Geo: Implement GraphQL endpoint for MR diffs

Implement the GraphQL endpoint as documented at:
https://docs.gitlab.com/ee/development/geo/framework.html#graphql-api

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/215138

doc/api/graphql/reference/gitlab_schema.graphql

@@ -6407,6 +6407,37 @@ type GeoNode {
   """
   internalUrl: String
 
+  """
+  Find merge request diff registries on this Geo node. Available only when
+  feature flag `geo_merge_request_diff_replication` is enabled
+  """
+  mergeRequestDiffRegistries(
+    """
+    Returns the elements in the list that come after the specified cursor.
+    """
+    after: String
+
+    """
+    Returns the elements in the list that come before the specified cursor.
+    """
+    before: String
+
+    """
+    Returns the first _n_ elements from the list.
+    """
+    first: Int
+
+    """
+    Filters registries by their ID
+    """
+    ids: [ID!]
+
+    """
+    Returns the last _n_ elements from the list.
+    """
+    last: Int
+  ): MergeRequestDiffRegistryConnection
+
   """
   The interval (in days) in which the repository verification is valid. Once expired, it will be reverified
   """
@@ -10199,6 +10230,86 @@ type MergeRequestCreatePayload {
   mergeRequest: MergeRequest
 }
 
+"""
+Represents the sync and verification state of a Merge Request diff
+"""
+type MergeRequestDiffRegistry {
+  """
+  Timestamp when the MergeRequestDiffRegistry was created
+  """
+  createdAt: Time
+
+  """
+  ID of the MergeRequestDiffRegistry
+  """
+  id: ID!
+
+  """
+  Error message during sync of the MergeRequestDiffRegistry
+  """
+  lastSyncFailure: String
+
+  """
+  Timestamp of the most recent successful sync of the MergeRequestDiffRegistry
+  """
+  lastSyncedAt: Time
+
+  """
+  ID of the Merge Request diff
+  """
+  mergeRequestDiffId: ID!
+
+  """
+  Timestamp after which the MergeRequestDiffRegistry should be resynced
+  """
+  retryAt: Time
+
+  """
+  Number of consecutive failed sync attempts of the MergeRequestDiffRegistry
+  """
+  retryCount: Int
+
+  """
+  Sync state of the MergeRequestDiffRegistry
+  """
+  state: RegistryState
+}
+
+"""
+The connection type for MergeRequestDiffRegistry.
+"""
+type MergeRequestDiffRegistryConnection {
+  """
+  A list of edges.
+  """
+  edges: [MergeRequestDiffRegistryEdge]
+
+  """
+  A list of nodes.
+  """
+  nodes: [MergeRequestDiffRegistry]
+
+  """
+  Information to aid in pagination.
+  """
+  pageInfo: PageInfo!
+}
+
+"""
+An edge in a connection.
+"""
+type MergeRequestDiffRegistryEdge {
+  """
+  A cursor for use in pagination.
+  """
+  cursor: String!
+
+  """
+  The item at the end of the edge.
+  """
+  node: MergeRequestDiffRegistry
+}
+
 """
 An edge in a connection.
 """

doc/api/graphql/reference/index.md

@@ -1497,6 +1497,21 @@ Autogenerated return type of MergeRequestCreate.
 | `errors` | String! => Array | Errors encountered during execution of the mutation. |
 | `mergeRequest` | MergeRequest | The merge request after mutation |
 
+### MergeRequestDiffRegistry
+
+Represents the sync and verification state of a Merge Request diff.
+
+| Field | Type | Description |
+| ----- | ---- | ----------- |
+| `createdAt` | Time | Timestamp when the MergeRequestDiffRegistry was created |
+| `id` | ID! | ID of the MergeRequestDiffRegistry |
+| `lastSyncFailure` | String | Error message during sync of the MergeRequestDiffRegistry |
+| `lastSyncedAt` | Time | Timestamp of the most recent successful sync of the MergeRequestDiffRegistry |
+| `mergeRequestDiffId` | ID! | ID of the Merge Request diff |
+| `retryAt` | Time | Timestamp after which the MergeRequestDiffRegistry should be resynced |
+| `retryCount` | Int | Number of consecutive failed sync attempts of the MergeRequestDiffRegistry |
+| `state` | RegistryState | Sync state of the MergeRequestDiffRegistry |
+
 ### MergeRequestPermissions
 
 Check permissions for the current user on a merge request.

