[Rd] Another issue with Sys.timezone

[Martin Maechler]

>>>>> 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 ?

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

--
Martin <Maechler at stat.math.ethz.ch>   http://stat.ethz.ch/~maechler
Seminar für Statistik, ETH Zürich
and R Core Team