Cancel ffmpeg (#11)

* Bug fix for issue #8: Cancel does not cancel ffmpeg.exe.
See #8.
When the calling program cancels the task by invoking Cancel() on the CancellationToken, send "q" to standard input of the ffmpeg.exe process.
This causes ffmpeg.exe to stop processing and shut down in an orderly way.

src/FFmpeg.NET/Extensions/ProcessExtensions.cs

@@ -1,33 +1,58 @@
-using System;
-using System.Diagnostics;
-using System.Threading;
-using System.Threading.Tasks;
-
-namespace FFmpeg.NET.Extensions
-{
-    public static class ProcessExtensions
-    {
-        public static Task<int> WaitForExitAsync(this Process process, Action<int> onException, CancellationToken cancellationToken = default)
-        {
-            var tcs = new TaskCompletionSource<int>();
-            if (cancellationToken != default)
-                cancellationToken.Register(tcs.SetCanceled);
-
-            process.EnableRaisingEvents = true;
-            process.Exited += (sender, e) =>
-            {
-                if (process.ExitCode != 0)
-                    onException?.Invoke(process.ExitCode);
-                tcs.TrySetResult(process.ExitCode);
-            };
-
-            var started = process.Start();
-            if (!started)
-                tcs.TrySetException(new InvalidOperationException($"Could not start process {process}"));
-
-            process.BeginErrorReadLine();
-
-            return tcs.Task;
-        }
-    }
-}
+using System;
+using System.Diagnostics;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace FFmpeg.NET.Extensions
+{
+    public static class ProcessExtensions
+    {
+        public static Task<int> WaitForExitAsync(this Process process, Action<int> onException, CancellationToken cancellationToken = default)
+        {
+            TaskCompletionSource<int> tcs = new TaskCompletionSource<int>();
+            if (cancellationToken != default)
+            {
+                cancellationToken.Register(() =>
+                {
+                    try
+                    {
+                        // Send "q" to ffmpeg, which will force it to stop (closing files).
+                        process.StandardInput.Write("q");
+                    }
+                    catch (InvalidOperationException)
+                    {
+                        // If the process doesn't exist anymore, ignore it.
+                    }
+                    finally
+                    {
+                        // Cancel the task. This will throw an exception to the calling program.
+                        // Exc.Message will be "A task was canceled."
+                        try
+                        {
+                            tcs.SetCanceled();
+                        }
+                        catch (Exception)
+                        {
+                        }
+                    }
+                });
+            }
+
+            process.EnableRaisingEvents = true;
+            process.Exited += (sender, e) =>
+            {
+                if (process.ExitCode != 0)
+                    onException?.Invoke(process.ExitCode);
+                tcs.TrySetResult(process.ExitCode);
+            };
+
+            var started = process.Start();
+            if (!started)
+                tcs.TrySetException(new InvalidOperationException($"Could not start process {process}"));
+
+            process.BeginErrorReadLine();
+
+            return tcs.Task;
+        }
+    }
+}

src/FFmpeg.NET/FFmpegProcess.cs

@@ -29,8 +29,24 @@ private async Task ExecuteAsync(ProcessStartInfo startInfo, FFmpegParameters par
                 ffmpegProcess.ErrorDataReceived += (sender, e) => OnData(new ConversionDataEventArgs(e.Data, parameters.InputFile, parameters.OutputFile));
                 ffmpegProcess.ErrorDataReceived += (sender, e) => FFmpegProcessOnErrorDataReceived(e, parameters, ref caughtException, messages);
 
-                await ffmpegProcess.WaitForExitAsync((exitCode) => OnException(messages, parameters, exitCode, caughtException), cancellationToken);
-
+                Task<int> task = null;
+                try
+                {
+                    task = ffmpegProcess.WaitForExitAsync((exitCode) => OnException(messages, parameters, exitCode, caughtException), cancellationToken);
+                    await task;
+                }
+                catch (Exception)
+                {
+                    // An exception occurs if the user cancels the operation by calling Cancel on the CancellationToken.
+                    // Exc.Message will be "A task was canceled." (in English).
+                    // task.IsCanceled will be true.
+                    if (task.IsCanceled)
+                    {
+                        throw new TaskCanceledException(task);
+                    }
+                    // I don't think this can occur, but if some other exception, rethrow it.
+                    throw;
+                }
                 if (caughtException != null || ffmpegProcess.ExitCode != 0)
                 {
                     OnException(messages, parameters, ffmpegProcess.ExitCode, caughtException);
@@ -49,7 +65,7 @@ private void OnException(List<string> messages, FFmpegParameters parameters, int
             OnConversionError(new ConversionErrorEventArgs(exception, parameters.InputFile, parameters.OutputFile));
         }
 
-        private string GetExceptionMessage(List<string> messages) 
+        private string GetExceptionMessage(List<string> messages)
             => messages.Count > 1
                 ? messages[1] + messages[0]
                 : string.Join(string.Empty, messages);
@@ -93,10 +109,11 @@ private void FFmpegProcessOnErrorDataReceived(DataReceivedEventArgs e, FFmpegPar
 
         private ProcessStartInfo GenerateStartInfo(string ffmpegPath, string arguments) => new ProcessStartInfo
         {
-            Arguments = "-nostdin -y -loglevel info " + arguments,
+            // -y overwrite output files
+            Arguments = "-y " + arguments,
             FileName = ffmpegPath,
             CreateNoWindow = true,
-            RedirectStandardInput = false,
+            RedirectStandardInput = true,
             RedirectStandardOutput = true,
             RedirectStandardError = true,
             UseShellExecute = false,
@@ -116,4 +133,4 @@ private void FFmpegProcessOnErrorDataReceived(DataReceivedEventArgs e, FFmpegPar
 
         private void OnData(ConversionDataEventArgs eventArgs) => Data?.Invoke(eventArgs);
     }
-}
+}