telnet: make it work through proxy tunnels

[icing]

Change telnet's antiquated corner places where it still used the connection socket directly. Use the transfer send call instead which goes the proper way thought the connection filters.

[Copilot]

🟡 Changes recommended

The new sends ignore partial and zero-byte writes, which can truncate Telnet negotiation messages.

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Pull request overview

Routes Telnet control messages through connection filters to support proxy tunnels.

Changes:

• Replaces direct socket writes with Curl_xfer_send().
• Reports filter-aware send errors.

File summaries

File	Description
lib/telnet.c	Routes Telnet negotiation and suboption writes through transfer filters.

Review details

Suppressed comments (4)

lib/telnet.c:706

• This send may consume fewer than 3 bytes, including zero bytes when the filter is blocked, while returning CURLE_OK. The code then sends the window-size body, producing a malformed subnegotiation stream. Retry the unsent header before proceeding to the body.

    result = Curl_xfer_send(data, tn->subbuffer, 3, FALSE, &nwritten);

lib/telnet.c:713

• The footer is discarded if Curl_xfer_send() reports a successful short or zero-byte write. This leaves the NAWS subnegotiation unterminated; retry until both footer bytes have been consumed and return any send failure to the transfer.

    result = Curl_xfer_send(data, tn->subbuffer + 7, 2, FALSE, &nwritten);

lib/telnet.c:1014

• This XDISPLOC response is silently truncated whenever the connection filter consumes fewer than len bytes, because nwritten is never checked. Loop or buffer the unconsumed suffix before treating the suboption as sent.

    result = Curl_xfer_send(data, temp, len, FALSE, &nwritten);

lib/telnet.c:1046

• This environment response can be much larger than one filter write, but the code ignores nwritten and returns success after a partial or zero-byte write. Preserve and retry the remainder so the Telnet subnegotiation is not truncated.

    result = Curl_xfer_send(data, temp, len, FALSE, &nwritten);

• Files reviewed: 1/1 changed files
• Comments generated: 3
• Review effort level: Balanced

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[icing]

What even is this?

[icing]

What am I doing here?

[icing]

If this were a human, violence would creep into my mind.

[bagder]

In the mean time, I made this: #22943

[icing]

This protocol handler, like all the pingpoing ones, is from another age and time. When Elves still wandered the earth and unicorns were roaming the forests...

[bagder]

the good old days! 🦄