Add SmtpClient.SendMailAsync overloads with cancellation

src/libraries/System.Net.Mail/ref/System.Net.Mail.cs

@@ -186,6 +186,8 @@ public void SendAsync(string from, string recipients, string subject, string bod
  public void SendAsyncCancel() { }
  public System.Threading.Tasks.Task SendMailAsync(System.Net.Mail.MailMessage message) { throw null; }
  public System.Threading.Tasks.Task SendMailAsync(string from, string recipients, string subject, string body) { throw null; }
+ public System.Threading.Tasks.Task SendMailAsync(System.Net.Mail.MailMessage message, System.Threading.CancellationToken cancellationToken) { throw null; }
+ public System.Threading.Tasks.Task SendMailAsync(string from, string recipients, string subject, string body, System.Threading.CancellationToken cancellationToken) { throw null; }
  }
  public enum SmtpDeliveryFormat
  {

src/libraries/System.Net.Mail/src/System/Net/Mail/SmtpClient.cs

@@ -786,17 +786,35 @@ public void SendAsyncCancel()
  public Task SendMailAsync(string from, string recipients, string subject, string body)
  {
  var message = new MailMessage(from, recipients, subject, body);
- return SendMailAsync(message);
+ return SendMailAsync(message, cancellationToken: default);
  }
 
  public Task SendMailAsync(MailMessage message)
  {
+ return SendMailAsync(message, cancellationToken: default);
+ }
+
+ public Task SendMailAsync(string from, string recipients, string subject, string body, CancellationToken cancellationToken)
+ {
+ var message = new MailMessage(from, recipients, subject, body);
+ return SendMailAsync(message, cancellationToken);
+ }
+
+ public Task SendMailAsync(MailMessage message, CancellationToken cancellationToken)
+ {
+ if (cancellationToken.IsCancellationRequested)
+ {
+ return Task.FromCanceled(cancellationToken);
+ }
+
  // Create a TaskCompletionSource to represent the operation
- var tcs = new TaskCompletionSource<object>();
+ var tcs = new TaskCompletionSource<object>(TaskCreationOptions.RunContinuationsAsynchronously);
+
+ CancellationTokenRegistration ctr = default;
 
  // Register a handler that will transfer completion results to the TCS Task
  SendCompletedEventHandler handler = null;
- handler = (sender, e) => HandleCompletion(tcs, e, handler);
+ handler = (sender, e) => HandleCompletion(tcs, ctr, e, handler);
  SendCompleted += handler;
 
  // Start the async operation.
@@ -810,20 +828,45 @@ public Task SendMailAsync(MailMessage message)
  throw;
  }
 
+ // Only register on the CT if HandleCompletion hasn't started to ensure the CTR is disposed
+ bool lockTaken = false;
+ try
+ {
+ Monitor.TryEnter(tcs, ref lockTaken);
+ if (lockTaken && !tcs.Task.IsCompleted)
+ {
+ ctr = cancellationToken.Register(s =>
+ {
+ ((SmtpClient)s).SendAsyncCancel();
+ }, this);
+ }
+ }
+ finally
+ {
+ if (lockTaken) Monitor.Exit(tcs);
+ }
+
  // Return the task to represent the asynchronous operation
  return tcs.Task;
  }
 
- private void HandleCompletion(TaskCompletionSource<object> tcs, AsyncCompletedEventArgs e, SendCompletedEventHandler handler)
+ private void HandleCompletion(TaskCompletionSource<object> tcs, CancellationTokenRegistration ctr, AsyncCompletedEventArgs e, SendCompletedEventHandler handler)
  {
  if (e.UserState == tcs)
  {
- try { SendCompleted -= handler; }
- finally
+ lock (tcs)
  {
- if (e.Error != null) tcs.TrySetException(e.Error);
- else if (e.Cancelled) tcs.TrySetCanceled();
- else tcs.TrySetResult(null);
+ try
+ {
+ SendCompleted -= handler;
+ ctr.Dispose();
+ }
+ finally
+ {
+ if (e.Error != null) tcs.TrySetException(e.Error);
+ else if (e.Cancelled) tcs.TrySetCanceled();
+ else tcs.TrySetResult(null);
+ }
  }
  }
  }