What is the purpose of this change?

Our users complained many times that rclone FTP client manifests random problems with completing a file upload.

As my investigations have shown, the bug triggers because jlaffaye/ftp does not properly nudge the fshttp anti-hanging timer on command channel after a file upload. I have a working fix for jlaffaye/ftp and backend/ftp which I will submit later.

This pull request solves a second-stage bug triggered by the above bug which can be fixed solely in rclone. Frequently when users reported the above bug we recommended them to simplify their FTP client settings, in particular set --ftp-concurrency=1. When the FTP backend's upload routine encounters a error, it will remove the failed file. The removal needs another connection but due to low concurrency setting it will wait for connection token forever.

This patch modifies the upload routine so that after error it will first release the failed connection and then remove the failed file and thus avoid the deadlock.
Due to presence of another bug I will include a relevant unit test with a dedicated pull request #5596, not here. I just hope that the change looks self-evident. @ncw ??

Was the change discussed in an issue or in the forum before?

• https://forum.rclone.org/t/rclone-copy-to-ftp-rclone-sends-a-quit-and-never-ends-itself/24046/10
• #5596

Checklist

• I have read the contribution guidelines.
• I have added tests for all changes in this PR if appropriate (see manual test below)
• I have added documentation for the changes if appropriate.
• All commit messages are in house style.
• I'm done, this Pull Request is ready for review :-)