Re: master d0c77a1: Remove some assumptions about timestamp format

[Paul Eggert <eggert@cs.ucla.edu>]

On 9/25/18 3:09 AM, Michael Albinus wrote:
> In the different tramp*.el files, '(0 0) is explicitly quoted as "don't
> know value". This is there for ages.
>
> If you want to change this in the tests, it shall be changed in
> tramp*.el as well. Maybe it would be even worth an own constant?
> Something like
>
> (defconst time-dont-know (seconds-to-time 0)
>    "A time value indicating an unknown time.")
>
> If you like, it could even have the value 0 instead of (seconds-to-time
> 0). But this would also introduce "some assumptions of Lisp code on
> timestamp format" - what you try to avoid.


A constant like that would make things clearer, yes. Either 0 or (0 0 0 
0) or (0 0) should do, so I suggest just 0 as it's simplest. While 
you're at it, you might consider changing the number from 0 to a 
negative value, say -1 (the value that POSIX uses for invalid time_t), 
since that's less likely to collide with actual file timestamps 
(timestamp 0 is far more common than timestamp -1 in real filesystems in 
my experience).

The Tramp code shouldn't care whether the constant is 0 or (0 0) or (0 0 
0 0) [or -1 or (-1 65535) or (-1 65535 0 0)], because the code should 
compare time stamps via time-less-p (or float-time, if the timestamps 
are known to be small like 0 or -1, or maybe we should add time-equal?), 
not via 'equal'. The idea is that the Lisp timestamp format has changed 
in the past and is likely to change in the future and one should not 
assume any particular format if one wants the code to be portable.