Fix race condition: set _canceled before SignalCore in ProcessWaitState

src/libraries/System.Diagnostics.Process/src/Microsoft/Win32/SafeHandles/SafeProcessHandle.Unix.cs

@@ -56,12 +56,9 @@ public int ProcessId
             }
         }
 
-        /// <summary>
-        /// Sets a value indicating whether the process has been terminated due to timeout or cancellation.
-        /// </summary>
-        private bool Canceled
+        private void Cancel()
         {
-            set => GetWaitState()._canceled = value;
+            GetWaitState().Cancel(this);
         }
 
         protected override bool ReleaseHandle()
@@ -132,7 +129,7 @@ private ProcessExitStatus WaitForExitOrKillOnTimeoutCore(int milliseconds)
 
             if (!waitState.WaitForExit(milliseconds))
             {
-                waitState._canceled = SignalCore(PosixSignal.SIGKILL);
+                waitState.Cancel(this);
                 waitState.WaitForExit(Timeout.Infinite);
             }
 

src/libraries/System.Diagnostics.Process/src/Microsoft/Win32/SafeHandles/SafeProcessHandle.Windows.cs

@@ -46,6 +46,13 @@ public int ProcessId
         /// </summary>
         private bool Canceled { get; set; }
 
+        private void Cancel()
+        {
+#pragma warning disable CA1416 // PosixSignal.SIGKILL is supported on Windows via SignalCore
+            Canceled = SignalCore(PosixSignal.SIGKILL);
+#pragma warning restore CA1416
+        }
+
         protected override bool ReleaseHandle()
         {
             return Interop.Kernel32.CloseHandle(handle);
@@ -673,9 +680,7 @@ private ProcessExitStatus WaitForExitOrKillOnTimeoutCore(int milliseconds)
             using Interop.Kernel32.ProcessWaitHandle processWaitHandle = new(this);
             if (!processWaitHandle.WaitOne(milliseconds))
             {
-#pragma warning disable CA1416 // PosixSignal.SIGKILL is supported on Windows via SignalCore
-                Canceled = SignalCore(PosixSignal.SIGKILL);
-#pragma warning restore CA1416
+                Cancel();
                 processWaitHandle.WaitOne(Timeout.Infinite);
             }
 

src/libraries/System.Diagnostics.Process/src/Microsoft/Win32/SafeHandles/SafeProcessHandle.cs

@@ -343,7 +343,7 @@ public async Task<ProcessExitStatus> WaitForExitOrKillOnCancellationAsync(Cancel
                             var (handle, tcs) = ((SafeProcessHandle, TaskCompletionSource<bool>))state!;
                             try
                             {
-                                handle.Canceled = handle.SignalCore(PosixSignal.SIGKILL);
+                                handle.Cancel();
                             }
                             catch (Exception ex)
                             {

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

@@ -5,6 +5,7 @@
 using System.Runtime.InteropServices;
 using System.Threading;
 using System.Threading.Tasks;
+using Microsoft.Win32.SafeHandles;
 
 namespace System.Diagnostics
 {
@@ -213,7 +214,17 @@ internal void ReleaseRef()
         /// <summary>Associated process is a child that can use the terminal.</summary>
         private readonly bool _usesTerminal;
         /// <summary>A value indicating whether the process has been terminated due to timeout or cancellation.</summary>
-        internal volatile bool _canceled;
+        internal bool _canceled;
+
+        internal void Cancel(SafeProcessHandle process)
+        {
+            lock (_gate)
+            {
+#pragma warning disable CA1416 // Signal is not supported on iOS/tvOS but Cancel is only called from supported platforms
+                _canceled = process.Signal(PosixSignal.SIGKILL);
+#pragma warning restore CA1416
+            }
+        }
 
         /// <summary>An in-progress or completed wait operation.</summary>
         /// <remarks>A completed task does not mean the process has exited.</remarks>