Setting SIGCHLD back to default (i.e. ignored) makes waitpid hang on an
old SIGCHLD that was still in the pipe?
This happens running test_important_plugin with developer=1:
(or with dev=0 and build-in plugins subscribed to "shutdown")
0 0x00007ff8336b6437 in __GI___waitpid (pid=-1, stat_loc=0x0, options=1) at ../sysdeps/unix/sysv/linux/waitpid.c:30
1 0x000055fb468f733a in sigchld_rfd_in (conn=0x55fb47c7cfc8, ld=0x55fb47bdce58) at lightningd/lightningd.c:785
2 0x000055fb469bcc6b in next_plan (conn=0x55fb47c7cfc8, plan=0x55fb47c7cfe8) at ccan/ccan/io/io.c:59
3 0x000055fb469bd80b in do_plan (conn=0x55fb47c7cfc8, plan=0x55fb47c7cfe8, idle_on_epipe=false) at ccan/ccan/io/io.c:407
4 0x000055fb469bd849 in io_ready (conn=0x55fb47c7cfc8, pollflags=1) at ccan/ccan/io/io.c:417
5 0x000055fb469bfa26 in io_loop (timers=0x55fb47c41198, expired=0x7ffdf4be9028) at ccan/ccan/io/poll.c:453
6 0x000055fb468f1be9 in io_loop_with_timers (ld=0x55fb47bdce58) at lightningd/io_loop_with_timers.c:21
7 0x000055fb46924817 in shutdown_plugins (ld=0x55fb47bdce58) at lightningd/plugin.c:2114
8 0x000055fb468f7c92 in main (argc=22, argv=0x7ffdf4be9228) at lightningd/lightningd.c:1195
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