Add support for the VT answerback capability (#17660)

The answerback feature allows for the user to define a message that the
terminal will transmit to the host whenever an `ENQ` (enquiry) control
character is received.

## Detailed Description of the Pull Request / Additional comments

In Windows Terminal, the message can be configured at the profile level
of the settings file, as a string property named `AnswerbackMessage`.

In ConHost, the message can be configured in the registry, again as a
string value with the name `AnswerbackMessage`.

## Validation Steps Performed

I've confirmed that the control is working as intended in both Windows
Terminal and ConHost using Vttest.

Closes #11946

.github/actions/spelling/expect/expect.txt

@@ -38,6 +38,7 @@ ANSISYS
 ANSISYSRC
 ANSISYSSC
 answerback
+ANSWERBACKMESSAGE
 antialiasing
 ANull
 anycpu

dep/Console/winconp.h

@@ -104,6 +104,7 @@ GetConsoleKeyboardLayoutNameW(
 #define CONSOLE_REGISTRY_DEFAULTFOREGROUND             L"DefaultForeground"
 #define CONSOLE_REGISTRY_DEFAULTBACKGROUND             L"DefaultBackground"
 #define CONSOLE_REGISTRY_TERMINALSCROLLING             L"TerminalScrolling"
+#define CONSOLE_REGISTRY_ANSWERBACKMESSAGE             L"AnswerbackMessage"
 // end V2 console settings
 
     /*

doc/cascadia/profiles.schema.json

@@ -3032,6 +3032,10 @@
           "description": "By default Windows treats Ctrl+Alt as an alias for AltGr. When altGrAliasing is set to false, this behavior will be disabled.",
           "type": "boolean"
         },
+        "answerbackMessage": {
+          "description": "The response that is sent when an ENQ control character is received.",
+          "type": "string"
+        },
         "source": {
           "description": "Stores the name of the profile generator that originated this profile.",
           "type": [

src/cascadia/TerminalCore/ICoreSettings.idl

@@ -14,6 +14,7 @@ namespace Microsoft.Terminal.Core
 
         Boolean SnapOnInput;
         Boolean AltGrAliasing;
+        String AnswerbackMessage;
 
         String StartingTitle;
         Boolean SuppressApplicationTitle;

src/cascadia/TerminalCore/Terminal.cpp

@@ -82,6 +82,7 @@ void Terminal::UpdateSettings(ICoreSettings settings)
 
     _snapOnInput = settings.SnapOnInput();
     _altGrAliasing = settings.AltGrAliasing();
+    _answerbackMessage = settings.AnswerbackMessage();
     _wordDelimiters = settings.WordDelimiters();
     _suppressApplicationTitle = settings.SuppressApplicationTitle();
     _startingTitle = settings.StartingTitle();

src/cascadia/TerminalCore/Terminal.hpp

@@ -135,6 +135,7 @@ class Microsoft::Terminal::Core::Terminal final :
     void SetTextAttributes(const TextAttribute& attrs) noexcept override;
     void SetSystemMode(const Mode mode, const bool enabled) noexcept override;
     bool GetSystemMode(const Mode mode) const noexcept override;
+    void ReturnAnswerback() override;
     void WarningBell() override;
     void SetWindowTitle(const std::wstring_view title) override;
     CursorType GetUserDefaultCursorStyle() const noexcept override;
@@ -371,6 +372,7 @@ class Microsoft::Terminal::Core::Terminal final :
 
     size_t _hyperlinkPatternId = 0;
 
+    std::wstring _answerbackMessage;
     std::wstring _workingDirectory;
 
     // This default fake font value is only used to check if the font is a raster font.

src/cascadia/TerminalCore/TerminalApi.cpp

@@ -71,6 +71,11 @@ bool Terminal::GetSystemMode(const Mode mode) const noexcept
     return _systemMode.test(mode);
 }
 
