Unmarshal an empty time object provides a problematic timezone

[aviadl]

Unmarshal a byte array that was generated by marshaling an empty time struct
If the unmarshal will receive an interface, this interface will eventually contain an empty time struct but with LMT timezone (that doesnt work properly in many cases)
If the unmarshal will receive a time struct, this will work properly

Expected Behavior

empty time object in utc timezone

Current Behavior

empty time object in LMT timezone

Possible Solution

add tm.isZero case to the flow of unmarshaling from interface
It does exists if a typed time struct is provided

Steps to Reproduce

See the following unit test:

func TestEmptyTimeMarshalWithInterface(t *testing.T) {
	a := time.Time{}
	b, err := msgpack.Marshal(a)
	if err != nil {
		t.Fatal(err)
	}
	var out interface{}
	err = msgpack.Unmarshal(b, &out)
	if err != nil {
		t.Fatal(err)
	}
	name, _ := out.(time.Time).Zone()
	if name != "UTC" {
		t.Fatal("Got wrong timezone")
	}

	var out2 time.Time
	err = msgpack.Unmarshal(b, &out2)
	if err != nil {
		t.Fatal(err)
	}
	name, _ = out2.Zone()
	if name != "UTC" {
		t.Fatal("Got wrong timezone")
	}
}

[pascal-zarrad]

Experiencing the same issue when using go-redis/cache.
Had to switch over to an alternative (JSON) as a workaround for now due to the issue.

[aviadl]

@pascal-zarrad you can check my PR above

[jaimem88]

https://github.com/eko/gocache recently upgraded to v5, and we are trying to upgrade the dependency as well in https://gitlab.com/gitlab-org/container-registry/-/merge_requests/1442.

Now we are encountering a similar issue where the time.Location is set as time.Local instead of UTC -> https://gitlab.com/gitlab-renovate-forks/container-registry/-/jobs/5419691095, see extract from the test output

expected:  DeletedAt:sql.NullTime{Time:time.Date(1, time.January, 1, 0, 0, 0, 0, time.UTC)
actual:       DeletedAt:sql.NullTime{Time:time.Date(1, time.January, 1, 0, 0, 0, 0, time.Local)

It's not the worst as the time is still kept as zero but our test fail when checking for equality.

cc @aviadl @vmihailenco