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encoding/gob: spin / crash #2267

bradfitz opened this issue Sep 17, 2011 · 3 comments

encoding/gob: spin / crash #2267

bradfitz opened this issue Sep 17, 2011 · 3 comments


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@bradfitz bradfitz commented Sep 17, 2011

Either the docs or implementation are wrong.

This crashes (after a few second pause burning CPU):

$ cat gob.go
package main

import (

type BlobRef struct {
    foo, bar string

func (br *BlobRef) String() string {
    if br == nil {
        return "<nil BlobRef>"
    return + "-" +

func (br *BlobRef) GobEncode() ([]byte, os.Error) {
        return []byte(br.String()), nil

func (br *BlobRef) GobDecode(b []byte) os.Error {
    if br == nil {
        panic("nil receiver in GobDecode")
    log.Printf("GobDecode(%q)", string(b)) = "foo" = "bar"
    return nil

func main() {
    br := &BlobRef{"foo", "bar"}
    log.Printf("blobref String = %q", br)

    buf := new(bytes.Buffer)
    e := gob.NewEncoder(buf)
    err := e.Encode(br)
        if err != nil {
        log.Fatalf("Encode: %v", err)

    d := gob.NewDecoder(buf)
        var got *BlobRef
        err = d.Decode(&got)
        if err != nil {
        log.Fatalf("Decode: %v", err)
    if got == nil {
                log.Fatalf("got == nil")

    if got.String() != br.String() {
                log.Fatalf("got = %q, want %q", got, br)
    log.Printf("got = %q", &got)

$ gorun gob.go
2011/09/16 17:30:30 blobref String = "foo-bar"
2011/09/16 17:30:30 GobEncode
2011/09/16 17:30:30 GobDecode("foo-bar")
unexpected fault address 0x0
throw: fault
[signal 0xb code=0x80 addr=0x0 pc=0x409172]

goroutine 1 [running]:
main.(*BlobRef).String(0xf840000740, 0x45e7b0, 0xf8400002b8, 0x0)
    /home/bradfitz/goplay/gob.go:18 +0x7d
    /home/bradfitz/goplay/gob.go:57 +0x427

$ hg identify
849002209017 tip


  1. gob.go (1089 bytes)
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@gopherbot gopherbot commented Sep 17, 2011

Comment 1 by robpike:

It's decoding incorrectly, probably due to an incorrect indirection calculation. The
value of got is not right - the strings inside are invalid, and the + operation is
looping. I want to debug this thoroughly - the runtime should protect itself better -
but in the meantime:
A simple fix for this example is to change got to be a BlobRef rather than a *BlobRef
(and remove the comparison against nil). That will work.
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Contributor Author

@bradfitz bradfitz commented Sep 17, 2011

Comment 2:

Yeah, I ended up finding that workaround, but it felt wrong.  Somewhat glad to hear it's
a real bug and I'm not confused.
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@robpike robpike commented Sep 20, 2011

Comment 3:

This issue was closed by revision 9ddc2b5.

Status changed to Fixed.

@bradfitz bradfitz added fixed labels Sep 20, 2011
@mikioh mikioh changed the title gob spin / crash encoding/gob: spin / crash Feb 26, 2015
@golang golang locked and limited conversation to collaborators Jun 24, 2016
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