The InputStateMachine should dispatch Intermediate characters (#1267)

The OutputStateMachine needs to collect "Intermediate" characters to be able to call [`Designate G0 Character Set`](https://invisible-island.net/xterm/ctlseqs/ctlseqs.html#h2-Controls-beginning-with-ESC) (as well as other sequences we don't yet support).

However, the InputStateMachine used by conpty to process input should _not_ collect these characters. The input engine uses `\x1b` as an indicator that a key was pressed with `Alt`. For keys like `/`, we want to dispatch the key immediately, instead of collecting it and leaving us in the Escape state.

OpenConsole.sln

@@ -149,6 +149,9 @@ EndProject
 Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "RendererVt", "src\renderer\vt\lib\vt.vcxproj", "{990F2657-8580-4828-943F-5DD657D11842}"
 EndProject
 Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "VtPipeTerm", "src\tools\vtpipeterm\VtPipeTerm.vcxproj", "{814DBDDE-894E-4327-A6E1-740504850098}"
+	ProjectSection(ProjectDependencies) = postProject
+		{9CBD7DFA-1754-4A9D-93D7-857A9D17CB1B} = {9CBD7DFA-1754-4A9D-93D7-857A9D17CB1B}
+	EndProjectSection
 EndProject
 Project("{8BC9CEB8-8B4A-11D0-8D11-00A0C91BC942}") = "ConEchoKey", "src\tools\echokey\ConEchoKey.vcxproj", "{814CBEEE-894E-4327-A6E1-740504850098}"
 EndProject

src/terminal/parser/IStateMachineEngine.hpp

@@ -52,6 +52,7 @@ namespace Microsoft::Console::VirtualTerminal
 
         virtual bool FlushAtEndOfString() const = 0;
         virtual bool DispatchControlCharsFromEscape() const = 0;
+        virtual bool DispatchIntermediatesFromEscape() const = 0;
     };
 
     inline IStateMachineEngine::~IStateMachineEngine() {}

src/terminal/parser/InputStateMachineEngine.cpp

@@ -883,6 +883,19 @@ bool InputStateMachineEngine::DispatchControlCharsFromEscape() const
     return true;
 }
 
+// Routine Description:
+// - Returns false if the engine wants to be able to collect intermediate
+//   characters in the Escape state. We do _not_ want to buffer any characters
+//   as intermediates, because we use ESC as a prefix to indicate a key was
+//   pressed while Alt was pressed.
+// Return Value:
+// - True iff we should dispatch in the Escape state when we encounter a
+//   Intermediate character.
+bool InputStateMachineEngine::DispatchIntermediatesFromEscape() const
+{
+    return true;
+}
+
 // Method Description:
 // - Retrieves the type of window manipulation operation from the parameter pool
 //      stored during Param actions.

src/terminal/parser/InputStateMachineEngine.hpp

@@ -66,6 +66,7 @@ namespace Microsoft::Console::VirtualTerminal
 
         bool FlushAtEndOfString() const override;
         bool DispatchControlCharsFromEscape() const override;
+        bool DispatchIntermediatesFromEscape() const override;
 
     private:
         const std::unique_ptr<IInteractDispatch> _pDispatch;

src/terminal/parser/OutputStateMachineEngine.cpp

@@ -1317,6 +1317,18 @@ bool OutputStateMachineEngine::DispatchControlCharsFromEscape() const
     return false;
 }
 
+// Routine Description:
+// - Returns false if the engine wants to be able to collect intermediate
+//   characters in the Escape state. We do want to buffer characters as
+//   intermediates. We need them for things like Designate G0 Character Set
+// Return Value:
+// - True iff we should dispatch in the Escape state when we encounter a
+//   Intermediate character.
+bool OutputStateMachineEngine::DispatchIntermediatesFromEscape() const
+{
+    return false;
+}
+
 // Routine Description:
 // - Converts a hex character to its equivalent integer value.
 // Arguments:

src/terminal/parser/OutputStateMachineEngine.hpp

@@ -58,6 +58,7 @@ namespace Microsoft::Console::VirtualTerminal
 
         bool FlushAtEndOfString() const override;
         bool DispatchControlCharsFromEscape() const override;
+        bool DispatchIntermediatesFromEscape() const override;
 
