Only flush during Present()

This is interesting, but didn't meaningfully move the needle

* one line flush change

32.443MB, 3.905s, 8.307MB/s
32.443MB, 3.938s, 8.238MB/s
32.443MB, 3.915s, 8.288MB/s

* with lock the buffer and flush on present

32.443MB, 3.891s, 8.339MB/s
32.443MB, 3.872s, 8.379MB/s
32.443MB, 3.877s, 8.368MB/s

src/renderer/vt/XtermEngine.cpp

@@ -112,7 +112,10 @@ XtermEngine::XtermEngine(_In_ wil::unique_hfile hPipe,
         // by prepending a cursor off.
         if (_lastCursorIsVisible != Tribool::False)
         {
-            _buffer.insert(0, "\x1b[?25l");
+            {
+                auto buffer{ _buffer.lock() };
+                buffer->insert(0, "\x1b[?25l");
+            }
             _lastCursorIsVisible = Tribool::False;
         }
         // If the cursor was NOT previously visible, then that's fine! we don't

src/renderer/vt/paint.cpp

@@ -94,22 +94,6 @@ using namespace Microsoft::Console::Types;
         RETURN_IF_FAILED(_MoveCursor(_deferredCursorPos));
     }
 
-    // If this frame was triggered because we encountered a VT sequence which
-    // required the buffered state to get printed, we don't want to flush this
-    // frame to the pipe. That might result in us rendering half the output of a
-    // particular frame (as emitted by the client).
-    //
-    // Instead, we'll leave this frame in _buffer, and just keep appending to
-    // it as needed.
-    if (_noFlushOnEnd) [[unlikely]]
-    {
-        _noFlushOnEnd = false;
-    }
-    else
-    {
-        RETURN_IF_FAILED(_Flush());
-    }
-
     return S_OK;
 }
 
@@ -122,8 +106,23 @@ using namespace Microsoft::Console::Types;
 // Return Value:
 // - S_FALSE since we do nothing.
 [[nodiscard]] HRESULT VtEngine::Present() noexcept
-{
-    return S_FALSE;
+{ // If this frame was triggered because we encountered a VT sequence which
+    // required the buffered state to get printed, we don't want to flush this
+    // frame to the pipe. That might result in us rendering half the output of a
+    // particular frame (as emitted by the client).
+    //
+    // Instead, we'll leave this frame in _buffer, and just keep appending to
+    // it as needed.
+    if (_noFlushOnEnd)
+        [[unlikely]]
+        {
+            _noFlushOnEnd = false;
+        }
+    else
+    {
+        RETURN_IF_FAILED(_Flush());
+    }
+    return S_OK;
 }
 
 [[nodiscard]] HRESULT VtEngine::ResetLineTransform() noexcept
@@ -552,10 +551,11 @@ using namespace Microsoft::Console::Types;
     // Write the actual text string. If we're using a soft font, the character
     // set should have already been selected, so we just need to map our internal
     // representation back to ASCII (handled by the _WriteTerminalDrcs method).
-    if (_usingSoftFont) [[unlikely]]
-    {
-        RETURN_IF_FAILED(VtEngine::_WriteTerminalDrcs({ _bufferLine.data(), cchActual }));
-    }
+    if (_usingSoftFont)
+        [[unlikely]]
+        {
+            RETURN_IF_FAILED(VtEngine::_WriteTerminalDrcs({ _bufferLine.data(), cchActual }));
+        }
     else
     {
         RETURN_IF_FAILED(VtEngine::_WriteTerminalUtf8({ _bufferLine.data(), cchActual }));

src/renderer/vt/state.cpp

@@ -93,7 +93,10 @@ try
 #endif
 
     // TODO GH10001: Replace me with REP
-    _buffer.append(n, c);
+    {
+        auto buffer{ _buffer.lock() };
+        buffer->append(n, c);
+    }
     return S_OK;
 }
 CATCH_RETURN();
@@ -129,7 +132,8 @@ CATCH_RETURN();
 
     try
     {
-        _buffer.append(str);
+        auto buffer{ _buffer.lock() };
+        buffer->append(str);
 
         return S_OK;
     }
@@ -140,8 +144,12 @@ CATCH_RETURN();
 {
     if (_hFile)
     {
-        auto fSuccess = !!WriteFile(_hFile.get(), _buffer.data(), gsl::narrow_cast<DWORD>(_buffer.size()), nullptr, nullptr);
-        _buffer.clear();
+        bool fSuccess = false;
+        {
+            auto buffer{ _buffer.lock() };
+            fSuccess = !!WriteFile(_hFile.get(), buffer->data(), gsl::narrow_cast<DWORD>(buffer->size()), nullptr, nullptr);
+            buffer->clear();
+        }
         if (!fSuccess)
         {
             _exitResult = HRESULT_FROM_WIN32(GetLastError());

src/renderer/vt/vtrenderer.hpp

@@ -20,6 +20,7 @@ Author(s):
 #include "tracing.hpp"
 #include <string>
 #include <functional>
+#include <til/mutex.h>
 
 // fwdecl unittest classes
 #ifdef UNIT_TESTING
@@ -93,7 +94,7 @@ namespace Microsoft::Console::Render
 
     protected:
         wil::unique_hfile _hFile;
-        std::string _buffer;
+        til::shared_mutex<std::string> _buffer;
 
         std::string _formatBuffer;
         std::string _conversionBuffer;