lua_pushstring failures result in leaked memory

[daurnimator]

If lua_pushstring longjmps out then the string is never freed.

e.g.

lua-systemd/src/login.c

Line 129 in f905300

lua_pushstring(L, session);

[daurnimator]

Possible solution via pcalling:

static int pointer_to_string (lua_State *L) {
    lua_pushstring(L, lua_topointer(L, 1))
    return 1;
}

static void pushstring_and_free (lua_State *L, char* str) {
    int err;
    lua_pushcfunction(L, pointer_to_string);
    lua_pushlightuserdata(L, str);
    err = lua_pcall(L, 1, 1, 0);
    free(str);
    if (LUA_OK != err) lua_error(L);
}

.....

    pushstring_and_free(L, str);

Uses lua_pushcfunction and lua_pushlightuserdata as they can't fail (as long as there is stack space).
In lua 5.1, lua_cpcall has to be used instead, however it doesn't leave return values on the stack; so needs gymnastics via the registry or something.

[daurnimator]

Alternative solution is via allocating a copy (via e.g. luaL_prepbuffsize), freeing the original, then pushing the buffer result.