Hello,

When forking, the finalization pipe file descriptors are
inherited. If the child process spawns a finalization thread, it will
use a copy of its parent finalization pipe file descriptors.

Hence, if the parent tries to stop its finalization thread, by forking
another process for instance, it may stop the child finalization thread
and hang forever waiting for its own finalization thread to stop.

Here's a small reproducer attached. On my machine, the program hangs
around iteration 100. Note that this has previously been discussed
here[1].

The attached patch should fix this by closing the finalization pipe file
descriptor copies in the child right after forking and opening a new
pipe by calling scm_init_finalizer_thread.

Thanks,

Mathieu

[1]: https://issues.guix.gnu.org/41948