vulnerabilities by link_type.

doc/api/graphql/reference/gitlab_schema.graphql

@@ -12233,6 +12233,11 @@ type Vulnerability {
     Returns the last _n_ elements from the list.
     """
     last: Int
+
+    """
+    Filter issue links by link type
+    """
+    linkType: [VulnerabilityIssueLinkType!]
   ): VulnerabilityIssueLinkConnection!
 
   """

doc/api/graphql/reference/gitlab_schema.json

@@ -36076,6 +36076,24 @@
               "name": "issueLinks",
               "description": "List of issue links related to the vulnerability",
               "args": [
+                {
+                  "name": "linkType",
+                  "description": "Filter issue links by link type",
+                  "type": {
+                    "kind": "LIST",
+                    "name": null,
+                    "ofType": {
+                      "kind": "NON_NULL",
+                      "name": null,
+                      "ofType": {
+                        "kind": "ENUM",
+                        "name": "VulnerabilityIssueLinkType",
+                        "ofType": null
+                      }
+                    }
+                  },
+                  "defaultValue": null
+                },
                 {
                   "name": "after",
                   "description": "Returns the elements in the list that come after the specified cursor.",

ee/app/graphql/resolvers/vulnerabilities/issue_links_resolver.rb

+# frozen_string_literal: true
+
+module Resolvers
+  module Vulnerabilities
+    class IssueLinksResolver < BaseResolver
+      type Types::Vulnerability::IssueLinkType, null: true
+
+      argument :link_type, [Types::Vulnerability::IssueLinkTypeEnum],
+               required: false,
+               description: 'Filter issue links by link type'
+
+      delegate :issue_links, to: :object, private: true
+
+      def resolve(link_type: nil, **)
+        issue_links.by_link_type(link_type)
+      end
+    end
+  end
+end

ee/app/graphql/types/vulnerability_type.rb

@@ -35,7 +35,8 @@ module Types
           resolve: -> (obj, _args, _ctx) { ::Gitlab::Routing.url_helpers.project_security_vulnerability_path(obj.project, obj) }
 
     field :issue_links, ::Types::Vulnerability::IssueLinkType.connection_type, null: false,
-          description: "List of issue links related to the vulnerability"
+          description: "List of issue links related to the vulnerability",
+          resolver: Resolvers::Vulnerabilities::IssueLinksResolver
 
     field :location, VulnerabilityLocationType, null: true,
           description: 'Location metadata for the vulnerability. Its fields depend on the type of security scan that found the vulnerability',

ee/app/models/vulnerabilities/issue_link.rb

@@ -17,5 +17,7 @@ module Vulnerabilities
                 message: N_('already has a "created" issue link')
               },
               if: :created?
+
+    scope :by_link_type, -> (link_type) { link_type ? where(link_type: link_type.downcase) : all }
   end
 end

ee/spec/graphql/resolvers/vulnerabilities/issue_links_resolver_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Resolvers::Vulnerabilities::IssueLinksResolver do
+  include GraphqlHelpers
+
+  describe '#resolve' do
+    let_it_be(:user) { create(:user) }
+    let_it_be(:vulnerability) { create(:vulnerability) }
+    let_it_be(:related_issue) { create(:vulnerabilities_issue_link, :related, vulnerability: vulnerability) }
+    let_it_be(:created_issue) { create(:vulnerabilities_issue_link, :created, vulnerability: vulnerability) }
+
+    subject { resolve(described_class, obj: vulnerability, args: filters, ctx: { current_user: user }) }
+
+    context 'when there is no filter given' do
+      let(:filters) { {} }
+
+      it { is_expected.to match_array([related_issue, created_issue]) }
+    end
+
+    context 'when the link_type filter is given' do
+      context 'when the filter is `CREATED`' do
+        let(:filters) { { link_type: 'CREATED' } }
+
+        it { is_expected.to match_array([created_issue]) }
+      end
+
+      context 'when the filter is `RELATED`' do
+        let(:filters) { { link_type: 'RELATED' } }
+
+        it { is_expected.to match_array([related_issue]) }
+      end
+    end
+  end
+end

ee/spec/models/vulnerabilities/issue_link_spec.rb

@@ -78,4 +78,29 @@ describe Vulnerabilities::IssueLink do
       end
     end
   end
+
+  describe '.by_link_type' do
+    let_it_be(:created_issue_link) { create(:vulnerabilities_issue_link, :created) }
+    let_it_be(:related_issue_link) { create(:vulnerabilities_issue_link, :related) }
+
+    subject { described_class.by_link_type(link_type).to_a }
+
+    context 'when the given argument is `nil`' do
+      let(:link_type) { nil }
+
+      it { is_expected.to match_array([created_issue_link, related_issue_link]) }
+    end
+
+    context 'when the given argument is an uppercase string enum value' do
+      let(:link_type) { 'CREATED' }
+
+      it { is_expected.to match_array([created_issue_link]) }
+    end
+
+    context 'when the given argument is an uppercase symbol enum value' do
+      let(:link_type) { :RELATED }
+
+      it { is_expected.to match_array([related_issue_link]) }
+    end
+  end
 end