From stephen.berman at gmx.net  Thu Oct 19 17:12:50 2017
From: stephen.berman at gmx.net (Stephen Berman)
Date: Thu, 19 Oct 2017 17:12:50 +0200
Subject: [Rd] Another issue with Sys.timezone
In-Reply-To: <23015.31941.397029.272777@stat.math.ethz.ch> (Martin Maechler's
 message of "Wed, 18 Oct 2017 18:09:41 +0200")
References: <871sm570d3.fsf@gmx.net>
 <23012.59579.35802.920274@stat.math.ethz.ch>
 <23015.31941.397029.272777@stat.math.ethz.ch>
Message-ID: <87r2tzmarx.fsf@gmx.net>

On Wed, 18 Oct 2017 18:09:41 +0200 Martin Maechler <maechler at stat.math.ethz.ch> wrote:

>>>>>> Martin Maechler <maechler at stat.math.ethz.ch>
>>>>>>     on Mon, 16 Oct 2017 19:13:31 +0200 writes:

(I also included a reply to part of this response of yours below.)

>>>>>> Stephen Berman <stephen.berman at gmx.net>
>>>>>>     on Sun, 15 Oct 2017 01:53:12 +0200 writes:
>
>>     > (I reported the test failure mentioned below to R-help but was advised
>>     > that this list is the right one to address the issue; in the meantime I
>>     > investigated the matter somewhat more closely, including searching
>>     > recent R-devel postings, since I haven't been following this list.)
>> 
>>     > Last May there were two reports here of problems with Sys.timezone, one
>>     > where the zoneinfo directory is in a nonstandard location
>>     > (https://stat.ethz.ch/pipermail/r-devel/2017-May/074267.html) and the
>>     > other where the system lacks the file /etc/localtime
>>     > (https://stat.ethz.ch/pipermail/r-devel/2017-May/074275.html).  My
>>     > system exhibits a third case: it lacks /etc/timezone and does not set TZ
>>     > systemwide, but it does have /etc/localtime, which is a copy of, rather
>>     > than a symlink to, a file under zoneinfo.  On this system Sys.timezone()
>>     > returns NA and the Sys.timezone test in reg-tests-1d fails.  However, on
>>     > my system I can get the (abbreviated) timezone in R by using as.POSIXlt,
>>     > e.g. as.POSIXlt(Sys.time())$zone.  If Sys.timezone took advantage of
>>     > this, e.g. as below, it would be useful on such systems as mine and the
>>     > regression test would pass.
>> 
>>     > my.Sys.timezone <- 
>>     > 	function (location = TRUE) 
>>     > {
>>     > 	tz <- Sys.getenv("TZ", names = FALSE)
>>     > 	if (!location || nzchar(tz)) 
>>     > 	    return(Sys.getenv("TZ", unset = NA_character_))
>>     > 	lt <- normalizePath("/etc/localtime")
>>     > 	if (grepl(pat <- "^/usr/share/zoneinfo/", lt) ||
>>     > 	    grepl(pat <- "^/usr/share/zoneinfo.default/", lt)) 
>>     > 	    sub(pat, "", lt)
>>     > 	else if (lt == "/etc/localtime")
>>     > 	    if (!file.exists("/etc/timezone"))
>>     > 		return(as.POSIXlt(Sys.time())$zone)
>>     > 	    else if (dir.exists("/usr/share/zoneinfo") && {
>>     > 		info <- file.info(normalizePath("/etc/timezone"), extra_cols = FALSE)
>>     > 		(!info$isdir && info$size <= 200L)
>>     > 	    } && {
>>     > 		tz1 <- tryCatch(readBin("/etc/timezone", "raw", 200L), 
>>     > 				error = function(e) raw(0L))
>>     > 		length(tz1) > 0L && all(tz1 %in% as.raw(c(9:10, 13L, 32:126)))
>>     > 	    } && {
>>     > 		tz2 <- gsub("^[[:space:]]+|[[:space:]]+$", "", rawToChar(tz1))
>>     > 		tzp <- file.path("/usr/share/zoneinfo", tz2)
>>     > 		file.exists(tzp) && !dir.exists(tzp) &&
>>     > 		    identical(file.size(normalizePath(tzp)), file.size(lt))
>>     > 	    }) 
>>     > 		tz2
>>     > 	    else NA_character_
>>     > }
>> 
>>     > One problem with this is that the zone component of as.POSIXlt only
>>     > holds the abbreviated timezone, not the Olson name.  
>> 
>> Yes, indeed.  So, really only for  Sys.timezone(location = FALSE)  this
>> should be given, for the default  location = TRUE   it should
>> still give NA (i.e. NA_character_)  in your setup.
>> 
>> Interestingly, the Windows versions of Sys.timezone(location =
>> FALSE) uses something like your proposal,  and I tend to think that
>> -- again only for location=FALSE -- this should be used on
>> on-Windows as well, at least instead of returning  NA  then.
>> 
>> Also for me on 3 different Linuxen (Fedora 24, F. 26, and ubuntu
>> 14.04 LTS), I get
>> 
>>   > Sys.timezone()
>>   [1] "Europe/Zurich"
>>   > Sys.timezone(FALSE)
>>   [1] NA
>>   > 
>> 
>> whereas on Windows I get Europe/Berlin for the first (why on
>> earth - I'm really in Zurich) and get  "CEST" ("Central European Summer Time") 
>> for the 2nd one instead of NA ... simply using a smarter version
>> of your proposal.   The windows source is
>> in R's source at  src/library/base/R/windows/system.R :
>> 
>> Sys.timezone <- function(location = TRUE)
>> {
>>     tz <- Sys.getenv("TZ", names = FALSE)
>>     if(nzchar(tz)) return(tz)
>>     if(location) return(.Internal(tzone_name()))
>>     z <- as.POSIXlt(Sys.time())
>>     zz <- attr(z, "tzone")
>>     if(length(zz) == 3L) zz[2L + z$isdst] else zz[1L]
>> }
>> 
>> >From what I read, the last three lines also work in your setup
>> where it seems zz would be of length 1, right ?

