Fix arr commands failing when the response body is empty (#709)

code/backend/Cleanuparr.Infrastructure.Tests/Features/Arr/RadarrClientTests.cs

@@ -143,6 +143,28 @@ public class RadarrClientTests
         _httpMessageHandler.CapturedRequests.ShouldBeEmpty();
     }
 
+    [Fact]
+    public async Task SearchItemsAsync_WithAnEmptyCommandBody_ReturnsEmpty()
+    {
+        // An arr answers some commands with an empty body.
+        _httpMessageHandler.SetupResponse((req, _) =>
+        {
+            if (req.Method == HttpMethod.Post && req.RequestUri!.AbsolutePath.EndsWith("/command"))
+            {
+                return Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK)
+                {
+                    Content = new StringContent(string.Empty, Encoding.UTF8, "application/json"),
+                });
+            }
+
+            return Task.FromResult(JsonNullResponse());
+        });
+
+        var ids = await _client.SearchItemsAsync(_arrInstance, new HashSet<SearchItem> { new() { Id = 10 } });
+
+        ids.ShouldBeEmpty();
+    }
+
     [Fact]
     public async Task SearchItemsAsync_PostsMoviesSearchCommandWithAllIds()
     {

code/backend/Cleanuparr.Infrastructure/Features/Arr/ArrClient.cs

@@ -1,3 +1,4 @@
+using System.Net;
 using Cleanuparr.Domain.Entities.Arr;
 using Cleanuparr.Domain.Entities.Arr.Queue;
 using Cleanuparr.Domain.Enums;
@@ -307,8 +308,20 @@ public abstract class ArrClient : IArrClient
         return response;
     }
     
+    /// <summary>
+    /// Reads the body of a response from an arr.
+    /// </summary>
+    /// <remarks>
+    /// An arr answers some requests with an empty body. An empty body gives null,
+    /// because each caller has a value for a null result.
+    /// </remarks>
     protected static async Task<T?> DeserializeStreamAsync<T>(HttpResponseMessage response, CancellationToken cancellationToken = default)
     {
+        if (response.StatusCode is HttpStatusCode.NoContent || response.Content.Headers.ContentLength is 0)
+        {
+            return default;
+        }
+
         await using Stream stream = await response.Content.ReadAsStreamAsync(cancellationToken);
         return await JsonSerializer.DeserializeAsync<T>(stream, CleanuparrJsonOptions.ExternalApiRead, cancellationToken);
     }