When non-blocking connect completes the error code can be read using
getsockopt(SO_ERROR). Doing this is important for identifying failure
to connect, especially if POLLERR and POLLHUP were not employed by the
user.
The QEMU iscsi block driver does not use POLLERR/POLLHUP and depends on
SO_ERROR to detect connection failure.
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
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