[lldb-dap] Improve the runInTerminal ux.

This updates lldb-dap to clear the screen when using `"console": "integratedTerminal"` or `"console": "externalTerminal"`.

VSCode will reuse the same terminal for a given debug configuration. After the process exits it will return to the shell but if the debug session is launched again it will be invoked in the same terminal. VSCode is sending the termainl the launch args as terminal input which means the terminal would now have a string like `lldb-dap --comm-file ... --launch-target ...` and the scrollback buffer from any previous output or shell commands used in the terminal.

To address this, I've updated LaunchRunInTerminalTarget to reset the cursor, clear the screen and clear the scrollback buffer to soft 'reset' the terminal prior to launching the process.

Author: ashgti

lldb/tools/lldb-dap/tool/lldb-dap.cpp

@@ -74,6 +74,7 @@ typedef int socklen_t;
 #include <netinet/in.h>
 #include <sys/socket.h>
 #include <sys/un.h>
+#include <termios.h>
 #include <unistd.h>
 #endif
 
@@ -121,6 +122,15 @@ class LLDBDAPOptTable : public llvm::opt::GenericOptTable {
 };
 } // anonymous namespace
 
+#define ESCAPE "\x1b"
+#define CSI ESCAPE "["
+// Move the cursor to 0,0
+#define ANSI_CURSOR_HOME CSI "H"
+// Clear the screen buffer
+#define ANSI_ERASE_SCREEN CSI "2J"
+// Clear the scroll back buffer
+#define ANSI_ERASE_SCROLLBACK CSI "3J"
+
 static void PrintHelp(LLDBDAPOptTable &table, llvm::StringRef tool_name) {
   std::string usage_str = tool_name.str() + " options";
   table.printHelp(llvm::outs(), usage_str.c_str(), "LLDB DAP", false);
@@ -261,6 +271,14 @@ static llvm::Error LaunchRunInTerminalTarget(llvm::opt::Arg &target_arg,
       files.push_back(files.back());
     if (llvm::Error err = SetupIORedirection(files))
       return err;
+  } else if ((isatty(STDIN_FILENO) != 0) &&
+             llvm::StringRef(getenv("TERM")).starts_with_insensitive("xterm")) {
+    // Clear the screen.
+    llvm::outs() << ANSI_CURSOR_HOME ANSI_ERASE_SCREEN ANSI_ERASE_SCROLLBACK;
+    // VSCode will reuse the same terminal for the same debug configuration
+    // between runs. Clear the input buffer prior to starting the new process so
+    // prior input is not carried forward to the new debug session.
+    tcflush(STDIN_FILENO, TCIFLUSH);
   }
 
   RunInTerminalLauncherCommChannel comm_channel(comm_file);