[Bug 1659950] Re: rsync -u --inplace --partial -a can't resume transfer

[Paride Legovini]

Thanks Nahuel! We're doing some retriage and cleanup of old bugs, and I
stumbled on this one :-)

I linked your upstream bug report to this bug, Launchpad will keep its
status in sync. You may have noticed that I didn't set the status of
this bug to Wontfix but to Opinion ("Doesn't fit with the project, but
can be discussed"), I think it fits better.

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https://bugs.launchpad.net/bugs/1659950

Title:
  rsync -u --inplace --partial -a can't resume transfer

Status in rsync:
  Unknown
Status in rsync package in Ubuntu:
  Opinion

Bug description:
  Suppose you have a file in hostA:

    hostA$ ls -l /tmp/files
     -rw-rw-r-- 2 root root 563016 Jan 10 15:01 test.txt

  You download it from the hostB using:

    hostB$ rsync -u --inplace --partial -a hostA::files/* .

  If the transfer is aborted, hostB will get only a partial file:

    hostB$ ls -l /tmp/files
     -rw-rw-r-- 2 root root   2024 Jan 11 18:00 test.txt
    
  BUT the ctime/mtime of hostB/test.txt now is NEWER than hostA/test.txt(and mtime == ctime). So, if you run the same rsync -u command again:

    hostB$ rsync -u --inplace --partial -a hostA::files/* .

  Rsync will SKIP THE FILE, because hostB/test.txt is "newer" than
  hostA/test.txt. So you CAN'T resume using rsync -u command, and you
  will think there are no differences.

  To avoid this bug, rsync must create the file with ctime=mtime=0. And
  if the file already exists before transfer, rsync -u must not change
  his current ctime/mtime. Ctime/mtime must be updated ONLY after the
  transfer was successfully completed.

  Note this is really need because there are scenarios where checksum
  comparison can't be used, only comparison by time. For example, to
  avoid deleting changes made in hostB to test.txt. Also I need to use
  --inplace.

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