add COMB support

[drakkan]

Thank you!

[codecov]

Codecov Report

Merging #203 (232b3a6) into master (fdb9611) will increase coverage by 0.76%.
The diff coverage is 77.33%.

@@            Coverage Diff             @@
##           master     #203      +/-   ##
==========================================
+ Coverage   72.62%   73.39%   +0.76%     
==========================================
  Files           9        9              
  Lines        1001     1060      +59     
==========================================
+ Hits          727      778      +51     
- Misses        194      202       +8     
  Partials       80       80              

Impacted Files	Coverage Δ
server.go	56.06% <ø> (ø)
handle_files.go	78.19% <75.00%> (+1.14%)	⬆️
client_handler.go	63.21% <100.00%> (+0.97%)	⬆️
handle_misc.go	95.58% <100.00%> (+0.06%)	⬆️

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[fclairamb]

LGTM. You're well on your way to building the server lib that supports the most features of the FTP protocol!

I didn't know the COMB command. That's actually a pretty useful one.

[drakkan]

This is a proprietary command supported in several commercial FTP server such as Titan, GlobalSCAPE etc. It is useful for local files but useless for S3/GCS/Azure backends.

I have almost finished my TODO. The last thing is ABOR support. This will change the server logic and should be reviewed and tested carefully.

I think for some optional interface could be useful to return a well know error and handle the request ourself in this case.

I mean something similar to this: if the ContentTyper interface implementation returns ErrNotImplemented the library will use its internal implementation. In our case it could be useful for hash, for example:

• you implement the Hasher interface
• if the hash is md5 and the backend is gdrive you fetch the hash in your application
• if you return ErrNotImplemented ftpserverlib will calculate the hash itself so you don't have to duplicate the library code in your application to calculate the hash for local fils.

Other minor things that I could contribute in future:

• ModeZ
• Allow to customize login ok and login fail messages (multilines)
• Ensure that the active data connection will be started from the same IP as the control connection (configurable)
• CISD command support