time: time.Time's UnmarshalJSON produces bad error message when given "null"

[gopherbot]

by dannyyoo@google.com:

The following demonstrates the problem:

---------------
package main

import (
    "encoding/json"
    "fmt"
    "bytes"
    "time"
)

type RegisterStructure struct {
    SignupTimestamp time.Time `json:"signuptimestamp,omitempty"`
}

func main() {
    requestBody := []byte(`{"signuptimestamp" : null}`)
    dec := json.NewDecoder(bytes.NewReader(requestBody))
    registerInstance := RegisterStructure{}
    err := dec.Decode(&registerInstance)
    if err != nil {
        fmt.Println(err)
    }
}
---------------

This reports the following error:

----------------------------
parsing time "null" as ""2006-01-02T15:04:05Z07:00"":
cannot parse "null" as """
----------------------------


Instead, what should probably happen is to set to the time.Time zero value.  This came
up in the course of answering on Stack Overflow:

http://stackoverflow.com/questions/26684752/json-decode-cannot-parse-timestamp-in-json-into-go-struct

[ianlancetaylor]

Comment 1:

Labels changed: added repo-main, release-go1.5.

[dspezia]

The documentation says:

"The JSON null value unmarshals into an interface, map, pointer, or slice by setting that Go value to nil. Because null is often used in JSON to mean “not present,” unmarshaling a JSON null into any other Go type has no effect on the value and produces no error."

My understanding is a null value bound to a time.Time should leave it unchanged (since a time.Time is a struct). Now, time.Time implements the json.Unmarshaller interface, and the implementation just parses a RFC3339 formatted string, without checking for JSON null values. In the JSON decoding engine, the call to the Unmarshaller interface takes precedence over null value management, which I guess is the origin of the problem.

There are two ways to fix it:

1. A minor change in decode.go can prevent an eventual Unmarshaller method to be called in case of null value, fully leading to the "null for non reference type, no change" semantic.

2. Proper management of null values in the time.Time unmarshaller method can be added.

I have a fix which seems to work fine for the first case, but I'm not 100% sure it is really the expected behavior. Can someone please advise?

[bradfitz]

JSON null probably decode to the zero value of time.Time{}

[dspezia]

Currently, it returns an error. See http://play.golang.org/p/4hwb51LBPG

The question is: should it decode to a zero value (making it a special case), or leave the value unchanged (as specified in the encoding/json package documentation)? And, in that case, should it be managed in encoding/json, or delegated to the unmarshaller implementation of time.Time?

Note that this is actually not specific to time.Time, I can produce a similar issue with any type implementing the Unmarshaller interface - see http://play.golang.org/p/Q6_XQjYIbn

[dspezia]

I think we are in the freeze now, so I suppose we will not change the behavior described in the documentation. Mailing a CL implementing my first solution.

[gopherbot]

CL https://golang.org/cl/9376 mentions this issue.