[Toybox] [PATCH] Support the %N coreutils/busybox extension to date.

[enh]

take 2 (switches to next_printf, fixes the %%N case):

 tests/date.test   | 14 ++++++++++++++
 toys/posix/date.c | 50 +++++++++++++++++++++++++++++++++++++++-----------
 2 files changed, 53 insertions(+), 11 deletions(-)


On Sun, Apr 16, 2017 at 4:22 PM, Rob Landley <rob at landley.net> wrote:

> On 04/16/2017 03:51 PM, enh wrote:
> > yeah, definitely a mess.
> >
> > note that i only did this because (a) existing tests were using it and
> > (b) busybox has it too. i've been assuming (perhaps incorrectly) that
> > busybox also tried to avoid needlessly implementing GNU extensions
> > unless they were actually useful to enough people.
>
> As far as I can tell the way people do this is indeed date %N. I've
> tried to do this before, but adding extra escapes to both input and
> output via an overlay mechanism turns out to be a hard problem to do right.
>
> (Really I want a generic mechanism to wrap arbitrary printf/scanf style
> things, but the problem space isn't necessarily set up that way. Can I
> chop strptime parsing into multiple strptime calls that _only_ modify
> certain fields each time while leaving the others alone? I was reading
> the posix page and man page stuff about that at one point, something
> said it only modified specific fields other than the ones you
> specified... but alas, I got distracted halfway through...)
>

one of the things i liked in Plan 9:
http://man.cat-v.org/plan_9/2/fmtinstall


> Checking my notes, I was attempting to add %s support to the input path.
> Except that %s pretty much _replaces_ strptime() because none of the
> other fields you set make sense in combination with that. (But %s could
> be in the middle of a context string describing the rest of the input;
> that I didn't make up, it's in the ubuntu one.) So %s is probably
> fiddlier than %N. Hmmm...
>
> In general I wince at merging half a solution to a larger problem,
> because it increases the surface area of the problem. (People start
> using the partial thing, then I break them when I try to fix the rest of
> it...) That said, if I'm not doing the full fix now and the partial fix
> is there and we need it... Sigh.


plus no-one else supports round trips as far as i can tell, so demand
hasn't been that great.


> >     Speaking of which, this patch adds %N to output but not to input, so
> >     date can still only _set_ nanoseconds using the @123456789.123456789
> >     syntax.
> >
> > isn't setting via arbitrary format a toybox extension?
>
> Walter Harms added it to busybox in 2006 (back when I maintained it):
>
>   http://lists.busybox.net/pipermail/busybox/2006-February/018203.html
>   https://git.busybox.net/busybox/commit/?id=c5789a6e234c
>
> I genericized his original idea a bit because "I want to convert data
> from one format to another" is awkward to solve otherwise. "This email's
> datestamp would be what in unix time"... I used to write little C
> programs to do that.
>
> I used to have a button that said "Only XT users know that January 1,
> 1980 was a tuesday". Toybox says:
>
>   $ ./date -D "%m %d %Y" -d "1 1 1980" +%A
>   Tuesday
>

(given that this question comes up in real life more than "show me the
whole calendar for 1856" or "show me the calendar for October 1856", i
never understood why cal will show you a whole year or a whole month but
not a single day.)


> If you can say %N in + but not in -D I'm doing it wrong. It's a tiny
> thing, but it bothers me.
>
> >     And you're not checking that "%%N is a literal %N". (Sigh. If
> >     we're going there, may I draw your attention to next_printf() in
> >     lib/lib.c? Example usage in seq and stat.)
> >
> > okay, i'll switch it over (assuming we actually want %N).
>
> Eh, we probably want it, I just winced at how expensive it was to
> implement and it's the wrong place to do it (belongs in libc) and if
> we're going there I want to do the full fix (get and set, and handling
> %% and %4N) so it isn't inconsistent...
>
> And it needs one of them Focused Review things that have been in short
> supply recently. (The easter weekend's helped me to dig out a bit. If I
> got a three day weekend every week, keeping up would be so much easier. :)


no extra day off here :-( at the other end of the spectrum, the London
office have a four-day weekend...


>
> >     It's kind of sad "ls" can't show nanoseconds. Or "stat". I understand
> >     why (nanoseconds were added when makefile dependencies started
> breaking
> >     as computers got faster and multiple build things happened in the
> same
> >     second; nothing else actually _needed_ them in the filesystem) but
> it's
> >     still half-assed.
> >
> > ~$ ls -ld --time-style=full-iso /
>
> --time-style is a can of worms (I made a design decision _not_ to
> support more than one output format, now there's a straightforward
> extension of the existing output format we want to add, but if I use the
> generic named selection mechanism for it the pressure to support all the
> rest of 'em will never stop. I lose the ability to draw a clear boundary
> line.)
>
> But --full-time is a synonym, which I can add _and_ make -ll do it too.
> (Repeat -l and it shows nanoseconds. I like having short options for
> things, but the letters are all taken here. Still, logically it's a
> "long long"... :)
>
> My concern here is maximizing the surface area of "I can write a script
> that runs under toybox and under ubuntu as well." Such as the current
> test suite stuff we're trying to do. I can write one using --full-time
> it works in both contexts.
>
> The problem with using ls for the touch test code is "ls -l" shows
> ownership information also, varying between systems. You can't output
> _just_ the info you want without that. We can do the awk/cut trick to
> filter it and only look at specific fields (todo: can we have spaces or
> tabs in usernames? You can in the realname field...).
>
> tl;dr there's still a call for "date %N".
>
> > i'm happy to rewrite the tests instead... i've never used %N personally,
> > and would just naturally use stat for this kind of thing.
>
> How do you get nanoseconds out of stat? (I'm happy to add a toybox
> extension for it, but relying on that breaks TEST_HOST or "make
> test_sed" on a non-toybox host.)


well, the _text_ form includes nanoseconds. but i guess you meant the
numeric-only form, in which case, no, i'm not aware of a way to do that.


> > as you say, the asymmetry is ugly, and the divergence from libc is ugly,
> > so if busybox added this "just because" maybe we shouldn't have it at
> all?
>
> It was useful, and still is. (Especially the ability to specify
> different input and output formats so you can convert between them.) And
> it's already implemented and working. :)
>
> Rob
>



-- 
Elliott Hughes - http://who/enh - http://jessies.org/~enh/
Android native code/tools questions? Mail me/drop by/add me as a reviewer.
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