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

[Nahuel Greco]

A reproducible test:

~$ cd /tmp/
/tmp$ mkdir a b
/tmp$ cd a
/tmp/a$ head -c 100000 /dev/urandom > test
/tmp/a$ ls -l test
-rw-r--r-- 1 nahuel nahuel 100000 Jan 27 18:27 test
/tmp/a$ cd ../b
/tmp/b$ timeout 3 rsync -u --inplace --partial --bwlimit=2k --progress -va ../a/test .
sending incremental file list
test
         32,768  32%    0.00kB/s    0:00:00  
rsync error: received SIGINT, SIGTERM, or SIGHUP (code 20) at rsync.c(632) [sender=3.1.1]
rsync error: received SIGINT, SIGTERM, or SIGHUP (code 20) at io.c(504) [generator=3.1.1]
/tmp/b$ ls -l test
-rw------- 1 nahuel nahuel 0 Jan 27 18:28 test
/tmp/b$ timeout 3 rsync -u --inplace --partial --bwlimit=2k --progress -va ../a/test .
sending incremental file list

sent 59 bytes  received 12 bytes  142.00 bytes/sec
total size is 100,000  speedup is 1,408.45
/tmp/b$ ls -l test
-rw------- 1 nahuel nahuel 0 Jan 27 18:28 test
nahuel at serge:/tmp/b$

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

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

Status in rsync package in Ubuntu:
  New

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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