Check ptr to string before use it

http/lib.c

@@ -91,6 +91,37 @@ tpl_term(int type, const char *str, size_t len, void *data)
 	}
 }
 
+/**
+ * This function exists because lua_tostring does not use
+ * __tostring metamethod, and this metamethod has to be used
+ * if we want to print Lua userdata correctly.
+ */
+static const char *
+luaT_tolstring(lua_State *L, int idx, size_t *len)
+{
+	if (!luaL_callmeta(L, idx, "__tostring")) {
+		switch (lua_type(L, idx)) {
+			case LUA_TNUMBER:
+			case LUA_TSTRING:
+				lua_pushvalue(L, idx);
+				break;
+			case LUA_TBOOLEAN: {
+				int val = lua_toboolean(L, idx);
+				lua_pushstring(L, val ? "true" : "false");
+				break;
+			}
+			case LUA_TNIL:
+				lua_pushliteral(L, "nil");
+				break;
+			default:
+				lua_pushfstring(L, "%s: %p", luaL_typename(L, idx),
+								lua_topointer(L, idx));
+		}
+	}
+
+	return lua_tolstring(L, -1, len);
+}
+
 static int
 lbox_httpd_escape_html(struct lua_State *L)
 {
@@ -109,11 +140,8 @@ lbox_httpd_escape_html(struct lua_State *L)
 	}
 
 	for (i = 1; i <= top; i++) {
-		if (lua_isnil(L, i)) {
-			luaL_addstring(&b, "nil");
-			continue;
-		}
-		const char *s = lua_tostring(L, i);
+		const char *s = luaT_tolstring(L, i, NULL);
+		int str_idx = lua_gettop(L);
 		for (; *s; s++) {
 			switch(*s) {
 				case '&':
@@ -136,6 +164,7 @@ lbox_httpd_escape_html(struct lua_State *L)
 					break;
 			}
 		}
+		lua_remove(L, str_idx);
 	}
 
 	luaL_pushresult(&b);

test/unit/lib_test.lua

@@ -37,6 +37,11 @@ g.test_template = function()
     local rendered = http_lib.template(template, { t = tt })
     t.assert(#rendered > 10000, "rendered size")
     t.assert_equals(rendered:sub(#rendered - 7, #rendered - 1), "</html>", "rendered eof")
+
+    -- gh-51 Incorrect arguments escaping leads to segfault
+    template = [[<%= {{continue}} %>"]]
+    local result = http_lib.template(template, {continue = '/'})
+    t.assert(result:find('\"') ~= nil)
 end
 
 g.test_parse_request = function()