Let a second concurrent first save replace the first one's state file [patch]

Navigation.Test/JsonFilePersistenceProviderTests.cs

@@ -56,6 +56,42 @@
 		Assert.AreEqual("c", string.Join(",", restored.GetForwardStack().Select(item => item.Id)));
 	}
 
+	[TestMethod]
+	public async Task ConcurrentFirstSaves_ToNewFile_BothSucceed()
+	{
+		// Two saves that both find no state file used to race on File.Move, and the loser threw
+		// "already exists". Loop, because the window is narrow
+		string directory = Path.Combine(Path.GetTempPath(), $"navigation-{Guid.NewGuid():N}");
+		try
+		{
+			for (int i = 0; i < 200; i++)
+			{
+				string filePath = Path.Combine(directory, $"state-{i}.json");
+				JsonFilePersistenceProvider<NavigationItem> provider = new(filePath);
+				Navigation<NavigationItem> first = new(null, provider);
+				first.NavigateTo(new NavigationItem("a", "A"));
+				Navigation<NavigationItem> second = new(null, provider);
+				second.NavigateTo(new NavigationItem("b", "B"));
+
+				await Task.WhenAll(
+					Task.Run(() => first.SaveStateAsync()),
+					Task.Run(() => second.SaveStateAsync())).ConfigureAwait(false);
+
+				Navigation<NavigationItem> restored = new(null, provider);
+				Assert.IsTrue(await restored.LoadStateAsync().ConfigureAwait(false), $"iteration {i}");
+				Assert.IsTrue(restored.Current?.Id is "a" or "b", $"iteration {i}");
+				Assert.IsEmpty(Directory.GetFiles(directory, $"state-{i}.json.*.tmp"), $"iteration {i}");
+			}
+		}
+		finally
+		{
+			if (Directory.Exists(directory))
+			{
+				Directory.Delete(directory, recursive: true);
+			}
+		}
+	}
+
 	private string[] GetTemporaryFiles() =>
 		Directory.GetFiles(Path.GetDirectoryName(_filePath!)!, $"{Path.GetFileName(_filePath!)}.*.tmp");
 

Navigation/Services/JsonFilePersistenceProvider.cs

@@ -74,14 +74,7 @@ public async Task SaveStateAsync(INavigationState<T> state, CancellationToken ca
 
 			cancellationToken.ThrowIfCancellationRequested();
 
-			if (File.Exists(_filePath))
-			{
-				File.Replace(tempPath, _filePath, destinationBackupFileName: null);
-			}
-			else
-			{
-				File.Move(tempPath, _filePath);
-			}
+			SwapIntoPlace(tempPath, _filePath);
 		}
 		catch
 		{
@@ -92,6 +85,34 @@ public async Task SaveStateAsync(INavigationState<T> state, CancellationToken ca
 
 	private static readonly UTF8Encoding Utf8NoBom = new(encoderShouldEmitUTF8Identifier: false);
 
+	/// <summary>
+	/// Moves a complete temporary file over the state file, whether or not the state file exists yet.
+	/// Two saves can both find no state file. Whichever moves second must replace the first one's file
+	/// rather than fail with "already exists", so the last writer wins as it does once the file exists.
+	/// </summary>
+	private static void SwapIntoPlace(string tempPath, string filePath)
+	{
+		if (File.Exists(filePath))
+		{
+			File.Replace(tempPath, filePath, destinationBackupFileName: null);
+			return;
+		}
+
+#if NETCOREAPP3_0_OR_GREATER
+		File.Move(tempPath, filePath, overwrite: true);
+#else
+		try
+		{
+			File.Move(tempPath, filePath);
+		}
+		catch (IOException) when (File.Exists(filePath))
+		{
+			// Another save created the file after the check above
+			File.Replace(tempPath, filePath, destinationBackupFileName: null);
+		}
+#endif
+	}
+
 	private static void TryDelete(string path)
 	{
 		try