fix: fix Downloader to dispose tempFile and use synchronous IO (#81)

Fixes coder/internal#598

There is a possible race where if the cancellation token is expired, `Download()` never gets called and the tempFile is never disposed of (at least until GC). We also switch to synchronous IO so that a pending overlapped write won't block the deletion.

These issues can cause races in our tests when we try to clean up the directory.

Author: spikecurtis

Tests.Vpn.Service/DownloaderTest.cs

@@ -442,23 +442,28 @@ public async Task CancelledWaitingForOther(CancellationToken ct)
     [CancelAfter(30_000)]
     public async Task CancelledInner(CancellationToken ct)
     {
+        var httpCts = CancellationTokenSource.CreateLinkedTokenSource(ct);
+        var taskCts = CancellationTokenSource.CreateLinkedTokenSource(ct);
         using var httpServer = new TestHttpServer(async ctx =>
         {
             ctx.Response.StatusCode = 200;
-            await ctx.Response.OutputStream.WriteAsync("test"u8.ToArray(), ct);
-            await ctx.Response.OutputStream.FlushAsync(ct);
-            await Task.Delay(TimeSpan.FromSeconds(5), ct);
+            await ctx.Response.OutputStream.WriteAsync("test"u8.ToArray(), httpCts.Token);
+            await ctx.Response.OutputStream.FlushAsync(httpCts.Token);
+            // wait up to 5 seconds.
+            await Task.Delay(TimeSpan.FromSeconds(5), httpCts.Token);
         });
         var url = new Uri(httpServer.BaseUrl + "/test");
         var destPath = Path.Combine(_tempDir, "test");
 
         var manager = new Downloader(NullLogger<Downloader>.Instance);
         // The "inner" Task should fail.
-        var smallerCt = new CancellationTokenSource(TimeSpan.FromSeconds(1)).Token;
+        var taskCt = taskCts.Token;
         var dlTask = await manager.StartDownloadAsync(new HttpRequestMessage(HttpMethod.Get, url), destPath,
-            NullDownloadValidator.Instance, smallerCt);
+            NullDownloadValidator.Instance, taskCt);
+        await taskCts.CancelAsync();
         var ex = Assert.ThrowsAsync<TaskCanceledException>(async () => await dlTask.Task);
-        Assert.That(ex.CancellationToken, Is.EqualTo(smallerCt));
+        Assert.That(ex.CancellationToken, Is.EqualTo(taskCt));
+        await httpCts.CancelAsync();
     }
 
     [Test(Description = "Validation failure")]

Vpn.Service/Downloader.cs

@@ -453,27 +453,25 @@ private async Task Start(CancellationToken ct = default)
         if (res.Content.Headers.ContentLength >= 0)
             TotalBytes = (ulong)res.Content.Headers.ContentLength;
 
-        FileStream tempFile;
-        try
-        {
-            tempFile = File.Create(TempDestinationPath, BufferSize,
-                FileOptions.Asynchronous | FileOptions.SequentialScan);
-        }
-        catch (Exception e)
-        {
-            _logger.LogError(e, "Failed to create temporary file '{TempDestinationPath}'", TempDestinationPath);
-            throw;
-        }
-
-        await Download(res, tempFile, ct);
+        await Download(res, ct);
         return;
     }
 
-    private async Task Download(HttpResponseMessage res, FileStream tempFile, CancellationToken ct)
+    private async Task Download(HttpResponseMessage res, CancellationToken ct)
     {
         try
         {
             var sha1 = res.Headers.Contains("ETag") ? SHA1.Create() : null;
+            FileStream tempFile;
+            try
+            {
+                tempFile = File.Create(TempDestinationPath, BufferSize, FileOptions.SequentialScan);
+            }
+            catch (Exception e)
+            {
+                _logger.LogError(e, "Failed to create temporary file '{TempDestinationPath}'", TempDestinationPath);
+                throw;
+            }
             await using (tempFile)
             {
                 var stream = await res.Content.ReadAsStreamAsync(ct);