One way to handle this is to switch tcp_read() to use the underlying file O_NONBLOCK
flag, as other socket operations do. And let SPLICE_F_NONBLOCK control the pipe output only.

Thanks Eric, this seems reasonable from my userspace perspective.

I admit I don't understand why SPLICE_F_NONBLOCK exists, it seems very
un-unixy to have a syscall completely ignore the NONBLOCK flag of the
fd it is called on. Ie setting NONBLOCK on the pipe itself does
nothing when using splice..

Jason