rr invents spurious syscall restart

[Keno]

I this test that I wrote while investigating #3285, rr changes the behavior of the execution: https://gist.github.com/Keno/eb3a40a9e63ba33c7d49b82a519c04f7

keno@antarctic:~/rr-build$ ./bin/desched_sigreturn
EXIT-SUCCESS
keno@antarctic:~/rr-build$ ./bin/rr record ./bin/desched_sigreturn
rr: Saving execution to trace directory `/home/keno/.local/share/rr/desched_sigreturn-16'.
FAILED at /home/keno/rr/src/test/desched_sigreturn.c:78: !(0) errno:0 (Success)
Aborted
keno@antarctic:~/rr-build$ ./bin/rr record -n ./bin/desched_sigreturn
rr: Saving execution to trace directory `/home/keno/.local/share/rr/desched_sigreturn-17'.
FAILED at /home/keno/rr/src/test/desched_sigreturn.c:78: !(0) errno:0 (Success)
Aborted

[Keno]

Never mind, I forgot there's a difference between kill and tgkill. This is just an artifact of the signal targeting the wrong task, which is a well-known edge case of rr.