New: Add Download Client validation for indexers

(cherry picked from commit e53b7f8c945e3597ca1719961e82540f1f01f0e9)

Closes #4246

src/Lidarr.Api.V1/Indexers/IndexerController.cs

@@ -1,5 +1,6 @@
 using Lidarr.Http;
 using NzbDrone.Core.Indexers;
+using NzbDrone.Core.Validation;
 
 namespace Lidarr.Api.V1.Indexers
 {
@@ -9,9 +10,10 @@ namespace Lidarr.Api.V1.Indexers
         public static readonly IndexerResourceMapper ResourceMapper = new ();
         public static readonly IndexerBulkResourceMapper BulkResourceMapper = new ();
 
-        public IndexerController(IndexerFactory indexerFactory)
+        public IndexerController(IndexerFactory indexerFactory, DownloadClientExistsValidator downloadClientExistsValidator)
             : base(indexerFactory, "indexer", ResourceMapper, BulkResourceMapper)
         {
+            SharedValidator.RuleFor(c => c.DownloadClientId).SetValidator(downloadClientExistsValidator);
         }
     }
 }

src/NzbDrone.Core/Validation/DownloadClientExistsValidator.cs

@@ -0,0 +1,27 @@
+using FluentValidation.Validators;
+using NzbDrone.Core.Download;
+
+namespace NzbDrone.Core.Validation
+{
+    public class DownloadClientExistsValidator : PropertyValidator
+    {
+        private readonly IDownloadClientFactory _downloadClientFactory;
+
+        public DownloadClientExistsValidator(IDownloadClientFactory downloadClientFactory)
+        {
+            _downloadClientFactory = downloadClientFactory;
+        }
+
+        protected override string GetDefaultMessageTemplate() => "Download Client does not exist";
+
+        protected override bool IsValid(PropertyValidatorContext context)
+        {
+            if (context?.PropertyValue == null || (int)context.PropertyValue == 0)
+            {
+                return true;
+            }
+
+            return _downloadClientFactory.Exists((int)context.PropertyValue);
+        }
+    }
+}