Sync ReadKeyProc thread with pipeline thread (#3294)

The pipeline thread was returning before the `ReadKeyProcThread` had finished processing the dummy input.
This was occurring somewhat frequently when `ReadLine` was invoked multiple times in a row creating a race condition.
With these changes, the pipeline thread will wait for dummy input to be received, dequeue the key, and continue with normal cancellation logic.

Author: SeeminglyScience

PSReadLine/ReadLine.cs

@@ -33,15 +33,11 @@ public partial class PSConsoleReadLine : IPSConsoleReadLineMockableMethods
     {
         private const int ConsoleExiting = 1;
 
-        private const int CancellationRequested = 2;
-
         // *must* be initialized in the static ctor
         // because the static member _clipboard depends upon it
         // for its own initialization
         private static readonly PSConsoleReadLine _singleton;
 
-        private static readonly CancellationToken _defaultCancellationToken = new CancellationTokenSource().Token;
-
         // This is used by PowerShellEditorServices (the backend of the PowerShell VSCode extension)
         // so that it can call PSReadLine from a delegate and not hit nested pipeline issues.
         #pragma warning disable CS0649
@@ -143,17 +139,7 @@ private void ReadOneOrMoreKeys()
                 }
                 while (_charMap.KeyAvailable)
                 {
-                    ConsoleKeyInfo keyInfo = _charMap.ReadKey();
-                    if (_cancelReadCancellationToken.IsCancellationRequested)
-                    {
-                        // If PSReadLine is running under a host that can cancel it, the
-                        // cancellation will come at a time when ReadKey is stuck waiting for input.
-                        // The next key press will be used to force it to return, and so we want to
-                        // discard this key since we were already canceled.
-                        continue;
-                    }
-
-                    var key = PSKeyInfo.FromConsoleKeyInfo(keyInfo);
+                    var key = PSKeyInfo.FromConsoleKeyInfo(_charMap.ReadKey());
                     _lastNKeys.Enqueue(key);
                     _queuedKeys.Enqueue(key);
                 }
@@ -170,10 +156,6 @@ private void ReadKeyThreadProc()
                     break;
 
                 ReadOneOrMoreKeys();
-                if (_cancelReadCancellationToken.IsCancellationRequested)
-                {
-                    continue;
-                }
 
                 // One or more keys were read - let ReadKey know we're done.
                 _keyReadWaitHandle.Set();
@@ -208,7 +190,6 @@ internal static PSKeyInfo ReadKey()
                     //   - a key is pressed
                     //   - the console is exiting
                     //   - 300ms timeout - to process events if we're idle
-                    //   - ReadLine cancellation is requested externally
                     handleId = WaitHandle.WaitAny(_singleton._requestKeyWaitHandles, 300);
                     if (handleId != WaitHandle.WaitTimeout)
                     {
@@ -292,10 +273,12 @@ internal static PSKeyInfo ReadKey()
                 throw new OperationCanceledException();
             }
 
-            if (handleId == CancellationRequested)
+            if (_singleton._cancelReadCancellationToken.IsCancellationRequested)
             {
-                // ReadLine was cancelled. Save the current line to be restored next time ReadLine
-                // is called, clear the buffer and throw an exception so we can return an empty string.
+                // ReadLine was cancelled. Dequeue the dummy input sent by the host, save the current
+                // line to be restored next time ReadLine is called, clear the buffer and throw an
+                // exception so we can return an empty string.
+                _singleton._queuedKeys.Dequeue();
                 _singleton.SaveCurrentLine();
                 _singleton._getNextHistoryIndex = _singleton._history.Count;
                 _singleton._current = 0;
@@ -331,9 +314,7 @@ private void PrependQueuedKeys(PSKeyInfo key)
         /// <returns>The complete command line.</returns>
         public static string ReadLine(Runspace runspace, EngineIntrinsics engineIntrinsics, bool? lastRunStatus)
         {
-            // Use a default cancellation token instead of CancellationToken.None because the
-            // WaitHandle is shared and could be triggered accidently.
-            return ReadLine(runspace, engineIntrinsics, _defaultCancellationToken, lastRunStatus);
+            return ReadLine(runspace, engineIntrinsics, CancellationToken.None, lastRunStatus);
         }
 
         /// <summary>
@@ -396,7 +377,6 @@ public static string ReadLine(
                     }
 
                     _singleton._cancelReadCancellationToken = cancellationToken;
-                    _singleton._requestKeyWaitHandles[2] = cancellationToken.WaitHandle;
                     return _singleton.InputLoop();
                 }
                 catch (OperationCanceledException)
@@ -877,7 +857,7 @@ private void DelayedOneTimeInitialize()
             _readKeyWaitHandle = new AutoResetEvent(false);
             _keyReadWaitHandle = new AutoResetEvent(false);
             _closingWaitHandle = new ManualResetEvent(false);
-            _requestKeyWaitHandles = new WaitHandle[] {_keyReadWaitHandle, _closingWaitHandle, null};
+            _requestKeyWaitHandles = new WaitHandle[] {_keyReadWaitHandle, _closingWaitHandle};
             _threadProcWaitHandles = new WaitHandle[] {_readKeyWaitHandle, _closingWaitHandle};
 
             // This is for a "being hosted in an alternate appdomain scenario" (the