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encoding/gob: serialize *string = "" and get *string = nil when deserializing #56316

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gbataille opened this issue Oct 19, 2022 · 2 comments
Closed

encoding/gob: serialize *string = "" and get *string = nil when deserializing #56316

gbataille opened this issue Oct 19, 2022 · 2 comments
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@gbataille
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What version of Go are you using (go version)?

$ go version
go version go1.18.6 darwin/amd64

Does this issue reproduce with the latest release?

yes

What operating system and processor architecture are you using (go env)?

go env Output
$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN="/Users/gbataille/go/bin"
GOCACHE="/Users/gbataille/Library/Caches/go-build"
GOENV="/Users/gbataille/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/gbataille/go/pkg/mod"
GOOS="darwin"
GOPATH="/Users/gbataille/go"
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go@1.18/1.18.6/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go@1.18/1.18.6/libexec/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.18.6"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/fs/21b7h4ms1k3669wh2kmjzj6w0000gp/T/go-build2442114366=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

if you deserialize a serialized struct which has a *string field and for which the value is a pointer to an empty string "", the deserialized value for the *string field is nil and not a pointer to the empty string anymore.

I think this is not how it should behave, should it?

type Bar struct {
	Str       string
	PStrNil   *string
	PStrEmpty *string
}

func TestTest(t *testing.T) {
	buf := new(bytes.Buffer)
	enc := gob.NewEncoder(buf)
	strEmpty := ""
	foo := Bar{
		Str:       "",
		PStrNil:   nil,
		PStrEmpty: &strEmpty,
	}
	err := enc.Encode(foo)
	require.NoError(t, err)
	fmt.Printf("%+v\n", foo)
	dec := gob.NewDecoder(buf)
	var res Bar
	err = dec.Decode(&res)
	require.NoError(t, err)
	fmt.Printf("%+v\n", res)
}

Gives, when run

=== RUN   TestTest
{Str: PStrNil:<nil> PStrEmpty:0xc000204bc0}
{Str: PStrNil:<nil> PStrEmpty:<nil>}
--- PASS: TestTest (0.00s)
PASS
ok      sandbox 0.295s
@robpike
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robpike commented Oct 19, 2022

Yes, that's how it works. The encoding has two properties that explain this:

  1. All pointers are flattened to their pointed-to values, as no pointers are transmitted.
  2. Zero values are not transmitted.

The same would happen if you had a field with a pointer to an integer zero, or any other value.

@gbataille
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ok I see. it's very confusing to me. I understand it similar to what protobuf does then. But in a protobuf definition, you don't have pointers, so it's clear.
Here it's feels non-obvious.

In any case, thanks for the quick answer

@golang golang locked and limited conversation to collaborators Oct 19, 2023
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@gbataille @robpike @gopherbot