Improve proxy cache behavior and diagnostics

tests/Jellyfin.Plugin.Multilang.Tests/ItemsProxyRequestBuilderTests.cs

@@ -68,6 +68,30 @@ public sealed class ItemsProxyRequestBuilderTests
         Assert.Contains("|SortName|Descending|L|20|10|fi-FI|", result.CacheKey);
     }
 
+    [Fact]
+    public void BuildUsesOneUpstreamKeyForDifferentLocalPagingControls()
+    {
+        var request = HttpRequest();
+
+        var first = ItemsProxyRequestBuilder.Build(
+            request,
+            "/Items?SortBy=SortName&StartIndex=0&Limit=100",
+            "token",
+            UserId,
+            multilangEnabled: true);
+        var second = ItemsProxyRequestBuilder.Build(
+            request,
+            "/Items?SortBy=SortName&StartIndex=100&Limit=100",
+            "token",
+            UserId,
+            multilangEnabled: true);
+
+        Assert.NotNull(first);
+        Assert.NotNull(second);
+        Assert.Equal(first.NormalizedUrlForCache, second.NormalizedUrlForCache);
+        Assert.NotEqual(first.CacheKey, second.CacheKey);
+    }
+
     [Fact]
     public void BuildLeavesSortAndPagingUpstreamWhenMultilangDisabled()
     {

tests/Jellyfin.Plugin.Multilang.Tests/ItemsProxyRequestCoalescerTests.cs

@@ -0,0 +1,52 @@
+using Jellyfin.Plugin.Multilang.Services;
+
+namespace Jellyfin.Plugin.Multilang.Tests;
+
+public sealed class ItemsProxyRequestCoalescerTests
+{
+    [Fact]
+    public async Task MatchingRequestsShareOneInFlightFetch()
+    {
+        var coalescer = new ItemsProxyRequestCoalescer();
+        var response = new TaskCompletionSource<ItemsProxyUpstreamResponse>(TaskCreationOptions.RunContinuationsAsynchronously);
+        var fetches = 0;
+
+        Task<ItemsProxyUpstreamResponse> Fetch(CancellationToken _) 
+        {
+            fetches++;
+            return response.Task;
+        }
+
+        var first = coalescer.GetOrFetchAsync("user|/Items", Fetch, CancellationToken.None);
+        var second = coalescer.GetOrFetchAsync("user|/Items", Fetch, CancellationToken.None);
+
+        Assert.Equal(1, fetches);
+        response.SetResult(new ItemsProxyUpstreamResponse(200, "{}", "application/json"));
+
+        var firstResult = await first;
+        var secondResult = await second;
+        Assert.False(firstResult.JoinedExistingRequest);
+        Assert.True(secondResult.JoinedExistingRequest);
+        Assert.Equal("{}", firstResult.Response.Body);
+        Assert.Equal(firstResult.Response, secondResult.Response);
+    }
+
+    [Fact]
+    public async Task CancellingOneWaiterDoesNotCancelTheSharedFetch()
+    {
+        var coalescer = new ItemsProxyRequestCoalescer();
+        var response = new TaskCompletionSource<ItemsProxyUpstreamResponse>(TaskCreationOptions.RunContinuationsAsynchronously);
+        using var cancellation = new CancellationTokenSource();
+
+        var cancelledWaiter = coalescer.GetOrFetchAsync("user|/Items", _ => response.Task, cancellation.Token);
+        var activeWaiter = coalescer.GetOrFetchAsync("user|/Items", _ => response.Task, CancellationToken.None);
+        cancellation.Cancel();
+
+        await Assert.ThrowsAnyAsync<OperationCanceledException>(() => cancelledWaiter);
+        response.SetResult(new ItemsProxyUpstreamResponse(200, "{}", "application/json"));
+
+        var result = await activeWaiter;
+        Assert.True(result.JoinedExistingRequest);
+        Assert.Equal(200, result.Response.StatusCode);
+    }
+}