Limit electrum rpc request line length

[agoodminute]

Summary

Connection::parse_requests read each incoming Electrum RPC request as a single line via read_until(b'\n', ...), with no limit on how large that line could grow before a newline arrived. This adds a bounded line reader and a configurable cap on request size.

Why

A request line with no upper bound means a single slow or unusually large request could grow the read buffer without limit before being rejected or completed. Capping the size up front keeps per-connection memory usage predictable regardless of what a client sends.

Changes

• New Connection::read_bounded_line — reads a \n-terminated request line, checking the accumulated size against the configured limit before extending the buffer, rather than only after the read completes.
• New config option --electrum-rpc-max-request-num-bytes (electrum_rpc_max_request_num_bytes), default 1 MiB. A request line exceeding the limit is rejected and the connection is dropped. 0 disables the limit.
• parse_requests / reader_thread now thread this limit through to the bounded reader instead of reading unbounded lines.

[EddieHouston]

lgtm 82ef302