[PATCH] Fix FreeBSD/Darwin and Linux sendfile

[Jesse Miller]

https://github.com/samba-team/samba/pull/76

Use poll() to emulate a blocking sendfile().

This avoids spinning on EAGAIN/EWOULDBLOCK under Linux implementation,
which greatly reduces CPU usage.

(https://lists.samba.org/archive/samba/2013-September/175838.html)

This fixes FreeBSD/Darwin implementation, which under most cases did
not work, it would break early when nwritten == 0.  Which would happen
anytime it spun on EAGAIN writing 0 bytes.

(https://forums.freebsd.org/threads/55017/)


I believe other platforms sys_sendfile() code will also spin, but I don't have access to those platforms for testing, so they remain unchanged.

Jesse

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