encoding/json: unmarshal regression for extra data error under goexperiment.jsonv2 #74614
Description
Go version
go1.25
Output of go env in your module/workspace:
n/aWhat did you do?
type T struct{ calledWith string }
func (t *T) UnmarshalJSON(b []byte) error {
t.calledWith = string(b)
return nil
}
func main() {
var v1, v2 T
in := []byte("0x")
err1 := jsonv1.Unmarshal(in, &v1)
fmt.Println(err1, v1)
err2 := jsonv1in2.Unmarshal(in, &v2)
fmt.Println(err2, v2)
}What did you see happen?
invalid character 'x' after top-level value {}
invalid character 'x' after top-level value {0}
What did you expect to see?
invalid character 'x' after top-level value {}
invalid character 'x' after top-level value {}
Notice that UnmarshalJSON was not called for v1, but is called for v1in2.
It seems that v1in2 eagerly handles any valid JSON prefix, and only reports an error for an invalid suffix after evaluation.
In contrast, v1 validates the entire JSON input up front.
The v1in2 behavior is an artifact of how it prioritizes streaming unmarshal.
It's unclear if this regression matters. I discovered it based on a test failure, but it has no consequential difference in production.
\cc @neild