Fixes <https://bugs.gnu.org/68087>.
* libguile/scmsigs.h (scm_i_signals_pre_fork, scm_i_signals_post_fork):
New declarations.
(scm_i_signal_delivery_thread): Change type to SCM..
* libguile/threads.c (scm_all_threads): Adjust accordingly and exclude
threads that have ‘t->exited’. Access ‘thread_count’ after grabbing
‘thread_admin_mutex’.
* libguile/posix.c (scm_fork): Add calls to ‘scm_i_signals_pre_fork’ and
‘scm_i_signals_post_fork’.
* libguile/scmsigs.c (signal_delivery_thread): Close signal_pipe[0] upon
exit and set it to -1.
(once): New file-global variable, moved from…
(scm_i_ensure_signal_delivery_thread): … here.
(stop_signal_delivery_thread, scm_i_signals_pre_fork)
(scm_i_signals_post_fork): New functions.
* test-suite/standalone/test-sigaction-fork: New file.
* test-suite/standalone/Makefile.am (check_SCRIPTS, TESTS): Add it.
