[fix] Fix HTML logger parallel file collision

src/Microsoft.TestPlatform.Extensions.HtmlLogger/HtmlLogger.cs

@@ -310,7 +310,7 @@ private void PopulateHtmlFile()
     {
         try
         {
-            var fileName = string.Format(CultureInfo.CurrentCulture, "{0}_{1}_{2}",
+            var fileName = string.Format(CultureInfo.InvariantCulture, "{0}_{1}_{2}",
                 Environment.GetEnvironmentVariable("UserName"), Environment.MachineName,
                 FormatDateTimeForRunName(DateTime.Now));
 
@@ -350,17 +350,19 @@ private void PopulateHtmlFile()
 
     private string GenerateUniqueFilePath(string fileName, string fileExtension)
     {
-        string fullFilePath;
         for (short i = 0; i < short.MaxValue; i++)
         {
             var fileNameWithIter = i == 0 ? fileName : Path.GetFileNameWithoutExtension(fileName) + $"[{i}]";
-            fullFilePath = Path.Combine(TestResultsDirPath!, $"TestResult_{fileNameWithIter}.{fileExtension}");
+            var fullFilePath = Path.Combine(TestResultsDirPath!, $"TestResult_{fileNameWithIter}.{fileExtension}");
+
             try
             {
-                using var _ = new FileStream(fullFilePath, FileMode.CreateNew, FileAccess.Write, FileShare.None);
+                // Use FileMode.CreateNew for atomic "create if not exists" to avoid
+                // cross-process race conditions when multiple vstest processes run in parallel.
+                using var _ = _fileHelper.GetStream(fullFilePath, FileMode.CreateNew, FileAccess.Write, FileShare.None);
                 return fullFilePath;
             }
-            catch (IOException) when (File.Exists(fullFilePath))
+            catch (IOException) when (_fileHelper.Exists(fullFilePath))
             {
                 // File already exists (another process created it), try next iteration.
             }

test/Microsoft.TestPlatform.Extensions.HtmlLogger.UnitTests/HtmlLoggerTests.cs

@@ -604,6 +604,35 @@ public void XmlFilePathShouldContainFractionalSecondsForCrossProcessUniqueness()
             $"Filename should end with yyyyMMdd_HHmmss.fffffff pattern: {fileName}");
     }
 
+    [TestMethod]
+    public void TestCompleteHandlerShouldRetryUniqueFileNameWhenCreateNewCollides()
+    {
+        var collisionInjected = false;
+
+        _mockFileHelper.Setup(x => x.Exists(It.IsAny<string>())).Returns(true);
+
+        _mockFileHelper.Setup(x => x.GetStream(It.IsAny<string>(), FileMode.CreateNew, FileAccess.Write, FileShare.None))
+            .Returns<string, FileMode, FileAccess, FileShare>((path, _, _, _) =>
+            {
+                if (!collisionInjected)
+                {
+                    collisionInjected = true;
+                    throw new IOException("collision");
+                }
+
+                return new Mock<Stream>().Object;
+            });
+
+        _mockFileHelper.Setup(x => x.GetStream(It.IsAny<string>(), FileMode.OpenOrCreate, FileAccess.ReadWrite))
+            .Returns(new Mock<Stream>().Object);
+
+        _htmlLogger.TestRunCompleteHandler(new object(), new TestRunCompleteEventArgs(null, false, true, null, null, null, TimeSpan.Zero));
+
+        Assert.IsNotNull(_htmlLogger.XmlFilePath);
+        Assert.Contains("[1].xml", _htmlLogger.XmlFilePath);
+        _mockFileHelper.Verify(x => x.GetStream(It.IsAny<string>(), FileMode.CreateNew, FileAccess.Write, FileShare.None), Times.AtLeast(2));
+    }
+
     private static TestCase CreateTestCase(string testCaseName)
     {
         return new TestCase(testCaseName, new Uri("some://uri"), "DummySourceFileName");