proposal: reflect/v2: make Value uncomparable

[dsnet]

Comparing two Value type does not do what the user expects. The type is documented as such:

Using == on two Values does not compare the underlying values they represent, but rather the contents of the Value structs. To compare two Values, compare the results of the Interface method.

However, it is still common to see users accidentally make this mistake. What's worse is that it "seems to work", giving a false sense of security.

s1, s2 := "hello", "goodbye"

v1a := reflect.ValueOf(&s1)
v1b := reflect.ValueOf(&s1)
v2 := reflect.ValueOf(&s2)

fmt.Println(v1a == v1a) // Prints true
fmt.Println(v1a == v1b) // Prints true
fmt.Println(v1a == v2) // Prints false

This is because the Value struct contains a flag field whose value may be different for each Value and is entirely an implementation detail of the reflect package.

Perhaps we should add a uncomparable type to prevent this misuse:

type Value struct {
	_ [0]func() // Prevents Value from being comparable and occupies 0 bytes
	typ *rtype
	ptr unsafe.Pointer
	flag
}

\cc @ianlancetaylor

[gopherbot]

CL https://golang.org/cl/36018 mentions this issue.

[ianlancetaylor]

Adding [0]func() is a clever idea, but unfortunately it breaks the Go 1 guarantee. We can't actually modify reflect.Value so that == does not work, as that will cause currently working programs to stop compiling.

[bradfitz]

What @ianlancetaylor said. It would break things like collecting a set of reflect.Values in a map.

[dsnet]

It would break things like collecting a set of reflect.Values in a map.

Is that so bad? It's funny that you bring up reflect.Values in a map. That was exactly what someone tried doing the other day and it "seemed to work".

However, I do agree that we probably can't do this in Go 1.

[bradfitz]

A map is fine in some cases. (e.g. using it as a set of things to process, and ranging over it to get the reflect.Values back)

Maybe worth a static analysis checker?

[dominikh]

In the subset of the Go corpus v0.01 that actually compiles, there are 33 instances of using == or != with two reflect.Value.

Of these, 12 compare against the zero value reflect.Value{}, as it is returned by, for example, MethodByName. 12 compare a reflect.Value against the result of reflect.Zero. The remaining 9 compare two generic reflect.Values.

Of the 21 incorrect comparisons, 5 compare reflect.Values derived from function values, which would not be comparable if converted to interface values first.

I've attached the list of matches. [fn] denotes a comparison of functions, [zero] denotes a comparison with the zero value, [zv] denotes a comparison with a value produced by reflect.Zero, and the rest are normal, though sometimes oddly written, comparisons.

[fn] github.com/Masterminds/glide/vendor/github.com/Masterminds/semver/constraints_test.go:47:9
[zero] github.com/cyfdecyf/cow/config.go:320:12
[zero] github.com/cyfdecyf/cow/config.go:348:12
[zero] github.com/cyfdecyf/cow/config.go:629:13
[zero] github.com/docker/docker/vendor/github.com/imdario/mergo/map.go:70:18
[zv] github.com/docker/machine/vendor/github.com/Azure/go-autorest/autorest/validation/validation.go:313:7
[zv] github.com/docker/machine/vendor/github.com/Azure/go-autorest/autorest/validation/validation.go:333:7
[zero] github.com/drone/drone/vendor/gopkg.in/gorp.v1/gorp.go:1763:7
[zero] github.com/ethereum/go-ethereum/vendor/github.com/robertkrimen/otto/type_go_slice.go:71:78
[zero] github.com/ethereum/go-ethereum/vendor/github.com/robertkrimen/otto/type_go_struct.go:41:70
[zero] github.com/ethereum/go-ethereum/vendor/github.com/robertkrimen/otto/type_go_struct.go:45:55
[fn] github.com/git-lfs/git-lfs/vendor/github.com/spf13/cobra/cobra_test.go:949:31
[zv] github.com/go-xorm/xorm/statement.go:418:19
[zv] github.com/go-xorm/xorm/statement.go:655:18
github.com/google/cadvisor/vendor/github.com/stretchr/testify/assert/assertions.go:41:19
github.com/google/cadvisor/vendor/github.com/stretchr/testify/assert/assertions.go:46:77
github.com/iris-contrib/formBinder/formBinder.go:40:14
github.com/iris-contrib/formBinder/formBinder.go:570:33
[zero] github.com/kataras/go-serializer/vendor/github.com/imdario/mergo/map.go:70:18
[zero] github.com/kataras/go-websocket/vendor/github.com/imdario/mergo/map.go:70:18
[zv] github.com/mitchellh/packer/vendor/github.com/gophercloud/gophercloud/params.go:61:46
[zv] github.com/mitchellh/packer/vendor/github.com/gophercloud/gophercloud/params.go:83:46
[zv] github.com/openshift/origin/vendor/github.com/Azure/go-autorest/autorest/validation/validation.go:313:7
[zv] github.com/openshift/origin/vendor/github.com/Azure/go-autorest/autorest/validation/validation.go:333:7
[zero] github.com/openshift/origin/vendor/github.com/imdario/mergo/map.go:70:18
[zv] github.com/prometheus/prometheus/vendor/github.com/Azure/go-autorest/autorest/validation/validation.go:313:7
[zv] github.com/prometheus/prometheus/vendor/github.com/Azure/go-autorest/autorest/validation/validation.go:333:7
[fn] github.com/sosedoff/pgweb/vendor/github.com/jmoiron/sqlx/sqlx.go:37:23
[fn] github.com/spf13/cobra/cobra_test.go:1135:31
[fn] github.com/tj/cobra/cobra_test.go:1136:31
[zero] k8s.io/heapster/vendor/github.com/imdario/mergo/map.go:70:18
[zv] k8s.io/heapster/vendor/github.com/rikatz/goryman/marshal.go:30:29
[zv] k8s.io/heapster/vendor/github.com/rikatz/goryman/marshal.go:93:29

[gopherbot]

Change https://golang.org/cl/308209 mentions this issue: go/tools: add vet check for direct comparion of reflect.Values