Both the snd_ackcb and snd_datacb callbacks were created and destroyed right after sending every packet.
Whenever TCP_REXMIT event occurred due to TCP send timeout, TCP_REXMIT was ignored because
snd_ackcb callback had been destroyed by the time.
The issue is fixed as follows:
- both the snd_ackcb and snd_datacb callbacks are combined into one snd_cb callback
(the same way as in tcp_send_unbuffered.c).
- the snd_cb callback lives until all requested data (via sendfile) is sent,
including all ACKs and possible retransmissions.
As a positive side effect of the code optimization / fix, sendfile TCP payload throughput is increased.
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