Add on_writable/dmtcp_send_space() send-side backpressure - #1
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dmtcp_send() short-writes once the 4 KB per-connection outbound buffer fills, but nothing signalled when an ACK freed room again - so any sender of a payload larger than the buffer (a file, a firmware image) had to poll on its own timer or thread, blind to the one event that matters. dmtcp_writable_handler_t is edge-triggered like POSIX EPOLLOUT: a short dmtcp_send() arms conn->writable_pending, the next space-reclaiming ACK fires the callback once and disarms it. A caller that never fills the buffer is never woken. dmtcp_send_space() reports the current free room so a handler can size its next chunk without calling back in. Delivery order within a segment is on_data -> on_writable -> terminal callback, and the callback is invoked after conn->lock is released, the same discipline every other callback here follows. Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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dmtcp_send() short-writes once the 4 KB per-connection outbound buffer fills, but nothing signalled when an ACK freed room again - so any sender of a payload larger than the buffer (a file, a firmware image) had to poll on its own timer or thread, blind to the one event that matters.
dmtcp_writable_handler_t is edge-triggered like POSIX EPOLLOUT: a short dmtcp_send() arms conn->writable_pending, the next space-reclaiming ACK fires the callback once and disarms it. A caller that never fills the buffer is never woken. dmtcp_send_space() reports the current free room so a handler can size its next chunk without calling back in.
Delivery order within a segment is on_data -> on_writable -> terminal callback, and the callback is invoked after conn->lock is released, the same discipline every other callback here follows.