The code made an implicit assumption that the 'len` variable passed
to the sendfile(2) syscall is not modified by the operating system
in case of an error other than EAGAIN or EINTR.
The man page leaves this unspecified on FreeBSD, DragonFly and
Darwin, so better check the error code which returns a valid
value in `len` explicitly (only EAGAIN and EINTR).
This fixes the test case for sendfile on DragonFly.
PR-URL: https://github.com/libuv/libuv/pull/466
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>
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