[Bug 12742] New: a proposal: fix bogus nanosecond mtimes on transfer (patch included)

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https://bugzilla.samba.org/show_bug.cgi?id=12742

            Bug ID: 12742
           Summary: a proposal: fix bogus nanosecond mtimes on transfer
                    (patch included)
           Product: rsync
           Version: 3.1.1
          Hardware: All
                OS: All
            Status: NEW
          Severity: minor
          Priority: P5
         Component: core
          Assignee: wayned at samba.org
          Reporter: gfedorov at unimelb.edu.au
        QA Contact: rsync-qa at samba.org

Created attachment 13152
  --> https://bugzilla.samba.org/attachment.cgi?id=13152&action=edit
a proposed patch to ignore bogus .st_mtim.tv_nsec values by resetting them to 0

This suggestion is actually in a grey area between a proposed enhancement and a
minor fix.

Basically utimensat() sometimes fails to transfer mtimes for some bogus mtime
values, so then rsync tries to transfer the same files over and over again.

However, a simple utime() call works just fine, what leads to the whole
suggestion.

I'm attaching a proposed patch for rsync 3.1.1, but I think v. 3.1.2 can be
adjusted in a similar vein ; not sure about 3.1.3, since I thought it is in
development.

Now I will go into details, and then briefly discuss the alternatives.


Details
=======

First of all, this is not strictly speaking an rsync problem: we are talking
about a marginally small number of files ( a few hundred of at least hundreds
of thousands, sometimes millions ), with their mtimes messed up possibly
because of some subtle fault in the filesystem implementation. 

However, these very few files make rsync to set return code RERR_PARTIAL (23)
for the whole transfer, what leaves us with no good options.

In my experience, on a Linux system with utimensat() / futimens() the latter
fails to transfer a negative .st_mtim.tv_nsec, or a value greater than
1000000000 with errno 22 (EINVAL).

However, since rsync 3.1.1 seems to compare only seconds, and rsync 3.1.2 seems
to still use only seconds comparison when nominating the files for a transfer,
it is very tempting to ignore bogus nanosecond values by setting them to zero,
because in that case futimens() will just do the job, and we'll have no further
transfer attempts.

I must admit that I do not know how and why these false mtime values appear; my
another guess is that there may be a minor bug in some of utimes()
implementations, since all the bogus nanosecond times that I see are a multiple
of a 1000.

However, struct timespec in my opinion is just a plain invitation for this kind
of error, since it defines tv_nsec as a long, which is plain insane: you do not
want to define a filesystem-related field to have anything but a fixed length
size, when at least for GCC a long is basically a machine word, that is, its
size depends on the CPU architecture. ( Windows long type definition is better
in that particular case, making it AFAIR an int32. )

Rsync, by the way, takes a much more reasonable approach by defining mod_nsec
in set_modtime() as an uint32 ; however, this sanity is incompatible with
insanity in the standard ( see e.g.
http://pubs.opengroup.org/onlinepubs/7908799/xsh/time.h.html ), what will of
course break the bogus nanosecond mtimes anyway.

For a discussion, please refer to a following rsync lists message:
https://lists.samba.org/archive/rsync/2017-April/031177.html , but in short --
nanosecond mtime values:

(a) are not supposed to exist, since we need to define at least UTIME_OMIT and
UTIME_NOW as long values, and have no other choice but to put these constants
somewhere outside of 0 .. 1 000 000 000 interval ;

(b) can still appear on a filesystem nevertheless due to some possible
imperfections in the OS code or filesystem implemenation ;

(c) are not really taken into account by rsync 3.1.1 / 3.1.2 when it compares
files ;

(d) however, can still break an rsync transfer -- that is, lead to an error
code 23.

Therefore, this request suggests that we ignore them on a transfer by setting
set them to 0.


Alternatives
============

One could of course recompile rsync to make it ignore ns precision at all, but
we can probably do much better -- that is, transfer all the proper mtime values
with the proper precision, and ignore nanoseconds for the messed up ones.

So an obvious alternative to changing mtimes would be to allow the user to
ignore them -- but since we still probably want to transfer mtimes for 99.99%
of files, this shall rather be an option to ignore a few mtime transfer errors,
and I will fill another enhancement request to address that.

Finally, running two subsequent rsync transfers -- one to transfer the file
contents ignoring mtimes, and another to transfer mtimes, where we will ignore
the return code, is possible, but has two drawbacks:

(1) this may double the time needed for the transfer: e.g. one one rsync
session runs overnight, there would be no place for another rsync session on
the same day ;

(2) rsync will still try to transfer the same files over and over again.

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