encoding/json: Decoding gives errPhase when unmarshaling a non-string literal into a TextUnmarshaler #9650

cespare · 2015-01-21T01:39:48Z

updated

When decoding a non-string literal into an encoding.TextUnmarshaler (such as net.IP), json.Unmarshal returns errPhase ("JSON decoder out of sync - data changing underfoot?").

Reading the code, it seems like it's supposed to return a type error when this happens
(source). But the errPhase ends up taking precedence and obscuring this message.

I'm not sure why UnmarshalText should only work for string literals, though.

Example play here.

Code:

package main

import (
  "encoding/json"
  "fmt"
  "net"
  "strconv"
)

type X int

func (x *X) UnmarshalText(b []byte) error {
  n, err := strconv.ParseInt(string(b), 10, 64)
  if err != nil {
    return err
  }
  *x = X(n)
  return nil
}

func main() {
  var x X
  err := x.UnmarshalText([]byte("123"))
  fmt.Println(x, err) // 123 <nil>

  // Works if the string is quoted
  err = json.Unmarshal([]byte(`"234"`), &x)
  fmt.Println(x, err) // 234 <nil>

  // Otherwise it gives an errPhase
  err = json.Unmarshal([]byte("123"), &x)
  fmt.Println(err) // JSON decoder out of sync - data changing underfoot?
  err = json.Unmarshal([]byte("true"), &x)
  fmt.Println(err) // JSON decoder out of sync - data changing underfoot?
  err = json.Unmarshal([]byte("null"), &x)
  fmt.Println(err) // JSON decoder out of sync - data changing underfoot?

  // Happens with real types like net.IP

  var ip net.IP
  err = json.Unmarshal([]byte("8080"), &ip)
  fmt.Println(err) // JSON decoder out of sync - data changing underfoot?

}

The text was updated successfully, but these errors were encountered:

awly · 2015-01-21T01:45:22Z

Note that this doesn't happen when json value is quoted as expected by TextUnmarshaler.

package main

import (
    "encoding/json"
    "errors"
    "fmt"
)

type X struct{}

func (x *X) UnmarshalText(b []byte) error {
    return errors.New("Hi!")
}

func main() {
    var x X
    err := json.Unmarshal([]byte("\"123\""), &x)
    fmt.Println(err) // Hi!
}

cespare · 2015-01-21T02:02:04Z

@alytvynov This bug report is for non-string literals. It is true that it passes the correct error for string literals.

I just noticed that this happens even when the TextUnmarshaler doesn't return an error. Updating the description now.

awly · 2015-01-21T02:04:16Z

@cespare sure, just pointing that out as a possible hint for debugging.

dominikh · 2015-07-15T07:00:27Z

@rsc: Two questions about your fix:

Can we infer from your fix that unmarshaling a non-string value, even null, into a TextUnmarshaler is always an error? That is, null can't be unmarshaled into a *net.IP, even though both *net.IP and net.IP can be nil?
The error now reads cannot unmarshal string into Go value of type [...] when trying to unmarshal null. Should null really be considered a string, or should an issue be filed for this?

cespare changed the title ~~encoding/json: Unmarshal gives errPhase on non-string literal when underlying TextUnmarshaler returns an error~~ encoding/json: Decoding gives errPhase when unmarshaling a non-string literal into a TextUnmarshaler Jan 21, 2015

rsc added this to the Go1.5Maybe milestone Apr 10, 2015

rsc mentioned this issue Jul 15, 2015

encoding/json: UnmarshalText confuses json.Unmarshal #8462

Closed

rsc closed this as completed in 671bddf Jul 15, 2015

mikioh modified the milestones: Go1.5, Go1.5Maybe Jul 16, 2015

golang locked and limited conversation to collaborators Jul 18, 2016

gopherbot added the FrozenDueToAge label Jul 18, 2016

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encoding/json: Decoding gives errPhase when unmarshaling a non-string literal into a TextUnmarshaler #9650

encoding/json: Decoding gives errPhase when unmarshaling a non-string literal into a TextUnmarshaler #9650

cespare commented Jan 21, 2015

awly commented Jan 21, 2015

cespare commented Jan 21, 2015

awly commented Jan 21, 2015

dominikh commented Jul 15, 2015

encoding/json: Decoding gives errPhase when unmarshaling a non-string literal into a TextUnmarshaler #9650

encoding/json: Decoding gives errPhase when unmarshaling a non-string literal into a TextUnmarshaler #9650

Comments

cespare commented Jan 21, 2015

awly commented Jan 21, 2015

cespare commented Jan 21, 2015

awly commented Jan 21, 2015

dominikh commented Jul 15, 2015