The control server wrote each reply with writeln!(out, "{r}") on the
bare TcpStream. serde_json formats a Value one piece at a time, and
each piece (a brace, a quotation mark, a key, a colon, a number) was
sent as a write of its own: the 17 MB document.inspect reply of a
document with 50,000 objects on one layer was sent in 7,857,200 writes.
On Windows 11, sending that reply to a client over loopback took 34 s,
and sending the 28 MB document.json reply took 57 s.
vectorcraft-cli mcp reads every reply from a running app over this
connection.
write_line now writes each reply, each refusal and the "too many
control connections" line through a 64 KiB BufWriter, then flushes
it. The 17 MB reply takes 262 writes. Sending it now takes 0.09 s,
and sending the 28 MB reply takes 0.2 s. The bytes are the same.
Docs: docs/target-app-parity.md.
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