[browser] Add test case for maxParallelDownloads (#104476)

src/mono/wasm/Wasm.Build.Tests/TestAppScenarios/MaxParallelDownloadsTests.cs

@@ -0,0 +1,48 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading.Tasks;
+using Xunit.Abstractions;
+using System.Text.RegularExpressions;
+using Xunit;
+
+#nullable enable
+
+namespace Wasm.Build.Tests.TestAppScenarios;
+
+public class MaxParallelDownloadsTests : AppTestBase
+{
+    public MaxParallelDownloadsTests(ITestOutputHelper output, SharedBuildPerTestClassFixture buildContext)
+        : base(output, buildContext)
+    {
+    }
+
+    [Theory]
+    [InlineData("Release", "1")]
+    [InlineData("Release", "4")]
+    public async Task NeverFetchMoreThanMaxAllowed(string config, string maxParallelDownloads)
+    {
+        CopyTestAsset("WasmBasicTestApp", "MaxParallelDownloadsTests", "App");
+        BuildProject(config);
+
+        var result = await RunSdkStyleAppForBuild(new(
+            Configuration: config,
+            TestScenario: "MaxParallelDownloads",
+            BrowserQueryString:  new Dictionary<string, string> { ["maxParallelDownloads"] = maxParallelDownloads }
+        ));
+        var resultTestOutput = result.TestOutput.ToList();
+        var regex = new Regex(@"Active downloads: (\d+)");
+        foreach (var line in resultTestOutput)
+        {
+            var match = regex.Match(line);
+            if (match.Success)
+            {
+                int activeDownloads = int.Parse(match.Groups[1].Value);
+                Assert.True(activeDownloads <= int.Parse(maxParallelDownloads), $"Active downloads exceeded the limit: {activeDownloads} > {maxParallelDownloads}");
+            }
+        }
+    }
+}

src/mono/wasm/testassets/WasmBasicTestApp/App/wwwroot/main.js

@@ -93,6 +93,26 @@ switch (testCase) {
         await dotnet.download();
         testOutput("download finished");
         break;
+    case "MaxParallelDownloads":
+        const maxParallelDownloads = params.get("maxParallelDownloads");
+        let activeFetchCount = 0;
+        const originalFetch2 = globalThis.fetch;
+        globalThis.fetch = async (...args) => {
+            activeFetchCount++;
+            testOutput(`Fetch started. Active downloads: ${activeFetchCount}`);
+            try {
+                const response = await originalFetch2(...args);
+                activeFetchCount--;
+                testOutput(`Fetch completed. Active downloads: ${activeFetchCount}`);
+                return response;
+            } catch (error) {
+                activeFetchCount--;
+                testOutput(`Fetch failed. Active downloads: ${activeFetchCount}`);
+                throw error;
+            }
+        };
+        dotnet.withConfig({ maxParallelDownloads: maxParallelDownloads });
+        break;
 }
 
 const { setModuleImports, getAssemblyExports, getConfig, INTERNAL } = await dotnet.create();
@@ -145,6 +165,7 @@ try {
             exit(0);
             break;
         case "DownloadThenInit":
+        case "MaxParallelDownloads":
             exit(0);
             break;
         default: