Fix Process.Kill(entireProcessTree: true) when intermediate child has KillOnParentExit

src/libraries/System.Diagnostics.Process/src/System/Diagnostics/Process.Unix.cs

@@ -95,46 +95,88 @@ private bool GetHasExited(bool refresh)
         }
 
         private void KillTree(ref List<Exception>? exceptions)
+        {
+            // First, stop and collect the entire process tree before killing any process.
+            // This is necessary because killing a parent process may cause children with
+            // PR_SET_PDEATHSIG (KillOnParentExit) to be killed by the kernel immediately,
+            // making it impossible to discover their descendants afterward.
+            List<Process> stoppedProcesses = [];
+            try
+            {
+                StopTree(ref exceptions, stoppedProcesses);
+            }
+            catch (Exception ex)
+            {
+                (exceptions ??= new List<Exception>()).Add(ex);
+            }
+            finally
+            {
+                // Kill all stopped processes even if StopTree threw partway through.
+                foreach (Process process in stoppedProcesses)
+                {
+                    int killResult = Interop.Sys.Kill(process._processId, Interop.Sys.GetPlatformSignalNumber(PosixSignal.SIGKILL));
+                    if (killResult != 0)
+                    {
+                        Interop.ErrorInfo errorInfo = Interop.Sys.GetLastErrorInfo();
+                        // Ignore 'process no longer exists' error.
+                        if (errorInfo.Error != Interop.Error.ESRCH)
+                        {
+                            (exceptions ??= new List<Exception>()).Add(new Win32Exception(errorInfo.RawErrno));
+                        }
+                    }
+
+                    if (process != this)
+                    {
+                        process.Dispose();
+                    }
+                }
+            }
+        }
+
+        /// <summary>
+        /// Recursively stops all processes in the tree (depth-first) and collects them into a flat list.
+        /// Stopping (SIGSTOP) before enumerating children ensures we can discover the entire tree
+        /// before any kill signals cause cascading terminations.
+        /// </summary>
+        /// <returns>
+        /// <see langword="true"/> if the process was successfully stopped and added to
+        /// <paramref name="stoppedProcesses"/>; <see langword="false"/> if the process had already
+        /// exited or could not be stopped, in which case the caller is responsible for disposing it.
+        /// </returns>
+        private bool StopTree(ref List<Exception>? exceptions, List<Process> stoppedProcesses)
         {
             // If the process has exited, we can no longer determine its children.
-            // If we know the process has exited, stop already.
             if (GetHasExited(refresh: false))
             {
-                return;
+                return false;
             }
 
             // Stop the process, so it won't start additional children.
             // This is best effort: kill can return before the process is stopped.
             int stopResult = Interop.Sys.Kill(_processId, Interop.Sys.GetPlatformSIGSTOP());
             if (stopResult != 0)
             {
-                Interop.Error error = Interop.Sys.GetLastError();
+                Interop.ErrorInfo errorInfo = Interop.Sys.GetLastErrorInfo();
                 // Ignore 'process no longer exists' error.
-                if (error != Interop.Error.ESRCH)
+                if (errorInfo.Error != Interop.Error.ESRCH)
                 {
-                    (exceptions ??= new List<Exception>()).Add(new Win32Exception());
+                    (exceptions ??= new List<Exception>()).Add(new Win32Exception(errorInfo.RawErrno));
                 }
-                return;
+                return false;
             }
 
-            List<Process> children = GetChildProcesses();
+            stoppedProcesses.Add(this);
 
-            int killResult = Interop.Sys.Kill(_processId, Interop.Sys.GetPlatformSignalNumber(PosixSignal.SIGKILL));
-            if (killResult != 0)
+            List<Process> children = GetChildProcesses();
+            foreach (Process childProcess in children)
             {
-                Interop.Error error = Interop.Sys.GetLastError();
-                // Ignore 'process no longer exists' error.
-                if (error != Interop.Error.ESRCH)
+                if (!childProcess.StopTree(ref exceptions, stoppedProcesses))
                 {
-                    (exceptions ??= new List<Exception>()).Add(new Win32Exception());
+                    childProcess.Dispose();
                 }
             }
 
-            foreach (Process childProcess in children)
-            {
-                childProcess.KillTree(ref exceptions);
-                childProcess.Dispose();
-            }
+            return true;
         }
 
         /// <summary>Discards any information about the associated process.</summary>

src/libraries/System.Diagnostics.Process/tests/KillOnParentExitTests.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Runtime.InteropServices;
+using System.Threading;
 using Microsoft.DotNet.RemoteExecutor;
 using Xunit;
 
@@ -145,6 +146,65 @@ public void KillOnParentExit_KillsTheChild_WhenParentIsKilled(bool enabled, bool
             }
         }
 
+        [ConditionalTheory(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
+        [InlineData(true)]
+        [InlineData(false)]
+        public void KillEntireProcessTree_KillsGrandchild_WhenIntermediateChildHasKillOnParentExit(bool enabled)
+        {
+            // Mimics the scenario: Process A -> Process B (KillOnParentExit) -> Process C
+            // Killing A with entireProcessTree:true should kill C regardless of KillOnParentExit setting.
+            RemoteInvokeOptions parentOptions = new() { CheckExitCode = false };
+            parentOptions.StartInfo.RedirectStandardOutput = true;
+            parentOptions.StartInfo.RedirectStandardInput = true;
+
+            using RemoteInvokeHandle parentHandle = RemoteExecutor.Invoke(
+                (enabledStr) =>
+                {
+                    // This is "Process A". Start "Process B" with KillOnParentExit.
+                    // Process B will start "Process C" and report C's PID.
+                    using Process child = CreateProcess(() =>
+                    {
+                        // This is "Process B". Start "Process C" (long-running, no KillOnParentExit).
+                        using Process grandChild = CreateProcessLong();
+                        grandChild.Start();
+                        Console.WriteLine(grandChild.Id);
+
+                        // Block until killed
+                        Thread.Sleep(Timeout.Infinite);
+                        return RemoteExecutor.SuccessExitCode;
+                    });
+                    child.StartInfo.KillOnParentExit = bool.Parse(enabledStr);
+                    child.StartInfo.RedirectStandardOutput = true;
+                    child.Start();
+
+                    // Read grandchild PID from Process B and forward to test
+                    string grandChildPidStr = child.StandardOutput.ReadLine();
+                    Console.WriteLine(grandChildPidStr);
+
+                    // Block until killed
+                    Thread.Sleep(Timeout.Infinite);
+                },
+                enabled.ToString(),
+                parentOptions);
+
+            int grandChildPid = int.Parse(parentHandle.Process.StandardOutput.ReadLine());
+            using Process grandchild = Process.GetProcessById(grandChildPid);
+
+            try
+            {
+                // Kill Process A with entireProcessTree: true
+                parentHandle.Process.Kill(entireProcessTree: true);
+
+                Assert.True(parentHandle.Process.WaitForExit(WaitInMS));
+                // The grandchild (Process C) should also be killed
+                Assert.True(grandchild.WaitForExit(WaitInMS));
+            }
+            finally
+            {
+                grandchild.Kill();
+            }
+        }
+
         [ConditionalTheory(typeof(RemoteExecutor), nameof(RemoteExecutor.IsSupported))]
         [InlineData(true, true)]
         [InlineData(true, false)]

src/native/libs/System.Native/pal_signal.c

@@ -507,7 +507,7 @@ void SystemNative_SetTerminalInvalidationHandler(TerminalInvalidationCallback ca
 
         installed = InstallSignalHandler(SIGCONT, SA_RESTART);
         assert(installed);
-        installed = InstallSignalHandler(SIGCHLD, SA_RESTART);
+        installed = InstallSignalHandler(SIGCHLD, SA_RESTART | SA_NOCLDSTOP);
         assert(installed);
         installed = InstallSignalHandler(SIGWINCH, SA_RESTART);
         assert(installed);
@@ -526,7 +526,7 @@ void SystemNative_RegisterForSigChld(SigChldCallback callback)
     {
         g_sigChldCallback = callback;
 
-        installed = InstallSignalHandler(SIGCHLD, SA_RESTART);
+        installed = InstallSignalHandler(SIGCHLD, SA_RESTART | SA_NOCLDSTOP);
         assert(installed);
     }
     pthread_mutex_unlock(&lock);