Avoid blocking UI via WebClient.OpenReadTaskAsync

This method can block synchronously. This change moves calls to the
thread pool, and limits the number of concurrent downloads to prevent
thread pool exhaustion.

Fixes gitextensions#5859.

Follows and replaces gitextensions#5868.

GitUI/Avatars/AvatarDownloader.cs

@@ -6,13 +6,17 @@
 using System.Security.Cryptography;
 using System.Text;
 using System.Text.RegularExpressions;
+using System.Threading;
 using System.Threading.Tasks;
 using GitCommands;
+using Microsoft.VisualStudio.Threading;
 
 namespace GitUI.Avatars
 {
     public sealed class AvatarDownloader : IAvatarProvider
     {
+        private const int MaxConcurrentDownloads = 10;
+
         /* A brief skim through the GitExtensions repo history shows GitHub emails with the following user names:
          *
          * 25421792+mserfli
@@ -25,6 +29,8 @@ public sealed class AvatarDownloader : IAvatarProvider
          */
         private static readonly Regex _gitHubEmailRegex = new Regex(@"^(\d+\+)?(?<username>[^@]+)@users\.noreply\.github\.com$", RegexOptions.Compiled | RegexOptions.IgnoreCase);
 
+        private static readonly SemaphoreSlim _downloadSemaphore = new SemaphoreSlim(initialCount: MaxConcurrentDownloads);
+
         private static readonly ConcurrentDictionary<Uri, (DateTime, Task<Image>)> _downloads = new ConcurrentDictionary<Uri, (DateTime, Task<Image>)>();
 
         /// <inheritdoc />
@@ -146,6 +152,15 @@ async Task<Image> DownloadImageAsync(Uri imageUrl)
 
                 async Task<Image> Download()
                 {
+                    // WebClient.OpenReadTaskAsync can block before returning a task, so make sure we
+                    // make such calls on the thread pool, and limit the number of concurrent requests.
+
+                    // Get onto background thread
+                    await TaskScheduler.Default;
+
+                    // Limit the number of concurrent download requests
+                    await _downloadSemaphore.WaitAsync();
+
                     try
                     {
                         using (var webClient = new WebClient { Proxy = WebRequest.DefaultWebProxy })
@@ -163,6 +178,10 @@ async Task<Image> Download()
                         // catch IO errors
                         Trace.WriteLine(ex.Message);
                     }
+                    finally
+                    {
+                        _downloadSemaphore.Release();
+                    }
 
                     return null;
                 }

GitUI/UserControls/AvatarControl.cs

@@ -104,6 +104,8 @@ private async Task UpdateAvatarAsync()
 
             if (!token.IsCancellationRequested)
             {
+                await this.SwitchToMainThreadAsync();
+
                 RefreshImage(image);
             }
         }

Plugins/Gource/GourceStart.cs

@@ -73,6 +73,9 @@ private void Button1Click(object sender, EventArgs e)
                     GitWorkingDir = WorkingDir.Text;
 
                     RunRealCmdDetached(GourcePath.Text, arguments);
+
+                    await this.SwitchToMainThreadAsync();
+
                     Close();
                 });
         }