With yt-dlp, cancellation requires killing the child process

src/YouTubeDownloadTool/Utils/Utils.cs

@@ -1,4 +1,6 @@
+using System.Diagnostics;
 using System.IO;
+using System.Reflection;
 
 namespace YouTubeDownloadTool;
 
@@ -40,4 +42,31 @@ await using (var file = tempFile.OpenStream())
 
         return fileLock;
     }
+
+    private static readonly Func<Process, Process[]?, IReadOnlyList<Process>>? GetChildProcesses =
+        typeof(Process).GetMethod("GetChildProcesses", BindingFlags.NonPublic | BindingFlags.Instance, null, new[] { typeof(Process[]) }, null)
+            ?.CreateDelegate<Func<Process, Process[]?, IReadOnlyList<Process>>>();
+
+    public static IReadOnlyList<Process>? TryFilterToChildProcesses(Process[] candidates, Process parentProcess)
+    {
+        return GetChildProcesses?.Invoke(parentProcess, candidates);
+    }
+
+    public static void TryKillImmediateChildrenWithProcessName(Process parentProcess, string childProcessName)
+    {
+        var allProcessesWithName = Process.GetProcessesByName(childProcessName);
+        try
+        {
+            if (TryFilterToChildProcesses(allProcessesWithName, parentProcess) is { } childProcesses)
+            {
+                foreach (var childProcess in childProcesses)
+                    childProcess.Kill();
+            }
+        }
+        finally
+        {
+            foreach (var process in allProcessesWithName)
+                process.Dispose();
+        }
+    }
 }

src/YouTubeDownloadTool/YTDlpTool.cs

@@ -126,7 +126,19 @@ public YTDlpTool(string executablePath, string ffmpegDirectory)
         process.BeginOutputReadLine();
         process.BeginErrorReadLine();
 
-        await using (cancellationToken.Register(process.Kill))
+        void Cancel()
+        {
+            var processName = process.ProcessName; // This can't be accessed after Process.Kill
+
+            // Prevent from starting any future child processes
+            process.Kill();
+
+            // Unlike youtube-dl, yt-dlp starts a child process with the same name (yt-dlp). Once it's done this,
+            // killing the process we started doesn't cancel anything.
+            Utils.TryKillImmediateChildrenWithProcessName(parentProcess: process, childProcessName: processName);
+        }
+
+        await using (cancellationToken.Register(Cancel))
             await process.WaitForExitAsync(CancellationToken.None);
 
         cancellationToken.ThrowIfCancellationRequested();