https://github.com/python/cpython/blob/4a08c4c469d36f99d3a5e0f17ad82ab35dcf2835/Modules/_posixsubprocess.c#L717

    child_exec(exec_array, argv, envp, cwd,
               p2cread, p2cwrite, c2pread, c2pwrite,
               errread, errwrite, errpipe_read, errpipe_write,
               close_fds, restore_signals, call_setsid,
               call_setgid, gid, call_setgroups, groups_size, groups,
               call_setuid, uid, child_umask, child_sigmask,
               py_fds_to_keep, preexec_fn, preexec_fn_args_tuple);

----------------

Py_NO_INLINE static void
child_exec(char *const exec_array[],
           char *const argv[],
           char *const envp[],
           const char *cwd,
           int p2cread, int p2cwrite,
           int c2pread, int c2pwrite,


-------------

So, calling child_exec pushes to stack GUARANTEED. This is the bug. In fact everything works, but is too weak and break at any moment.

Again. Please comment calling vfork() for now. And re-implement child part later.