clean-temp: make multithread-safe

[Bruno Haible]

With these two patches, the clean-temp module becomes multithread-safe.

The difficult part here is to avoid that two 'close (fd)' calls, one in
normal thread code and one in a signal handler, have the effect that an
unrelated file descriptor, just open()ed in another thread, could be closed.
This issue is solved through the 'asyncsafe-spin' module, as explained in
<https://lists.gnu.org/archive/html/bug-gnulib/2020-07/msg00005.html>.


2020-07-04  Bruno Haible  <bruno@clisp.org>

        clean-temp: Make multithread-safe, part 2.
        * lib/fatal-signal.h: Include <signal.h>.
        (get_fatal_signal_set): New declaration.
        * lib/fatal-signal.c (get_fatal_signal_set): New function.
        * lib/clean-temp.c: Include asyncsafe-spin.h, gl_linked_list.h.
        (struct closeable_fd): New type.
        (fatal_signal_set): New variable.
        (init_fatal_signal_set): New function.
        (asyncsafe_close, asyncsafe_fclose_variant): New functions.
        (cleanup_action): Invoke asyncsafe_close instead of close.
        (create_temp_dir): Invoke init_fatal_signal_set.
        (register_fd): Use a plain linked list. Add a 'struct closeable_fd *'
        element.
        (unregister_fd): Remove function.
        (close_temp): Cleanup descriptors list on the fly. Invoke
        init_fatal_signal_set. Invoke asyncsafe_close instead of close.
        (fclose_variant_temp): New function.
        (fclose_temp, fwriteerror_temp, close_stream_temp): Use it.
        * modules/clean-temp (Depends-on): Add asyncsafe-spin, linked-list.

2020-07-04  Bruno Haible  <bruno@clisp.org>

        clean-temp: Make multithread-safe, part 1.
        * lib/clean-temp.c: Include glthread/lock.h.
        (cleanup_list_lock): New variable.
        (register_temp_file, unregister_temp_file, register_temp_subdir,
        unregister_temp_subdir, cleanup_temp_dir_contents): Use it.
        (create_temp_dir): Likewise. Don't free the old array.
        (descriptors_lock): New variable.
        (register_fd, unregister_fd): Use it.
        * modules/clean-temp (Depends-on): Add lock.

0001-clean-temp-Make-multithread-safe-part-1.patch
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0002-clean-temp-Make-multithread-safe-part-2.patch
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