write_queued_bytes_without_blocking() used to allocate a Vector sized
to the entire queued response buffer and memcpy every queued byte into
it, on every curl on_data_received callback. When the client pipe was
slower than the network, the buffer grew, and each arriving chunk
triggered a full copy of everything still queued. With enough pending
data this added up to tens of gigabytes of memcpy per request and
stalled RequestServer for tens of seconds.
Drain the stream in a loop using peek_some_contiguous() + send()
directly from the underlying chunk, and discard exactly what the
socket accepted. No intermediate buffer, no copy. The loop exits on
EAGAIN and enables the writer notifier, matching the previous
back-pressure behavior.
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