         void SetTerminalConnection(Microsoft::Console::ITerminalOutputConnection* const pTtyConnection,
                                    std::function<bool()> pfnFlushToTerminal);

src/terminal/parser/stateMachine.cpp

@@ -853,8 +853,16 @@ void StateMachine::_EventEscape(const wchar_t wch)
     }
     else if (s_IsIntermediate(wch))
     {
-        _ActionCollect(wch);
-        _EnterEscapeIntermediate();
+        if (_pEngine->DispatchIntermediatesFromEscape())
+        {
+            _ActionEscDispatch(wch);
+            _EnterGround();
+        }
+        else
+        {
+            _ActionCollect(wch);
+            _EnterEscapeIntermediate();
+        }
     }
     else if (s_IsCsiIndicator(wch))
     {

src/terminal/parser/ut_parser/InputEngineTest.cpp

@@ -229,6 +229,7 @@ class Microsoft::Console::VirtualTerminal::InputEngineTest
     TEST_METHOD(CSICursorBackTabTest);
     TEST_METHOD(AltBackspaceTest);
     TEST_METHOD(AltCtrlDTest);
+    TEST_METHOD(AltIntermediateTest);
 
     friend class TestInteractDispatch;
 };
@@ -765,3 +766,63 @@ void InputEngineTest::AltCtrlDTest()
     Log::Comment(NoThrowString().Format(L"Processing \"\\x1b\\x04\""));
     _stateMachine->ProcessString(seq);
 }
+
+void InputEngineTest::AltIntermediateTest()
+{
+    // Tests GH#1209. When we process a alt+key combination where the key just
+    // so happens to be an intermediate character, we should make sure that an
+    // immediately subsequent ctrl character is handled correctly.
+    TestState testState;
+
+    // We'll test this by creating both a TerminalInput and an
+    // InputStateMachine, and piping the KeyEvents generated by the
+    // InputStateMachine into the TerminalInput.
+    std::wstring expectedTranslation{};
+
+    // First create the callback TerminalInput will call - this will be
+    // triggered second, after both the state machine and the TerminalInput have
+    // translated the characters.
+    auto pfnTerminalInputCallback = [&](std::deque<std::unique_ptr<IInputEvent>>& inEvents) {
+        // Get all the characters:
+        std::wstring wstr = L"";
+        for (auto& ev : inEvents)
+        {
+            if (ev->EventType() == InputEventType::KeyEvent)
+            {
+                auto& k = static_cast<KeyEvent&>(*ev);
+                auto wch = k.GetCharData();
+                wstr += wch;
+            }
+        }
+
+        VERIFY_ARE_EQUAL(expectedTranslation, wstr);
+    };
+    TerminalInput terminalInput{ pfnTerminalInputCallback };
+
+    // Create the callback that's fired when the state machine wants to write
+    // input. We'll take the events and put them straight into the
+    // TerminalInput.
+    auto pfnInputStateMachineCallback = [&](std::deque<std::unique_ptr<IInputEvent>>& inEvents) {
+        for (auto& ev : inEvents)
+        {
+            terminalInput.HandleKey(ev.get());
+        }
+    };
+    auto inputEngine = std::make_unique<InputStateMachineEngine>(new TestInteractDispatch(pfnInputStateMachineCallback, &testState));
+    auto stateMachine = std::make_unique<StateMachine>(inputEngine.release());
+    VERIFY_IS_NOT_NULL(stateMachine);
+    testState._stateMachine = stateMachine.get();
+
+    // Write a Alt+/, Ctrl+e pair to the input engine, then take it's output and
+    // run it through the terminalInput translator. We should get ^[/^E back
+    // out.
+    std::wstring seq = L"\x1b/";
+    expectedTranslation = seq;
+    Log::Comment(NoThrowString().Format(L"Processing \"\\x1b/\""));
+    stateMachine->ProcessString(seq);
+
+    seq = L"\x05"; // 0x05 is ^E
+    expectedTranslation = seq;
+    Log::Comment(NoThrowString().Format(L"Processing \"\\x05\""));
+    stateMachine->ProcessString(seq);
+}