feat: make MODE Z configurable

[drakkan]

This is a non-standard FTP extension and is not supported by major clients:

https://trac.filezilla-project.org/ticket/2977
https://winscp.net/forum/viewtopic.php?t=25351

Additionally, it could potentially increase the attack surface, although I have not been able to fully verify this:

$ lftp ftp://127.0.0.1:2121
lftp 127.0.0.1:~> debug on;
lftp 127.0.0.1:~> set ftp:use-mode-z yes;
lftp 127.0.0.1:~> login a password
lftp a@127.0.0.1:~> ls
**** zlib inflate error: incorrect header check
ls: Errore fatale: zlib inflate error: incorrect header check
lftp a@127.0.0.1:/> quit

Do you know of any compatible client? Which client has it been tested with?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.15%. Comparing base (d7a1353) to head (f9f44d2).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #604      +/-   ##
==========================================
+ Coverage   87.14%   87.15%   +0.01%     
==========================================
  Files          12       12              
  Lines        1828     1830       +2     
==========================================
+ Hits         1593     1595       +2     
  Misses        157      157              
  Partials       78       78              

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[fclairamb]

Hi @drakkan , long time no see. Yes, this should be disabled by default. I don't even remember where this feature comes from. It has been almost two years since I created the PR around it. There are a few clients supporting it. I haven't fully tested it, I just want to see if claude code could finish this feature for me (I had a good understanding on how to fix it but not the motivation to make the core changes) and it did.

[drakkan]

Hi @drakkan , long time no see.

Yes, sorry. Things with SFTPGo are going well, but it’s a very busy period and I don’t have time for anything else at the moment.

Yes, this should be disabled by default. I don't even remember where this feature comes from. It has been almost two years since I created the PR around it. There are a few clients supporting it. I haven't fully tested it, I just want to see if claude code could finish this feature for me (I had a good understanding on how to fix it but not the motivation to make the core changes) and it did.

Ok, let’s see if anyone uses it and reports a bug 😄 I just want to avoid introducing a breaking change for SFTPGo OSS users. In SFTPGo Enterprise, I’m currently using a fork of ftpserverlib with some very specific changes, and others that could be upstreamed in the future.