time: Location() reports UTC inconsistently

[dallbee]

The Location() method on a Time struct reports UTC as "local" when acquired from /etc/localtime, but "UTC" when acquired from the TZ environment variable.

This seems related to:
#45960

However, this inconsistency is produced without any parse or marshal methods.

What version of Go are you using (go version)?

$ go version
go version go1.17.1 linux/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE="on"
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/dylan/.cache/go-build"
GOENV="/home/dylan/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/dylan/go/pkg/mod"
GOOS="linux"
GOPATH="/home/dylan/go"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.17.1"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build817213888=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Using the following program (playground link):

func main() {
    x := time.Now()
    fmt.Println(x.Location())
}

# with localtime set to UTC
$ doas ln -sf /usr/share/zoneinfo/UTC /etc/localtime

# load from /etc/localtime by unsetting TZ
$ env -u TZ go run main.go
Local

# load from TZ
$ env TZ="UTC" go run main.go
UTC
$ env TZ="" go run main.go
UTC

What did you expect to see?

Consistent location behavior between environment variable and /etc/localtime methods of setting time.

What did you see instead?

The output of Location() changes based on where it obtained the timezone information.

Additional Notes

The internal documentation on the Time struct's loc variable is incorrect:

go/src/time/time.go

Line 146 in 79159f2

// All UTC times are represented with loc==nil, never loc==&utcLoc.

After String(), the loc field becomes non-nil, and is set to UTC:
https://play.golang.org/p/rqgA876kFIW

These are just internal docs, so I'm not suggesting this itself is a bug. The bug report is about the Location behavior noted in prior sections.

[dr2chase]

@rsc
I don't know if this is a bug or WAI, but it is nicely described.

[ianlancetaylor]

If TZ is not set, the location is always set to Local. That seems reasonable and consistent. We don't want to try to figure out exactly which timezone corresponds to /etc/localtime. That is true even in the particular case that /etc/localtime happens to be UTC.

If TZ is set to a non-empty string, then that is used to determine the location. That also seems reasonable.

I think the only question here is how to handle TZ being set to the empty string. Right now that is the same as TZ being set to UTC. That is consistent with the C library behavior.

[dallbee]

@ianlancetaylor thanks for you thoughts. Agreed on your 2nd and 3rd points.

We don't want to try to figure out exactly which timezone corresponds to /etc/localtime. That is true even in the particular case that /etc/localtime happens to be UTC

Respectfully, why not? Go already reads /etc/localtime in this case, so there's not any syscall being avoided.

For comparison, in C:

#include <stdio.h>
#include <time.h>

int main () {
   time_t rawtime;
   struct tm *info;
   char buffer[80];

   time(&rawtime);
   info = localtime(&rawtime);

   strftime(buffer,80,"%Z", info);
   printf("Timezone: %s\n", buffer);
}

$ doas ln -sf /usr/share/zoneinfo/UTC /etc/localtime
$ env -u TZ ./a.out
Timezone: UTC
$ env TZ="" ./a.out
Timezone: UTC
$ env TZ="UTC" ./a.out
Timezone: UTC

# strace of reads to /etc/localtime
$ env -u TZ strace ./a.out 2>&1 | grep localtime
openat(AT_FDCWD, "/etc/localtime", O_RDONLY|O_CLOEXEC) = 3
$ env TZ="" strace ./a.out 2>&1 | grep localtime
$ env TZ="UTC" strace ./a.out 2>&1 | grep localtime

[ianlancetaylor]

Respectfully, why not? Go already reads /etc/localtime in this case, so there's not any syscall being avoided.

It's a fair question. My only answer is that is how the time package has worked since the current version was introduced in https://golang.org/cl/5392041 in 2011. It means that time.LoadLocation("Local") gives you a timezone whose name is Local that represents the local time zone. It lets us do the right thing on systems like Windows and Plan 9 that don't require the IANA database.

Changing this now would have a decent chance of breaking working Go programs. I think that any such change would have to through the proposal process (https://golang.org/s/proposal-process).

[dallbee]

Changing this now would have a decent chance of breaking working Go programs. I think that any such change would have to through the proposal process (https://golang.org/s/proposal-process).

Sensible. I brought it up because it leads to some interesting scenarios, such as reflect.DeepEqual not considering the time from time.Now() and a parsed equivalent to be equal even if they've had their clocks rounded - the timezone can still differ. Of course, the answer is to use Equal, so it's a minor point.

As for a proposal, I don't think there's a way to change this in a backward compatible manner. I'm not even sure I'd suggest any changes to the existing documentation.

Perhaps, for this one, this closed issue thread will suffice. A future soul might save a few minutes when they're investigating the oddity.

Glad to know it's at least working as designed, Thanks!