+void Terminal::ReturnAnswerback()
+{
+    ReturnResponse(_answerbackMessage);
+}
+
 void Terminal::WarningBell()
 {
     _pfnWarningBell();

src/cascadia/TerminalSettingsModel/MTSMSettings.h

@@ -81,6 +81,7 @@ Author(s):
     X(int32_t, HistorySize, "historySize", DEFAULT_HISTORY_SIZE)                                                                                               \
     X(bool, SnapOnInput, "snapOnInput", true)                                                                                                                  \
     X(bool, AltGrAliasing, "altGrAliasing", true)                                                                                                              \
+    X(hstring, AnswerbackMessage, "answerbackMessage")                                                                                                         \
     X(hstring, Commandline, "commandline", L"%SystemRoot%\\System32\\cmd.exe")                                                                                 \
     X(Microsoft::Terminal::Control::ScrollbarState, ScrollState, "scrollbarState", Microsoft::Terminal::Control::ScrollbarState::Visible)                      \
     X(Microsoft::Terminal::Control::TextAntialiasingMode, AntialiasingMode, "antialiasingMode", Microsoft::Terminal::Control::TextAntialiasingMode::Grayscale) \

src/cascadia/TerminalSettingsModel/Profile.idl

@@ -73,6 +73,7 @@ namespace Microsoft.Terminal.Settings.Model
         INHERITABLE_PROFILE_SETTING(Int32, HistorySize);
         INHERITABLE_PROFILE_SETTING(Boolean, SnapOnInput);
         INHERITABLE_PROFILE_SETTING(Boolean, AltGrAliasing);
+        INHERITABLE_PROFILE_SETTING(String, AnswerbackMessage);
         INHERITABLE_PROFILE_SETTING(BellStyle, BellStyle);
 
         INHERITABLE_PROFILE_SETTING(Windows.Foundation.Collections.IMap<String COMMA String>, EnvironmentVariables);

src/cascadia/TerminalSettingsModel/TerminalSettings.cpp

@@ -284,6 +284,7 @@ namespace winrt::Microsoft::Terminal::Settings::Model::implementation
         _HistorySize = profile.HistorySize();
         _SnapOnInput = profile.SnapOnInput();
         _AltGrAliasing = profile.AltGrAliasing();
+        _AnswerbackMessage = profile.AnswerbackMessage();
 
         // Fill in the remaining properties from the profile
         _ProfileName = profile.Name();

src/cascadia/TerminalSettingsModel/TerminalSettings.h

@@ -86,6 +86,7 @@ namespace winrt::Microsoft::Terminal::Settings::Model::implementation
 
         INHERITABLE_SETTING(Model::TerminalSettings, bool, SnapOnInput, true);
         INHERITABLE_SETTING(Model::TerminalSettings, bool, AltGrAliasing, true);
+        INHERITABLE_SETTING(Model::TerminalSettings, hstring, AnswerbackMessage);
         INHERITABLE_SETTING(Model::TerminalSettings, til::color, CursorColor, DEFAULT_CURSOR_COLOR);
         INHERITABLE_SETTING(Model::TerminalSettings, Microsoft::Terminal::Core::CursorStyle, CursorShape, Core::CursorStyle::Vintage);
         INHERITABLE_SETTING(Model::TerminalSettings, uint32_t, CursorHeight, DEFAULT_CURSOR_HEIGHT);

src/cascadia/inc/ControlProperties.h

@@ -38,6 +38,7 @@
     X(int32_t, InitialCols, 80)                                                                                   \
     X(bool, SnapOnInput, true)                                                                                    \
     X(bool, AltGrAliasing, true)                                                                                  \
+    X(winrt::hstring, AnswerbackMessage)                                                                          \
     X(winrt::hstring, WordDelimiters, DEFAULT_WORD_DELIMITERS)                                                    \
     X(bool, CopyOnSelect, false)                                                                                  \
     X(bool, FocusFollowMouse, false)                                                                              \

src/host/outputStream.cpp

@@ -146,6 +146,16 @@ bool ConhostInternalGetSet::GetSystemMode(const Mode mode) const
     }
 }
 
