The problem was that system calls got restarted after a signal,
instead of returning EINTR. Thus the read builtin, had no way to
know that a signal occured that could change the course of execution.
Since the code has sprinkled checks for EINTR all over the place,
it is supposed to work properly with non restartable syscalls.
The fix is to use siginterrupt(signo, 1), before setting a signal
handler, to make sure that system calls don't get restarted.
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