ftp: split ftp_state_use_port into sub functions

[bagder]

For readability and reduced complexity.

[Copilot]

Pull request overview

This PR refactors the large ftp_state_use_port() function into seven smaller, focused sub-functions to improve code readability and reduce complexity. The refactoring splits the original ~220-line function into logical steps: parsing the FTPPORT string, determining the default host, resolving the host, opening a socket, binding the socket, listening, and sending the FTP command. The main function now orchestrates these steps with clear error handling between each phase.

Changes:

• Extracted parsing logic into ftp_port_parse_string() to handle the CURLOPT_FTPPORT string format
• Extracted socket setup and FTP protocol commands into dedicated helper functions (ftp_port_open_socket, ftp_port_bind_socket, ftp_port_listen, ftp_port_send_command)
• Extracted fallback logic into ftp_port_default_host() and ftp_port_resolve_host()

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[bagder]

augment review

[augmentcode]

🤖 Augment PR Summary

Summary: Refactors active-mode FTP setup by splitting ftp_state_use_port() into focused helper functions to reduce complexity.

Changes:

• Extracted parsing of CURLOPT_FTPPORT into ftp_port_parse_string() (address/interface + optional port range).
• Added ftp_port_default_host() fallback to derive a local IP from the control connection when no host is specified.
• Centralized DNS resolving into ftp_port_resolve_host().
• Moved socket creation into ftp_port_open_socket() and binding logic into ftp_port_bind_socket() (including non-local bind fallback).
• Split listen and command send steps into ftp_port_listen() and ftp_port_send_command().
• Kept existing behavior of installing the listening socket on SECONDARYSOCKET via Curl_conn_tcp_listen_set() and enabling data-SSL when configured.

Technical Notes: Error handling/cleanup remains centralized in ftp_state_use_port() (DNS unlink, state transition to FTP_STOP, and socket close on failure).

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[augmentcode]

Review completed. 2 suggestions posted.

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[bagder]

augment review

[augmentcode]

Review completed. 1 suggestions posted.

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[bagder]

@aisle-analyzer

[aisle-research-bot]

🔒 Aisle Security Analysis

We found 1 potential security issue(s) in this PR:

#	Severity	Title
1	🔵 Low	Incorrect parsing of unbracketed IPv6 in CURLOPT_FTPPORT may trigger unintended DNS resolution and wrong PORT/EPRT target

See details in the comment below.

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Analyzed PR: #20685 at commit f568b60