+// Routine Description:
+// - Sends the configured answerback message in response to an ENQ query.
+// Return Value:
+// - <none>
+void ConhostInternalGetSet::ReturnAnswerback()
+{
+    const auto& gci = ServiceLocator::LocateGlobals().getConsoleInformation();
+    ReturnResponse(gci.GetAnswerbackMessage());
+}
+
 // Routine Description:
 // - Sends a notify message to play the "SystemHand" sound event.
 // Return Value:

src/host/outputStream.hpp

@@ -40,6 +40,7 @@ class ConhostInternalGetSet final : public Microsoft::Console::VirtualTerminal::
     void SetSystemMode(const Mode mode, const bool enabled) override;
     bool GetSystemMode(const Mode mode) const override;
 
+    void ReturnAnswerback() override;
     void WarningBell() override;
 
     void SetWindowTitle(const std::wstring_view title) override;

src/host/settings.cpp

@@ -765,6 +765,11 @@ void Settings::SetTerminalScrolling(const bool terminalScrollingEnabled) noexcep
     _TerminalScrolling = terminalScrollingEnabled;
 }
 
+std::wstring_view Settings::GetAnswerbackMessage() const noexcept
+{
+    return _answerbackMessage;
+}
+
 // Determines whether our primary renderer should be DirectX or GDI.
 // This is based on user preference and velocity hold back state.
 bool Settings::GetUseDx() const noexcept

src/host/settings.hpp

@@ -176,6 +176,8 @@ class Settings
     bool IsTerminalScrolling() const noexcept;
     void SetTerminalScrolling(const bool terminalScrollingEnabled) noexcept;
 
+    std::wstring_view GetAnswerbackMessage() const noexcept;
+
     bool GetUseDx() const noexcept;
     bool GetCopyColor() const noexcept;
     SettingsTextMeasurementMode GetTextMeasurementMode() const noexcept;
@@ -236,5 +238,6 @@ class Settings
     bool _fInterceptCopyPaste;
 
     bool _TerminalScrolling;
+    WCHAR _answerbackMessage[32] = {};
     friend class RegistrySerialization;
 };

src/propslib/RegistrySerialization.cpp

@@ -60,6 +60,7 @@ const RegistrySerialization::_RegPropertyMap RegistrySerialization::s_PropertyMa
     { _RegPropertyType::Dword,          CONSOLE_REGISTRY_CURSORTYPE,                    SET_FIELD_AND_SIZE(_CursorType)                  },
     { _RegPropertyType::Boolean,        CONSOLE_REGISTRY_INTERCEPTCOPYPASTE,            SET_FIELD_AND_SIZE(_fInterceptCopyPaste)         },
     { _RegPropertyType::Boolean,        CONSOLE_REGISTRY_TERMINALSCROLLING,             SET_FIELD_AND_SIZE(_TerminalScrolling)           },
+    { _RegPropertyType::String,         CONSOLE_REGISTRY_ANSWERBACKMESSAGE,             SET_FIELD_AND_SIZE(_answerbackMessage)           },
     { _RegPropertyType::Boolean,        CONSOLE_REGISTRY_USEDX,                         SET_FIELD_AND_SIZE(_fUseDx)                      },
     { _RegPropertyType::Boolean,        CONSOLE_REGISTRY_COPYCOLOR,                     SET_FIELD_AND_SIZE(_fCopyColor)                  },
     { _RegPropertyType::Dword,          L"TextMeasurement",                             SET_FIELD_AND_SIZE(_textMeasurement)             },

src/terminal/adapter/ITermDispatch.hpp

@@ -63,6 +63,7 @@ class Microsoft::Console::VirtualTerminal::ITermDispatch
     virtual bool SetAnsiMode(const bool ansiMode) = 0; // DECANM
     virtual bool SetTopBottomScrollingMargins(const VTInt topMargin, const VTInt bottomMargin) = 0; // DECSTBM
     virtual bool SetLeftRightScrollingMargins(const VTInt leftMargin, const VTInt rightMargin) = 0; // DECSLRM
+    virtual bool EnquireAnswerback() = 0; // ENQ
     virtual bool WarningBell() = 0; // BEL
     virtual bool CarriageReturn() = 0; // CR
     virtual bool LineFeed(const DispatchTypes::LineFeedType lineFeedType) = 0; // IND, NEL, LF, FF, VT