ee/app/finders/geo/merge_request_diff_registry_finder.rb

+# frozen_string_literal: true
+
+module Geo
+  class MergeRequestDiffRegistryFinder
+    include FrameworkRegistryFinder
+  end
+end

ee/app/graphql/resolvers/geo/merge_request_diff_registries_resolver.rb

+# frozen_string_literal: true
+
+module Resolvers
+  module Geo
+    class MergeRequestDiffRegistriesResolver < BaseResolver
+      include RegistriesResolver
+    end
+  end
+end

ee/app/graphql/types/geo/geo_node_type.rb

@@ -22,6 +22,11 @@ module Types
       field :selective_sync_shards, type: [GraphQL::STRING_TYPE], null: true, description: 'The repository storages whose projects should be synced, if `selective_sync_type` == `shards`'
       field :selective_sync_namespaces, ::Types::NamespaceType.connection_type, null: true, method: :namespaces, description: 'The namespaces that should be synced, if `selective_sync_type` == `namespaces`'
       field :minimum_reverification_interval, GraphQL::INT_TYPE, null: true, description: 'The interval (in days) in which the repository verification is valid. Once expired, it will be reverified'
+      field :merge_request_diff_registries, ::Types::Geo::MergeRequestDiffRegistryType.connection_type,
+            null: true,
+            resolver: ::Resolvers::Geo::MergeRequestDiffRegistriesResolver,
+            description: 'Find merge request diff registries on this Geo node',
+            feature_flag: :geo_merge_request_diff_replication
       field :package_file_registries, ::Types::Geo::PackageFileRegistryType.connection_type,
             null: true,
             resolver: ::Resolvers::Geo::PackageFileRegistriesResolver,

ee/app/graphql/types/geo/merge_request_diff_registry_type.rb

+# frozen_string_literal: true
+
+module Types
+  module Geo
+    # rubocop:disable Graphql/AuthorizeTypes because it is included
+    class MergeRequestDiffRegistryType < BaseObject
+      include ::Types::Geo::RegistryType
+
+      graphql_name 'MergeRequestDiffRegistry'
+      description 'Represents the sync and verification state of a Merge Request diff'
+
+      field :merge_request_diff_id, GraphQL::ID_TYPE, null: false, description: 'ID of the Merge Request diff'
+    end
+  end
+end

ee/spec/finders/geo/merge_request_diff_registry_finder_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Geo::MergeRequestDiffRegistryFinder do
+  it_behaves_like 'a framework registry finder', :geo_merge_request_diff_registry
+end

ee/spec/graphql/resolvers/geo/merge_request_diff_registries_resolver_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Resolvers::Geo::MergeRequestDiffRegistriesResolver do
+  it_behaves_like 'a Geo registries resolver', :geo_merge_request_diff_registry
+end

ee/spec/graphql/types/geo/geo_node_type_spec.rb

@@ -11,7 +11,8 @@ RSpec.describe GitlabSchema.types['GeoNode'] do
       repos_max_capacity verification_max_capacity
       container_repositories_max_capacity sync_object_storage
       selective_sync_type selective_sync_shards selective_sync_namespaces
-      minimum_reverification_interval package_file_registries
+      minimum_reverification_interval merge_request_diff_registries
+      package_file_registries
       terraform_state_registries terraform_state_version_registries
     ]
 

ee/spec/graphql/types/geo/merge_request_diff_registry_type_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe GitlabSchema.types['MergeRequestDiffRegistry'] do
+  it_behaves_like 'a Geo registry type'
+
+  it 'has the expected fields (other than those included in RegistryType)' do
+    expected_fields = %i[merge_request_diff_id]
+
+    expect(described_class).to have_graphql_fields(*expected_fields).at_least
+  end
+end

ee/spec/requests/api/graphql/geo/registries_spec.rb

@@ -3,6 +3,13 @@
 require 'spec_helper'
 
 RSpec.describe 'Gets registries' do
+  it_behaves_like 'gets registries for', {
+    field_name: 'mergeRequestDiffRegistries',
+    registry_class_name: 'MergeRequestDiffRegistry',
+    registry_factory: :geo_merge_request_diff_registry,
+    registry_foreign_key_field_name: 'mergeRequestDiffId'
+  }
+
   it_behaves_like 'gets registries for', {
     field_name: 'packageFileRegistries',
     registry_class_name: 'PackageFileRegistry',