proposal: time: List timezones

[Merovius]

I'd like to propose to add a function to the time package that lists the set of installed timezones. For a good user experience, I want my service to provide auto completion or a drop-down for a user to select their timezone; currently, the time package only supports probing, which would make for a bad user-experience.

In theory, this could be provided outside of the stdlib by walking the corresponding directory trees; however, for different systems, different directories are used, so this would require duplicating those lists and keeping them in sync with the stdlib manually (otherwise you might claim to support a timezone, which you then not actually support; again, a bad user experience).

I'd write this myself, but wanted to first get an opinion on whether this would be accepted.

System details

go version go1.8.1 linux/amd64
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH=""
GORACE=""
GOROOT="/home/axelw/go"
GOTOOLDIR="/home/axelw/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build049658130=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
GOROOT/bin/go version: go version go1.8.1 linux/amd64
GOROOT/bin/go tool compile -V: compile version go1.8.1 X:framepointer
uname -sr: Linux 4.4.0-72-generic
Distributor ID:	Ubuntu
Description:	Ubuntu 14.04.5 LTS
Release:	14.04
Codename:	trusty
/lib/x86_64-linux-gnu/libc.so.6: GNU C Library (Ubuntu EGLIBC 2.19-0ubuntu6.11) stable release version 2.19, by Roland McGrath et al.
gdb --version: GNU gdb (GDB) 7.9-gg19

[rsc]

Someone would have to put readdir etc into package time (which cannot import os), at the bare minimum. Then the implementation would have to read the TZ database directory and open and read every file? Or maybe just list them and let the caller open them?

It's true that package time knows best where it will search from LoadLocation, and returning the directory is not super-helpful on Windows or other systems that have it in the zip file in GOROOT. So you can't write this function outside the standard library.

On the other hand it seems very specialized.

[Merovius]

I did not think about the restrictions about importing packages; that, indeed, complicates things and changes the tradeoffs significantly. TBH, I don't find it that specialized. I'd argue that most software that exposes timezones to users probably wants this, or risk being very frustrating to users. I know I often don't know what to call the timezone I'm about; what major city to use (and daylight savings time makes zone abbreviations pretty useless too, AIUI). But yeah, I can see how it's probably specialized enough to not justify the tradeoffs under these circumstances.

I'd still love to see this (maybe in x/, which doesn't have the restriction on imports but would still have reasonable guarantees to stay in sync with the stdlib?), but understand if you decide to not do it.

[bradfitz]

Related issue: #20629 ("time: Add function to generate a Location struct")

[rsc]

Note that if we support listing timezones by default that complicates #20629. On the other hand, if we support #20629, maybe developers can use that and their own "list their source" instead of having listing be part of the standard time API.

[rsc]

Will close this in favor of #20629.

[rogpeppe]

@rsc

On the other hand, if we support #20629, maybe developers can use that and their own "list their source" instead of having listing be part of the standard time API

I'm not sure that's possible with the eventually chosen solution to #20629, is it? The tzdata package doesn't seem to provide any way to acquire any of the actual data or enumerate the possible locations. Should this issue be reopened, or am I missing something?

[ianlancetaylor]

As far as I can see basically the only thing people need to implement this is the list of directories, which on Unix systems is:

	"/usr/share/zoneinfo/",
	"/usr/share/lib/zoneinfo/",
	"/usr/lib/locale/TZ/",
	runtime.GOROOT() + "/lib/time/zoneinfo.zip",

I think the question is: is it useful for the time package to export that list?

[rogpeppe]

As far as I can see basically the only thing people need to implement this is the list of directories

That's not quite true, at least any more, because the data can also be in the tzdata package. I see the problem with adding directory-reading functionality to the time package, and adding some kind of listing functionality to tzdata seems wrong because it's important that it's only imported at the top level and we wouldn't want the "list time zones" functionality to have the side effect of ignoring system-supplied time zones.

I wonder about a new package (time/zones? time/tz?) that exports just a single function, List, that lists the names of all the available time zones.

I'll reopen this issue for now, as it's clearly not entirely resolved.