src/terminal/adapter/ITerminalApi.hpp

@@ -64,6 +64,7 @@ namespace Microsoft::Console::VirtualTerminal
         virtual void SetSystemMode(const Mode mode, const bool enabled) = 0;
         virtual bool GetSystemMode(const Mode mode) const = 0;
 
+        virtual void ReturnAnswerback() = 0;
         virtual void WarningBell() = 0;
         virtual void SetWindowTitle(const std::wstring_view title) = 0;
         virtual void UseAlternateScreenBuffer(const TextAttribute& attrs) = 0;

src/terminal/adapter/adaptDispatch.cpp

@@ -2476,6 +2476,18 @@ bool AdaptDispatch::SetLeftRightScrollingMargins(const VTInt leftMargin,
     return true;
 }
 
+// Routine Description:
+// - ENQ - Directs the terminal to send the answerback message.
+// Arguments:
+// - None
+// Return Value:
+// - True.
+bool AdaptDispatch::EnquireAnswerback()
+{
+    _api.ReturnAnswerback();
+    return true;
+}
+
 // Routine Description:
 // - BEL - Rings the warning bell.
 //    Causes the terminal to emit an audible tone of brief duration.

src/terminal/adapter/adaptDispatch.hpp

@@ -101,6 +101,7 @@ namespace Microsoft::Console::VirtualTerminal
                                           const VTInt bottomMargin) override; // DECSTBM
         bool SetLeftRightScrollingMargins(const VTInt leftMargin,
                                           const VTInt rightMargin) override; // DECSLRM
+        bool EnquireAnswerback() override; // ENQ
         bool WarningBell() override; // BEL
         bool CarriageReturn() override; // CR
         bool LineFeed(const DispatchTypes::LineFeedType lineFeedType) override; // IND, NEL, LF, FF, VT

src/terminal/adapter/termDispatch.hpp

@@ -56,6 +56,7 @@ class Microsoft::Console::VirtualTerminal::TermDispatch : public Microsoft::Cons
     bool SetAnsiMode(const bool /*ansiMode*/) override { return false; } // DECANM
     bool SetTopBottomScrollingMargins(const VTInt /*topMargin*/, const VTInt /*bottomMargin*/) override { return false; } // DECSTBM
     bool SetLeftRightScrollingMargins(const VTInt /*leftMargin*/, const VTInt /*rightMargin*/) override { return false; } // DECSLRM
+    bool EnquireAnswerback() override { return false; } // ENQ
     bool WarningBell() override { return false; } // BEL
     bool CarriageReturn() override { return false; } // CR
     bool LineFeed(const DispatchTypes::LineFeedType /*lineFeedType*/) override { return false; } // IND, NEL, LF, FF, VT

src/terminal/adapter/ut_adapter/adapterTest.cpp

@@ -118,6 +118,11 @@ class TestGetSet final : public ITerminalApi
         return _systemMode.test(mode);
     }
 
+    void ReturnAnswerback()
+    {
+        Log::Comment(L"ReturnAnswerback MOCK called...");
+    }
+
     void WarningBell() override
     {
         Log::Comment(L"WarningBell MOCK called...");

src/terminal/parser/OutputStateMachineEngine.cpp

@@ -49,9 +49,7 @@ bool OutputStateMachineEngine::ActionExecute(const wchar_t wch)
     switch (wch)
     {
     case AsciiChars::ENQ:
-        // GH#11946: At some point we may want to add support for the VT
-        // answerback feature, which requires responding to an ENQ control
-        // with a user-defined reply, but until then we just ignore it.
+        _dispatch->EnquireAnswerback();
         break;
     case AsciiChars::BEL:
         _dispatch->WarningBell();