From 69b71a69197de0cb6f7f58f5d7c55d9a9a6e529d Mon Sep 17 00:00:00 2001
From: Roberto Ierusalimschy <roberto@inf.puc-rio.br>
Date: Mon, 26 Oct 2020 11:15:51 -0300
Subject: [PATCH] _PROMPT can have non-string values

'get_prompt' uses 'luaL_tolstring' to convert _PROMPT or _PROMPT2
value to a string. That conversion may invoke a '__tostring'
metamethod.
---
 lua.c           | 16 ++++++++++------
 testes/main.lua | 27 +++++++++++++++++++++++++++
 2 files changed, 37 insertions(+), 6 deletions(-)

diff --git a/lua.c b/lua.c
index 454ce12f..b5b884b6 100644
--- a/lua.c
+++ b/lua.c
@@ -416,14 +416,18 @@ static int handle_luainit (lua_State *L) {
 
 
 /*
-** Returns the string to be used as a prompt by the interpreter.
+** Return the string to be used as a prompt by the interpreter. Leave
+** the string (or nil, if using the default value) on the stack, to keep
+** it anchored.
 */
 static const char *get_prompt (lua_State *L, int firstline) {
-  const char *p;
-  lua_getglobal(L, firstline ? "_PROMPT" : "_PROMPT2");
-  p = lua_tostring(L, -1);
-  if (p == NULL) p = (firstline ? LUA_PROMPT : LUA_PROMPT2);
-  return p;
+  if (lua_getglobal(L, firstline ? "_PROMPT" : "_PROMPT2") == LUA_TNIL)
+    return (firstline ? LUA_PROMPT : LUA_PROMPT2);  /* use the default */
+  else {  /* apply 'tostring' over the value */
+    const char *p = luaL_tolstring(L, -1, NULL);
+    lua_remove(L, -2);  /* remove original value */
+    return p;
+  }
 }
 
 /* mark in error messages for incomplete statements */
diff --git a/testes/main.lua b/testes/main.lua
index d2d602de..56959abd 100644
--- a/testes/main.lua
+++ b/testes/main.lua
@@ -287,6 +287,33 @@ RUN([[lua "-e_PROMPT='%s'" -i < %s > %s]], prompt, prog, out)
 local t = getoutput()
 assert(string.find(t, prompt .. ".*" .. prompt .. ".*" .. prompt))
 
+-- using the prompt default
+prepfile[[ --
+a = 2
+]]
+RUN([[lua -i < %s > %s]], prog, out)
+local t = getoutput()
+prompt = "> "    -- the default
+assert(string.find(t, prompt .. ".*" .. prompt .. ".*" .. prompt))
+
+
+-- non-string prompt
+prompt =
+  "local C = 0;\z
+   _PROMPT=setmetatable({},{__tostring = function () \z
+     C = C + 1; return C end})"
+prepfile[[ --
+a = 2
+]]
+RUN([[lua -e "%s" -i < %s > %s]], prompt, prog, out)
+local t = getoutput()
+assert(string.find(t, [[
+1 --
+2a = 2
+3
+]], 1, true))
+
+
 -- test for error objects
 prepfile[[
 debug = require "debug"