Map cancelled request body uploads to OperationCanceledException in AndroidMessageHandler

src/Mono.Android/Xamarin.Android.Net/AndroidMessageHandler.cs

@@ -202,16 +202,16 @@ void DisposeStream ()
 					stream.Dispose ();
 			}
 
-			static bool ShouldMapToCancellation (Exception ex, CancellationToken cancellationToken)
-			{
-				return cancellationToken.IsCancellationRequested &&
-					ex is global::System.IO.IOException
-						or Java.IO.IOException
-						or InvalidDataException
-						or ObjectDisposedException
-						or WebException;
-			}
+		}
 
+		static bool ShouldMapToCancellation (Exception ex, CancellationToken cancellationToken)
+		{
+			return cancellationToken.IsCancellationRequested &&
+				ex is global::System.IO.IOException
+					or Java.IO.IOException
+					or InvalidDataException
+					or ObjectDisposedException
+					or WebException;
 		}
 
 		internal const string LOG_APP = "monodroid-net";
@@ -788,6 +788,11 @@ protected virtual async Task WriteRequestContentToOutput (HttpRequestMessage req
 			var stream = await request.Content.ReadAsStreamAsync ().ConfigureAwait (false);
 			try {
 				await stream.CopyToAsync(httpConnection.OutputStream!, 4096, cancellationToken).ConfigureAwait(false);
+			} catch (Exception ex) when (ShouldMapToCancellation (ex, cancellationToken)) {
+				// When the caller cancels the request while the body is being uploaded, the connection
+				// is disconnected which surfaces as a transport exception (e.g. "Socket closed"). Map it
+				// to an OperationCanceledException so callers observe cancellation instead of a WebException.
+				throw new System.OperationCanceledException ("Request body upload was canceled.", ex, cancellationToken);
 			} finally {
 				//
 				// Rewind the stream to beginning in case the HttpContent implementation

tests/Mono.Android-Tests/Mono.Android-Tests/Xamarin.Android.Net/AndroidMessageHandlerCancellationTests.cs

@@ -1,6 +1,7 @@
 #nullable enable
 
 using System;
+using System.IO;
 using System.Net;
 using System.Net.Http;
 using System.Net.Sockets;
@@ -19,6 +20,7 @@ namespace Xamarin.Android.NetTests
 	public class AndroidMessageHandlerCancellationTests
 	{
 		const int StalledResponseContentLength = 1024 * 1024;
+		const int UploadContentLength = 16 * 1024 * 1024;
 		const int BodyReadBlockDelayMilliseconds = 250;
 		const int PromptCancellationTimeoutMilliseconds = 3000;
 
@@ -58,6 +60,27 @@ public async Task ResponseContentReadBodyReadCancellationIsPrompt ()
 			await AssertCanceledPromptly (readTask, server.ReleaseResponseBody).ConfigureAwait (false);
 		}
 
+		[Test]
+		public async Task RequestBodyUploadCancellationIsPrompt ()
+		{
+			using var uploadServer = new StalledRequestServer ();
+			using var handler = new AndroidMessageHandler ();
+			using var client = new HttpClient (handler);
+			using var cts = new CancellationTokenSource ();
+			using var request = new HttpRequestMessage (HttpMethod.Put, $"http://localhost:{uploadServer.Port}/upload") {
+				// A large body ensures the socket send buffer fills while the server stalls reading it,
+				// so the upload is still in progress when the caller cancels. The content streams the
+				// bytes in small chunks instead of allocating the whole body up front.
+				Content = new StreamingContent (UploadContentLength),
+			};
+
+			Task sendTask = client.SendAsync (request, HttpCompletionOption.ResponseHeadersRead, cts.Token);
+
+			await WaitForBodyReadToBlock (uploadServer.BodyStartedTask).ConfigureAwait (false);
+			cts.Cancel ();
+			await AssertCanceledPromptly (sendTask, uploadServer.ReleaseRequestBody).ConfigureAwait (false);
+		}
+
 		[Test]
 		public async Task ResponseHeadersReadBodyReadCancellationIsPrompt ()
 		{
@@ -199,5 +222,106 @@ async Task ObserveServerTask ()
 				await serverTask.ConfigureAwait (false);
 			}
 		}
+
+		sealed class StreamingContent : HttpContent
+		{
+			readonly long length;
+
+			public StreamingContent (long length)
+			{
+				this.length = length;
+			}
+
+			protected override async Task SerializeToStreamAsync (Stream stream, System.Net.TransportContext? context)
+			{
+				var buffer = new byte [4096];
+				long remaining = length;
+				while (remaining > 0) {
+					int toWrite = (int) Math.Min (remaining, buffer.Length);
+					await stream.WriteAsync (buffer, 0, toWrite).ConfigureAwait (false);
+					remaining -= toWrite;
+				}
+			}
+
+			protected override bool TryComputeLength (out long computedLength)
+			{
+				computedLength = length;
+				return true;
+			}
+		}
+
+		sealed class StalledRequestServer : IDisposable
+		{
+			readonly TcpListener listener;
+			readonly TaskCompletionSource<bool> bodyStarted = new TaskCompletionSource<bool> (TaskCreationOptions.RunContinuationsAsynchronously);
+			readonly TaskCompletionSource<bool> releaseBody = new TaskCompletionSource<bool> (TaskCreationOptions.RunContinuationsAsynchronously);
+			readonly Task serverTask;
+
+			public StalledRequestServer ()
+			{
+				listener = new TcpListener (IPAddress.Loopback, 0);
+				listener.Start ();
+				Port = ((IPEndPoint) listener.LocalEndpoint).Port;
+
+				serverTask = StallRequestBody ();
+			}
+
+			public int Port { get; }
+
+			public Task BodyStartedTask => bodyStarted.Task;
+
+			public void ReleaseRequestBody ()
+			{
+				releaseBody.TrySetResult (true);
+			}
+
+			public void Dispose ()
+			{
+				ReleaseRequestBody ();
+				try {
+					listener.Stop ();
+				} catch (Exception ex) {
+					Console.WriteLine ($"Exception while stopping the stalled request server: {ex}");
+				}
+			}
+
+			async Task StallRequestBody ()
+			{
+				try {
+					using var client = await listener.AcceptTcpClientAsync ().ConfigureAwait (false);
+					using var stream = client.GetStream ();
+
+					// Read just the request headers so the upload phase begins, then stop reading the body
+					// to keep the socket send buffer full on the client side until released.
+					await ReadRequestHeaders (stream).ConfigureAwait (false);
+					bodyStarted.TrySetResult (true);
+
+					await releaseBody.Task.ConfigureAwait (false);
+				} catch (Exception ex) {
+					if (!BodyStartedTask.IsCompleted) {
+						bodyStarted.TrySetException (ex);
+						return;
+					}
+					Console.WriteLine ($"Exception while stalling the request body: {ex}");
+				}
+			}
+
+			static async Task ReadRequestHeaders (NetworkStream stream)
+			{
+				var buffer = new byte [1];
+				int consecutiveLineEndChars = 0;
+				while (consecutiveLineEndChars < 4) {
+					int read = await stream.ReadAsync (buffer, 0, 1).ConfigureAwait (false);
+					if (read == 0)
+						break;
+
+					byte b = buffer [0];
+					if (b == '\r' || b == '\n')
+						consecutiveLineEndChars++;
+					else
+						consecutiveLineEndChars = 0;
+				}
+			}
+		}
 	}
 }