encoding/gob: (or maybe time.Time): unexplained error with time.Time value

[robpike]

http://play.golang.org/p/cNlOycuLzl is this:

package main

import (
    "bytes"
    "encoding/gob"
    "time"
)

type Update struct {
    Time        time.Time   // ***  Note1 : Works if change from time.Time -> int ***//
}

type Time struct {
    T   *time.Time
}

type PC struct {
    Id            int64 
    Update []Update  // ***  Note2 : Works if comment out one of  Update or LastUpdate ***//
    LastUpdate    Time 
}

func main() {
    buf := new(bytes.Buffer)
    enc := gob.NewEncoder(buf)
    var err error
    if err = enc.Encode(PC{}); err != nil {
        println("A", err.Error())
        return
    }
    dec := gob.NewDecoder(bytes.NewBuffer(buf.Bytes()))
    if err = dec.Decode(new(PC)); err != nil {
        println("B", err.Error())
        return
    }
}

It prints "B extra data in buffer. Removing either of the time.Time values from the
struct or replacing them with ints clears up the error. Something is awry with gots and
time.

[kylelemons]

Comment 1:

It doesn't seem to be related to time.  It seems to be related to two
GobEncoder/GobDecoders in the same value where one is a pointer and the other isn't:
package main
import (
    "bytes"
    "encoding/gob"
)
type T struct{}
func (T) GobEncode() ([]byte, error) {
    return []byte{0xDE, 0xAE, 0xAD, 0xBE, 0xEF}, nil
}
func (T) GobDecode([]byte) error {
    return nil
}
type PC struct {
    A     T
    B *T
}
func main() {
    buf := new(bytes.Buffer)
    enc := gob.NewEncoder(buf)
    var err error
    if err = enc.Encode(PC{}); err != nil {
        println("A", err.Error())
        return
    }
    dec := gob.NewDecoder(bytes.NewBuffer(buf.Bytes()))
    if err = dec.Decode(new(PC)); err != nil {
        println("B", err.Error())
        return
    }
}
http://play.golang.org/p/7MfiN4ToAZ

[kortschak]

Comment 2:

It looks like the arrangement of pointer/non-pointer fields makes a difference. If the
pointer field is first, there is no problem, in other positions the bug appears.

[kylelemons]

Comment 3:

Dan is correct.  It appears to happen because GobEncoders are treated specially and as
such, the *T is encoded immediately, despite the T having already been encoded.  The
"extra data in buffer" error is actually masking the "gob: duplicate type received"
error (which we might also want to address).
I don't know if this is the proper fix, but it makes my test pass:
https://golang.org/cl/7085051

[adg]

Comment 4:

This issue was closed by revision bc1152a.

Status changed to Fixed.