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Here's my initial fix.
If fcntl(errpipe_write, F_DUPFD, 3) is widely available this could be shrunk a bit to avoid the for loop potentially calling dup a few times and tracking the wasted fds to close later.
Otherwise if it isn't I'd rather not bother with F_DUPFD as this code takes the optimal path when F_DUPFD_CLOEXEC is supported (true on all modern linux systems) and should cause no harm beyond a couple extra dup and close syscalls otherwise.
Note: Linux pipe2() support appears in kernels a few revisions after F_DUPFD_CLOEXEC support so the good behavior when pipe2 exists will be kept in this situation as that also means F_DUPFD_CLOEXEC support should exist. The code will work regardless of that (incase someone has a frankenkernel, or other OSes choose to implement one but not the other).