breaking change in go 1.9 about time marshal/unmarshal

[yiminc-zz]

Please answer these questions before submitting your issue. Thanks!

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

go version
go version go1.9.1 darwin/amd64

Does this issue reproduce with the latest release?

Yes

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

go env
GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/yiminc/gocode"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.9.1/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.9.1/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/f6/2zwvng355csd30gym37w1yph0000gn/T/go-build213006030=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

Marshal and Unmarshal a time.Time instance, they become different.
The use case is we use testify to setup mock expectation with a time as expected parameter. Then the tested code internally would do a marshal/unmarshal, and pass the value to the mock call. But the mock call would fail because the expectation does not met. The testify uses reflect.DeepEqual to compare if the parameters are match or not.

However, if you marshal/unmarshal a zero time, then before == after.

https://play.golang.org/p/dX_knLlRY-

This used to work fine with go 1.8

What did you expect to see?

a time instance after marshal/unmarshal (on same machine) should not change.

What did you see instead?

a time instance after marshal/unmarshal (on same machine) are different.

[artyom]

Possible duplicate of #20876, please see its discussion.

[ianlancetaylor]

This is expected behavior. You should not in general use reflect.DeepEqual to compare time.Time values. You should use time.Equal instead. The time package docs say:

// Note that the Go == operator compares not just the time instant but
// also the Location and the monotonic clock reading. See the
// documentation for the Time type for a discussion of equality
// testing for Time values.

This discussion of == applies to reflect.DeepEqual instead.

It is true that this became less likely to work in Go 1.9, due to the introduction of monotonic time values. But it was never fully safe even before Go 1.9, due to the way that timezones are handled.

Closing because this isn't going to change.

[yiminc-zz]

Understand DeepEqual behavior. The question is more about why the time are changed after marshal/unmarshal.

[ianlancetaylor]

I assume because of the monotonic time values. See the "Monotonic Clock" section at https://golang.org/pkg/time.

[yiminc-zz]

Don't understand why do we need to change the monotonic clock over marshal/unmarshal process?

[ianlancetaylor]

The monotonic time is meaningless in a different process, so we don't marshal it at all.

[calvn]

Interestingly, time comparison with == or reflect.DeepEqual after using time.Truncate(0) seem to work on MacOS, but not in linux/Go Playground. I haven't figured out why yet, but thought it might be useful to report this behavior.

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/cleung/Library/Caches/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/cleung/go"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.10.1/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.10.1/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/1x/c_x5s2h90mzdyx4bzz3_dsx80000gn/T/go-build019210901=/tmp/go-build -gno-record-gcc-switches -fno-common"

Updated Go Playground using time.Truncate(0) https://play.golang.org/p/XWurzn_3NTZ:

********************
BEFORE: 0001-01-01 00:00:00 +0000 UTC
AFTER : 0001-01-01 00:00:00 +0000 UTC
BEFORE.Equals(AFTER) : true
BEFORE == AFTER : true
BEFORE DeepEqual AFTER : true
********************
BEFORE: 2009-11-10 23:00:00 +0000 UTC
AFTER : 2009-11-10 23:00:00 +0000 UTC
BEFORE.Equals(AFTER) : true
BEFORE == AFTER : false
BEFORE DeepEqual AFTER : false

Program exited.

MacOS locally:

$ go run main.go
********************
BEFORE: 0001-01-01 00:00:00 +0000 UTC
AFTER : 0001-01-01 00:00:00 +0000 UTC
BEFORE.Equals(AFTER) : true
BEFORE == AFTER : true
BEFORE DeepEqual AFTER : true
********************
BEFORE: 2018-04-02 12:56:10.065154507 -0400 EDT
AFTER : 2018-04-02 12:56:10.065154507 -0400 EDT
BEFORE.Equals(AFTER) : true
BEFORE == AFTER : true
BEFORE DeepEqual AFTER : true

[alexandrestein]

@calvn I have a similar behavior with Arch Linux. Go is installed via default package manage pacman.

go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/alexandre/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/alexandre/go"
GOPROXY=""
GORACE=""
GOROOT="/usr/lib/go"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
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-build737376362=/tmp/go-build -gno-record-gcc-switches"

If I run the same code inside a Docker it behave as Playground.