[lldb] Correctly restore the cursor column after resizing the statusline

lldb/include/lldb/Core/Debugger.h

@@ -133,6 +133,8 @@ class Debugger : public std::enable_shared_from_this<Debugger>,
 
   void SetAsyncExecution(bool async);
 
+  CursorPosition GetIOHandlerCursorPosition();
+
   lldb::FileSP GetInputFileSP() { return m_input_file_sp; }
   File &GetInputFile() { return *m_input_file_sp; }
 

lldb/include/lldb/Core/IOHandler.h

@@ -10,6 +10,7 @@
 #define LLDB_CORE_IOHANDLER_H
 
 #include "lldb/Host/Config.h"
+#include "lldb/Host/Terminal.h"
 #include "lldb/Utility/CompletionRequest.h"
 #include "lldb/Utility/Flags.h"
 #include "lldb/Utility/Predicate.h"
@@ -113,6 +114,10 @@ class IOHandler {
 
   virtual const char *GetHelpPrologue() { return nullptr; }
 
+  virtual CursorPosition GetCursorPosition() const {
+    return {std::nullopt, std::nullopt};
+  }
+
   int GetInputFD();
 
   int GetOutputFD();
@@ -404,6 +409,8 @@ class IOHandlerEditline : public IOHandler {
 
   void PrintAsync(const char *s, size_t len, bool is_stdout) override;
 
+  virtual CursorPosition GetCursorPosition() const override;
+
 private:
 #if LLDB_ENABLE_LIBEDIT
   bool IsInputCompleteCallback(Editline *editline, StringList &lines);

lldb/include/lldb/Host/Editline.h

@@ -35,6 +35,7 @@
 #include <vector>
 
 #include "lldb/Host/StreamFile.h"
+#include "lldb/Host/Terminal.h"
 #include "lldb/lldb-private.h"
 
 #if !defined(_WIN32) && !defined(__ANDROID__)
@@ -267,6 +268,8 @@ class Editline {
 
   size_t GetTerminalHeight() { return m_terminal_height; }
 
+  CursorPosition GetCursorPosition();
+
 private:
   /// Sets the lowest line number for multi-line editing sessions.  A value of
   /// zero suppresses line number printing in the prompt.

lldb/include/lldb/Host/Terminal.h

@@ -169,6 +169,11 @@ class TerminalState {
   lldb::pid_t m_process_group = -1;       ///< Cached process group information.
 };
 
+struct CursorPosition {
+  std::optional<unsigned> cols;
+  std::optional<unsigned> rows;
+};
+
 } // namespace lldb_private
 
 #endif // LLDB_HOST_TERMINAL_H

lldb/source/Core/Debugger.cpp

@@ -1240,6 +1240,14 @@ void Debugger::DispatchInputEndOfFile() {
     reader_sp->GotEOF();
 }
 
+CursorPosition Debugger::GetIOHandlerCursorPosition() {
+  std::lock_guard<std::recursive_mutex> guard(m_io_handler_stack.GetMutex());
+  IOHandlerSP reader_sp(m_io_handler_stack.Top());
+  if (reader_sp)
+    return reader_sp->GetCursorPosition();
+  return {std::nullopt, std::nullopt};
+}
+
 void Debugger::ClearIOHandlers() {
   // The bottom input reader should be the main debugger input reader.  We do
   // not want to close that one here.

lldb/source/Core/IOHandler.cpp

@@ -634,6 +634,14 @@ void IOHandlerEditline::GotEOF() {
 #endif
 }
 
+CursorPosition IOHandlerEditline::GetCursorPosition() const {
+#if LLDB_ENABLE_LIBEDIT
+  if (m_editline_up)
+    return m_editline_up->GetCursorPosition();
+#endif
+  return {std::nullopt, std::nullopt};
+}
+
 void IOHandlerEditline::PrintAsync(const char *s, size_t len, bool is_stdout) {
 #if LLDB_ENABLE_LIBEDIT
   if (m_editline_up) {

lldb/source/Core/Statusline.cpp

@@ -28,6 +28,7 @@
 #define ANSI_TO_START_OF_ROW ESCAPE "[%u;1f"
 #define ANSI_REVERSE_VIDEO ESCAPE "[7m"
 #define ANSI_UP_ROWS ESCAPE "[%dA"
+#define ANSI_SET_COLUMN_N ESCAPE "[%uG"
 
 using namespace lldb;
 using namespace lldb_private;
@@ -103,19 +104,35 @@ void Statusline::UpdateScrollWindow(ScrollWindowMode mode) {
       (mode == DisableStatusline) ? m_terminal_height : m_terminal_height - 1;
 
   LockedStreamFile locked_stream = stream_sp->Lock();
+
+  if (mode == EnableStatusline) {
+    // Get the cursor position before we potentially change the cursor position.
+    CursorPosition cursor_position = m_debugger.GetIOHandlerCursorPosition();
+
+    // Move everything on the screen up to make space for the statusline. This
+    // is going to move the cursor to the start of the next line which we need
+    // to undo.
+    locked_stream << '\n';
+
+    // First move the cursor back up. We can't use ANSI_SAVE/RESTORE_CURSOR
+    // here, because the old and new position differ if everything on the screen
+    // moved up.
+    locked_stream.Printf(ANSI_UP_ROWS, 1);
+
+    // Finally move the cursor back to the correct column, if the IOHandler was
+    // able to tell us where that was.
+    if (cursor_position.cols)
+      locked_stream.Printf(ANSI_SET_COLUMN_N, *cursor_position.cols);
+  }
+
+  // Adjust the scroll window.
   locked_stream << ANSI_SAVE_CURSOR;
   locked_stream.Printf(ANSI_SET_SCROLL_ROWS, scroll_height);
   locked_stream << ANSI_RESTORE_CURSOR;
-  switch (mode) {
-  case EnableStatusline:
-    // Move everything on the screen up.
-    locked_stream.Printf(ANSI_UP_ROWS, 1);
-    locked_stream << '\n';
-    break;
-  case DisableStatusline:
+
+  if (mode == DisableStatusline) {
     // Clear the screen below to hide the old statusline.
     locked_stream << ANSI_CLEAR_BELOW;
-    break;
   }
 }
 

lldb/source/Host/common/Editline.cpp

@@ -398,6 +398,20 @@ int Editline::GetLineIndexForLocation(CursorLocation location, int cursor_row) {
   return line;
 }
 
+CursorPosition Editline::GetCursorPosition() {
+  if (!m_editline)
+    return {};
+
+  const LineInfoW *info = el_wline(m_editline);
+  if (!info)
+    return {};
+
+  const size_t editline_cursor_col =
+      (int)((info->cursor - info->buffer) + GetPromptWidth()) + 1;
+
+  return {editline_cursor_col, std::nullopt};
+}
+
 void Editline::MoveCursor(CursorLocation from, CursorLocation to) {
   const LineInfoW *info = el_wline(m_editline);
   int editline_cursor_position =