time: Altered time.Time state from calling IsDST or Zone

[myshkin5]

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

$ go version
go version go1.17.6 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=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/dschultz/.cache/go-build"
GOENV="/home/dschultz/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/dschultz/workspace/apiserver-go/pkg/mod"
GONOPROXY="github.com/aspenmesh/*"
GONOSUMDB="github.com/aspenmesh/*"
GOOS="linux"
GOPATH="/home/dschultz/workspace/apiserver-go"
GOPRIVATE="github.com/aspenmesh/*"
GOPROXY="https://proxy.golang.org"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.17.6"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/dschultz/workspace/apiserver/go.mod"
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-build2743734130=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Observing bizarre state mutations around the location in an instance of time.Time. By calling IsDST (which should be read-only, right?), the state of an instance is changed. Subsequent instances instantiated after the call to IsDST have an updated location and altered state.

https://go.dev/play/p/bq0qA7JMJIz

With output as follows:

before 11810389586750117723, after 12359862718078847701, new 12359862718078847701, pre 11810389586750117723

Showing that the hash of the instance changes after calling IsDST (Zone produces the same results). The hash of a new instance created after the call to IsDST on the first instance equals the new hash.

What did you expect to see?

A consistent hash and unaltered state when calling a seemingly read-only function.

What did you see instead?

A mysteriously changing hash value.

[ericlagergren]

What is the bug here? The time package explicitly says you should not do this:

// Note that the Go == operator compares not just the time instant but also the
// Location and the monotonic clock reading. Therefore, Time values should not
// be used as map or database keys without first guaranteeing that the
// identical Location has been set for all values, which can be achieved
// through use of the UTC or Local method, and that the monotonic clock reading
// has been stripped by setting t = t.Round(0). In general, prefer t.Equal(u)
// to t == u, since t.Equal uses the most accurate comparison available and
// correctly handles the case when only one of its arguments has a monotonic
// clock reading.

[seankhliao]

The location (time.Local) is only initialized on first use. As this isn't exposed, there's no guarantee on internal values.

Closing as working as intended.

[myshkin5]

Thanks for the explanation and the documentation is quite clear. Just rather odd behavior. Calling UTC and/or Local had no effect. Adding this init function solved the problem:

func init() {
	_ = time.Unix(0, 0).IsDST()
}