Those line do indeed work here, but zz has three elements:

> attributes(as.POSIXlt(Sys.time()))$tzone
[1] ""     "CET"  "CEST"

>> I'd really propose to use these 3 lines in the non-Windows
>> version of Sys.timezone .. at the end *instead* of NA_character_
>> (or a slightly safer version which gives  NA_character_ if zz is
>> of length 0 {e.g. if there is no "tzone" attribute}.
>> 
>>     > i don't know how to
>>     > get the Olson name using only R functions, but maybe it would be good
>>     > enough to return the abbreviated timezone where possible, e.g. as above.
>>     > (On my system I can get the Olson name of the timezone in R with a shell
>>     > pipeline, e.g.: system("find /usr/share/zoneinfo/ -type f | xargs md5sum
>>     > | grep $(md5sum /etc/localtime | cut -d ' ' -f 1) | head -n 1 | cut -d
>>     > '/' -f 5,6"), but the last part of this is tailored to my configuration
>>     > and the whole thing is not OS-neutral, so it isn't suitable for
>>     > Sys.timezone.)
>> 
>>     > Steve Berman
>> 
>> Definitely not.  I still recommend you think of a more portable
>> solution for the   `location = TRUE` (default) case in Sys.timezone().
>> Returning the non-location form (e.g "CEST") when something like
>> "Europe/Zurich" is expected is really not a good idea,
>> and you are lucky that the regression test passes "accidentally" ...
>> 
>> Martin
>
> In the mean time, I have committed a common version (Windows and
> non-Windows) of  Sys.timezone()  to the R development sources
> (aka "R-devel").
>
> That now uses  as.POSIXlt(Sys.time())  very similarly to the
> above "Windows only" case,  but __only__ for  'location=FALSE'
> which is not the default.

Thanks, I think that's definitely better than returning NA when
`location' is false...

> The most current development source is always available (via
> 'svn' or alternatively for browsing via your web browser) from
>
>    https://svn.r-project.org/R/trunk/src/library/base/R/datetime.R

...however, I tried the test that failed for me during `make check' now
with this new definition of Sys.timezone() by pasting the definition (as
new.Sys.timezone()) and the two lines of the test code into the R console,
and this is what happened:

   > new.Sys.timezone()
   > new.Sys.timezone(FALSE)
   [1] "CEST"
   > (S.t <- new.Sys.timezone())
   NULL
   > if(is.na(S.t) || !nzchar(S.t)) stop("could not get timezone")
   Error in if (is.na(S.t) || !nzchar(S.t)) stop("could not get timezone") : 
     missing value where TRUE/FALSE needed
   In addition: Warning message:
   In is.na(S.t) : is.na() applied to non-(list or vector) of type 'NULL'

This is because `location' is true but all the if-clauses in the body
following `if(location)' are false, so it returns NULL.  If you add the
line `else NA_character_' below the line `tz2', then NA is returned and
the test fails as before instead of as above.

> As you say yourself, the above system("... xargs md5sum ...")
> using workaround is really too platform specific  but I'd guess
> there should be a less error prone way to get the long timezone
> name on your system ...

If I understand the zic(8) man page, the files in /usr/share/zoneinfo
should contain this information, but I don't know how to extract it,
since these are compiled files.  And since on my system /etc/localtime
is a copy of one of these compiled files, I don't know of any other way
to recover the location name without comparing it to those files.

> If that remains "contained" (i.e. small) and works with files
> and R's files tools -- e.g. file.*() ones [but not system()],
> I'd consider a patch to the above source file
> (sent by you to the R-devel mailing list --- or after having
>  gotten an account there by asking, via bug report & patch
>  attachment at https://bugs.r-project.org/ )

If comparing file size sufficed, that would be easy to do in R;
unfortunately, it is not sufficient, since some files designating
different time zones in /usr/share/zoneinfo do have the same size.  So
the only alternative I can think of is to compare bytes, e.g. with
md5sum or with cmp.  Is there some way to do this in R without using
system()?

Steve Berman



[Rd] Another issue with Sys.timezone
Stephen Berman