Bug: rsync erroneously changes modification time

[max.power at clovermail.net]

How exactly does rsync determine that the copy has the incorrect  
timestamp and not the source file?
Does it assume that the copy must be incorrect or are there other  
criteria that have to be considered?


Quoting Kevin Korb via rsync <rsync at lists.samba.org>:

> Whenever you use --times (included in --archive) rsync will fix
> incorrect time stamps.  The only thing --size-only is doing is keeping
> the incorrect data instead of replacing it.
>
> The purpose of these options is to "fix" a copy done in a way that did
> not preserve timestamps but the data is known to have not changed.
> These options allow rsync to correct the incorrect timestamps without
> even looking inside of the files.  If you are not 100% sure your file
> data matches you should not be using --size-only.
>
> On 06/11/2017 09:28 AM, max.power--- via rsync wrote:
>> When a file of same length already exists at the destination then the
>> command 'rsync --archive --size-only' (--archive is same as -rlptgoD)
>> may change the modification time of the destination file even if no
>> modification was made.
>>
>> Type the following commands in a terminal in order to reproduce the
>> problem:
>>
>> $ mkdir source
>> $ mkdir target
>> $ echo "file one" > source/file
>> $ echo "file two" > target/file
>> $ srcstamp=201111110000.11
>> $ tgtstamp=200001010000.00
>> $ touch -t $srcstamp source/file
>> $ touch -t $tgtstamp target/file
>> $ stat -c "%y %s" source/file
>> $ stat -c "%y %s" target/file
>>
>> Notice, that although the file in source/ and target/ folder contain
>> different content they do not  differ in size. Thus, there following
>> command should not make any changes:
>>
>> $ rsync -rlptgoD --size-only source/ target
>>
>> There was indeed no backup performed, which can be verified by viewing
>> the contents of the files:
>>
>> $ cat source/file
>> $ cat target/file
>>
>> However, the modification time of the target file was updated to the
>> same modification time as file in source/:
>>
>> $ stat -c "%y %s" source/file
>> $ stat -c "%y %s" target/file
>>
>> Since the file was not modified the modification time should not be
>> changed. Omitting the '-t' option in above rsync does not solve the
>> problem because when a backup is actually performed then the
>> modification time will simply be set to the current system time instead
>> of the modification time of the source file.
>>
>> The same behaviour can be observed when using the flags -rlptgoD (or
>> --archive) with the --checksum flag. Although it is highly unlikely that
>> the files will differ if the checksums are the same, the modficaition
>> should still not be changed for consistency reasons.
>>
>>
>>
>>
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> --
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