Make sure DCS strings are flushed to conpty without delay (#17195)

When the `DCS` passthrough code was first implemented, it relied on the
`ActionPassThroughString` method flushing the given string immediately.
However, that has since stopped being the case, so `DCS` operations end
up being delayed until the entire sequence has been parsed.

This PR fixes the issue by introducing a `flush` parameter to force an
immediate flush on the `ActionPassThroughString` method, as well as the
`XtermEngine::WriteTerminalW` method that it calls.

## Validation Steps Performed

I've confirmed that the test case in issue #17111 now updates the color
table as soon as each color entry is parsed, instead of delaying the
updates until the end of the sequence.

Closes #17111

(cherry picked from commit 9c16c5c)
Service-Card-Id: 92501226
Service-Version: 1.21

src/renderer/vt/XtermEngine.cpp

@@ -520,9 +520,10 @@ CATCH_RETURN();
 //      proper utf-8 string, depending on our mode.
 // Arguments:
 // - wstr - wstring of text to be written
+// - flush - set to true if the string should be flushed immediately
 // Return Value:
 // - S_OK or suitable HRESULT error from either conversion or writing pipe.
-[[nodiscard]] HRESULT XtermEngine::WriteTerminalW(const std::wstring_view wstr) noexcept
+[[nodiscard]] HRESULT XtermEngine::WriteTerminalW(const std::wstring_view wstr, const bool flush) noexcept
 {
     RETURN_IF_FAILED(_fUseAsciiOnly ?
                          VtEngine::_WriteTerminalAscii(wstr) :
@@ -535,8 +536,11 @@ CATCH_RETURN();
     // cause flickering (where we've buffered some state but not the whole
     // "frame" as specified by the app). We'll just immediately buffer this
     // sequence, and flush it when the render thread comes around to paint the
-    // frame normally.
-
+    // frame normally, unless a flush has been explicitly requested.
+    if (flush)
+    {
+        _flushImpl();
+    }
     return S_OK;
 }
 

src/renderer/vt/XtermEngine.hpp

@@ -51,7 +51,7 @@ namespace Microsoft::Console::Render
 
         [[nodiscard]] HRESULT InvalidateScroll(const til::point* const pcoordDelta) noexcept override;
 
-        [[nodiscard]] HRESULT WriteTerminalW(const std::wstring_view str) noexcept override;
+        [[nodiscard]] HRESULT WriteTerminalW(const std::wstring_view str, const bool flush) noexcept override;
 
         [[nodiscard]] HRESULT SetWindowVisibility(const bool showOrHide) noexcept override;
 

src/renderer/vt/vtrenderer.hpp

@@ -78,7 +78,7 @@ namespace Microsoft::Console::Render
         [[nodiscard]] HRESULT RequestCursor() noexcept;
         [[nodiscard]] HRESULT InheritCursor(const til::point coordCursor) noexcept;
         [[nodiscard]] HRESULT WriteTerminalUtf8(const std::string_view str) noexcept;
-        [[nodiscard]] virtual HRESULT WriteTerminalW(const std::wstring_view str) noexcept = 0;
+        [[nodiscard]] virtual HRESULT WriteTerminalW(const std::wstring_view str, const bool flush = false) noexcept = 0;
         void SetTerminalOwner(Microsoft::Console::VirtualTerminal::VtIo* const terminalOwner);
         void SetResizeQuirk(const bool resizeQuirk);
         void SetLookingForDSRCallback(std::function<void(bool)> pfnLooking) noexcept;

src/terminal/adapter/adaptDispatch.cpp

@@ -4882,7 +4882,7 @@ ITermDispatch::StringHandler AdaptDispatch::_CreatePassthroughHandler()
                 {
                     buffer += L'\\';
                 }
-                engine.ActionPassThroughString(buffer);
+                engine.ActionPassThroughString(buffer, true);
                 buffer.clear();
             }
             return !endOfString;

src/terminal/parser/IStateMachineEngine.hpp

@@ -35,7 +35,7 @@ namespace Microsoft::Console::VirtualTerminal
         virtual bool ActionPrint(const wchar_t wch) = 0;
         virtual bool ActionPrintString(const std::wstring_view string) = 0;
 
-        virtual bool ActionPassThroughString(const std::wstring_view string) = 0;
+        virtual bool ActionPassThroughString(const std::wstring_view string, const bool flush = false) = 0;
 
         virtual bool ActionEscDispatch(const VTID id) = 0;
         virtual bool ActionVt52EscDispatch(const VTID id, const VTParameters parameters) = 0;

src/terminal/parser/InputStateMachineEngine.cpp

@@ -276,9 +276,10 @@ bool InputStateMachineEngine::ActionPrintString(const std::wstring_view string)
 //      string of characters given.
 // Arguments:
 // - string - string to dispatch.
+// - flush - not applicable to the input state machine.
 // Return Value:
 // - true iff we successfully dispatched the sequence.
-bool InputStateMachineEngine::ActionPassThroughString(const std::wstring_view string)
+bool InputStateMachineEngine::ActionPassThroughString(const std::wstring_view string, const bool /*flush*/)
 {
     if (_pDispatch->IsVtInputEnabled())
     {

src/terminal/parser/InputStateMachineEngine.hpp

@@ -142,7 +142,7 @@ namespace Microsoft::Console::VirtualTerminal
 
         bool ActionPrintString(const std::wstring_view string) override;
 
-        bool ActionPassThroughString(const std::wstring_view string) override;
+        bool ActionPassThroughString(const std::wstring_view string, const bool flush) override;
 
         bool ActionEscDispatch(const VTID id) override;
 

src/terminal/parser/OutputStateMachineEngine.cpp

@@ -180,14 +180,15 @@ bool OutputStateMachineEngine::ActionPrintString(const std::wstring_view string)
 //      we don't know what to do with it)
 // Arguments:
 // - string - string to dispatch.
+// - flush - set to true if the string should be flushed immediately.
 // Return Value:
 // - true iff we successfully dispatched the sequence.
-bool OutputStateMachineEngine::ActionPassThroughString(const std::wstring_view string)
+bool OutputStateMachineEngine::ActionPassThroughString(const std::wstring_view string, const bool flush)
 {
     auto success = true;
     if (_pTtyConnection != nullptr)
     {
-        const auto hr = _pTtyConnection->WriteTerminalW(string);
+        const auto hr = _pTtyConnection->WriteTerminalW(string, flush);
         LOG_IF_FAILED(hr);
         success = SUCCEEDED(hr);
     }

src/terminal/parser/OutputStateMachineEngine.hpp

@@ -38,7 +38,7 @@ namespace Microsoft::Console::VirtualTerminal
 
         bool ActionPrintString(const std::wstring_view string) override;
 
-        bool ActionPassThroughString(const std::wstring_view string) override;
+        bool ActionPassThroughString(const std::wstring_view string, const bool flush) override;
 
         bool ActionEscDispatch(const VTID id) override;
 

src/terminal/parser/ut_parser/StateMachineTest.cpp

@@ -59,7 +59,7 @@ class Microsoft::Console::VirtualTerminal::TestStateMachineEngine : public IStat
         return true;
     };
 
-    bool ActionPassThroughString(const std::wstring_view string) override
+    bool ActionPassThroughString(const std::wstring_view string, const bool /*flush*/) override
     {
         passedThrough += string;
         return true;