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dominikh opened this issue Aug 6, 2024 · 6 comments
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encoding/json: nested unmarshaling mangles errors #68750

dominikh opened this issue Aug 6, 2024 · 6 comments
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NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.

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@dominikh
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dominikh commented Aug 6, 2024

Go version

go version devel go1.24-96f7159705 Tue Jul 30 20:12:39 2024 +0000 linux/amd64

Output of go env in your module/workspace:

GO111MODULE=''
GOARCH='amd64'
GOBIN=''
GOCACHE='/home/dominikh/.cache/go-build'
GOENV='/home/dominikh/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFLAGS=''
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMODCACHE='/home/dominikh/prj/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/dominikh/prj'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/dominikh/prj/go'
GOSUMDB='sum.golang.org'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/home/dominikh/prj/go/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='devel go1.24-96f7159705 Tue Jul 30 20:12:39 2024 +0000'
GODEBUG=''
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/dominikh/.config/go/telemetry'
GCCGO='gccgo'
GOAMD64='v1'
AR='ar'
CC='gcc'
CXX='g++'
CGO_ENABLED='1'
GOMOD='/home/dominikh/prj/src/example.com/go.mod'
GOWORK=''
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
PKG_CONFIG='pkg-config'
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/nix-shell.pip13R/go-build4163698412=/tmp/go-build -gno-record-gcc-switches'

What did you do?

https://play.golang.org/p/Ypv2xaFDAOs

package main

import (
	"encoding/json"
	"fmt"
)

type Foo struct {
	Bar Bar
}

type Bar struct{}

func (*Bar) UnmarshalJSON(data []byte) error {
	var dst struct {
		K int `json:"k"`
	}
	err := json.Unmarshal(data, &dst)
	fmt.Println(err)
	return err
}

func main() {
	var f Foo
	fmt.Println(json.Unmarshal([]byte(`{"Bar": {"k": "v"}}`), &f))
}

What did you see happen?

json: cannot unmarshal string into Go struct field .k of type int
json: cannot unmarshal string into Go struct field Foo.Bar of type int

What did you expect to see?

A less misleading error returned by the call to json.Unmarshal in main. The type of Foo.Bar is not int and we no longer know where the error actually happened.
@gabyhelp
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gabyhelp commented Aug 6, 2024

Related Issues and Documentation

(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)

@dominikh
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dominikh commented Aug 6, 2024

Thanks bot. This is #61337, which was closed as a duplicate of #11858. I disagree with that decision. While they're related by what causes them, they're two different problems, and this issue will probably be easier to fix than #11858.

@dr2chase
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dr2chase commented Aug 6, 2024

@dsnet @mvdan

@GodsBoss GodsBoss mentioned this issue Aug 7, 2024
Merged
@cagedmantis cagedmantis added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Aug 12, 2024
@cagedmantis
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cc @rsc

@j2gg0s
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j2gg0s commented Aug 14, 2024 •
edited
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We should join err.Field with d.errContext.FieldContext if err.Field is not empty?

@j2gg0s j2gg0s mentioned this issue Aug 14, 2024
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@gopherbot
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Change https://go.dev/cl/605455 mentions this issue: encoding/json: optimize UnmarshalTypeError's field stack for nested unmarshal

j2gg0s added a commit to j2gg0s/go that referenced this issue Aug 14, 2024
@j2gg0s
encoding/json: optimize type error's field stack for nested unmarshal.
c9a6f13 
When people return UnmarshalTypeError in UnmarshalJSON,
we should append error's Field to FieldStack.

Fixes golang#68750
j2gg0s added a commit to j2gg0s/go that referenced this issue Aug 15, 2024
@j2gg0s
encoding/json: merge FieldStack if the error's Field exists.
18796ad 
When people return UnmarshalTypeError in UnmarshalJSON,
we should append error's Field to FieldStack.

Fixes golang#68750
@gabyhelp gabyhelp mentioned this issue Aug 19, 2024
Closed
@gabyhelp gabyhelp mentioned this issue Aug 30, 2024
Closed
@gabyhelp gabyhelp mentioned this issue Feb 27, 2025
Closed
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encoding/json: merge FieldStack if the error's Field exists. j2gg0s/go
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@dominikh @cagedmantis @dr2chase @gopherbot @j2gg0s @gabyhelp