[LLDB-lua] modify Lua's 'print' to respect 'io.stdout'

[tammela]

Summary

This patch changes the implementation of Lua's print() function to
respect io.stdout.

The original implementation uses lua_writestring() internally, which is
hardcoded to stdout.

[JDevlieghere]

This is great, it's been bothering me a lot. Thank Pedro!

Could you please add a test case (something similar to `lldb/test/Shell/ScriptInterpreter/Lua/io.test should be fine).

[tammela]

Add test case

[JDevlieghere]

LGTM

lldb/source/Plugins/ScriptInterpreter/Lua/Lua.h

 //===-- ScriptInterpreterLua.h ----------------------------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef liblldb_Lua_h_
 #define liblldb_Lua_h_
 
 #include "lldb/lldb-types.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Support/Error.h"
 
 #include "lua.hpp"
 
 #include <mutex>
 
 namespace lldb_private {
 
 extern "C" {
 int luaopen_lldb(lua_State *L);
 }
 
 class Lua {
 public:
-  Lua() : m_lua_state(luaL_newstate()) {
-    assert(m_lua_state);
-    luaL_openlibs(m_lua_state);
-    luaopen_lldb(m_lua_state);
-  }
-
-  ~Lua() {
-    assert(m_lua_state);
-    lua_close(m_lua_state);
-  }
+  Lua();
+  ~Lua();
 
   llvm::Error Run(llvm::StringRef buffer);
   llvm::Error LoadModule(llvm::StringRef filename);
   llvm::Error ChangeIO(FILE *out, FILE *err);
 
 private:
   lua_State *m_lua_state;
 };
 
 } // namespace lldb_private
 
 #endif // liblldb_Lua_h_

lldb/source/Plugins/ScriptInterpreter/Lua/Lua.cpp

 //===-- Lua.cpp -----------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "Lua.h"
 #include "lldb/Host/FileSystem.h"
 #include "lldb/Utility/FileSpec.h"
 #include "llvm/Support/FormatVariadic.h"
 
 using namespace lldb_private;
 using namespace lldb;
 
+static int lldb_print(lua_State *L) {
+  int n = lua_gettop(L);
+  lua_getglobal(L, "io");
+  lua_getfield(L, -1, "stdout");
+  lua_getfield(L, -1, "write");
+  for (int i = 1; i <= n; i++) {
+    lua_pushvalue(L, -1); // write()
+    lua_pushvalue(L, -3); // io.stdout
+    luaL_tolstring(L, i, nullptr);
+    lua_pushstring(L, i != n ? "\t" : "\n");
+    lua_call(L, 3, 0);
+  }
+  return 0;
+}
+
+Lua::Lua() {
+  m_lua_state = luaL_newstate();
+  assert(m_lua_state);
+  luaL_openlibs(m_lua_state);
+  luaopen_lldb(m_lua_state);
+  lua_pushcfunction(m_lua_state, lldb_print);
+  lua_setglobal(m_lua_state, "print");
+}
+
+Lua::~Lua() {
+  assert(m_lua_state);
+  lua_close(m_lua_state);
+}
+
 llvm::Error Lua::Run(llvm::StringRef buffer) {
   int error =
       luaL_loadbuffer(m_lua_state, buffer.data(), buffer.size(), "buffer") ||
       lua_pcall(m_lua_state, 0, 0, 0);
   if (error == LUA_OK)
     return llvm::Error::success();
 
   llvm::Error e = llvm::make_error<llvm::StringError>(