[Bug time/30200] time sometimes appears to go backwards

["eggert at cs dot ucla.edu" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=30200

eggert at cs dot ucla.edu changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |eggert at cs dot ucla.edu

--- Comment #11 from eggert at cs dot ucla.edu ---
(In reply to Florian Weimer from comment #10)
> Is this actually causing application problems...?

In theory any sequence of timestamps is something that a (perverse) sysadmin
could inflict on the user by constantly futzing with the system clock, and
hence applications that generate out-of-order timestamps are allowed by POSIX.

In practice out-of-order timestamps are problematic. Users will likely be
confused, for example, if a log of a thread's actions contain out-of-order
timestamps even when the system clock was not reset.

I looked at a few applications.

* Git can issue out-of-order timestamps. It's complicated enough that I judged
it simpler to fix the problem than worry about how serious it is, and filed a
proposed patch to do that
<https://lore.kernel.org/git/20230319064353.686226-1-eggert@cs.ucla.edu/T/#u>.

* GNU Make can issue out-of-order timestamps in a debugging log - not a serious
problem but it could be annoying. I filed a proposed patch
<https://lists.gnu.org/r/bug-make/2023-03/msg00081.html>.

* Gnulib can generate out-of-order timestamps when parsing old-fashioned time
strings, causing glitches in Gnulib-using commands like 'touch 01010000 file'.
This is not serious (hardly anybody uses those old POSIX-specified strings that
do not specify year numbers) and I don't know whether it's a POSIX violation. I
fixed that problem <https://lists.gnu.org/r/bug-gnulib/2023-03/msg00028.html>
and the fix should propagate into future releases of Coreutils etc.

* GNU Emacs proper does not have the problem with Glibc (it may have it with
MS-Windows but that’s out of scope here). One Emacs auxiliary program
‘movemail’ may have the issue since it does call ‘time’. I haven’t investigated
further.

The result of this brief investigation:

* There are problems.

* Effects observed so far are small (e.g., logs out of order).

* GNU Make (my first worry, due to its extensive use of timestamps) seems to be
mostly OK.

* Any big effects are surely either rare, or hard to trace back to the problem
source (or both); if big effects were common and easily traced back, we’d know
already.

As an application developer I’d rather see ‘time’ fixed to match ‘gettimeofday’
/ CLOCK_REALTIME / etc., as I worry that there might be larger, rare effects
that this little survey didn’t catch.

-- 
You are receiving this mail because:
You are on the CC list for the bug.