spanner: Different behavior between string and spanner.NullString when unmarshal from json

[CuberL]

Client

Spanner

Environment

Local

Go Environment

$ go version
go version go1.17.2 darwin/amd64
$ go env

Code

package main

import (
	"encoding/json"
	"fmt"

	"cloud.google.com/go/spanner"
)

type Example struct {
	FieldA spanner.NullString `json:"field_a"`
	FieldB string             `json:"field_b"`
}

func main() {
	exampleStr := []byte(`
{
	"field_a": "{\"sub_a\": \"value_1\"}",
        "field_b": "{\"sub_b\": \"value_2\"}"
}
`)
	e := &Example{}
	json.Unmarshal(exampleStr, e)
	fmt.Println(e.FieldA)
	fmt.Println(e.FieldB)
}

Expected behavior

Output:

{"sub_a": "value_1"}
{"sub_b": "value_2"}

Actual behavior

{\"sub_a\": \"value_1\"}
{"sub_b": "value_2"}

Additional context

google-cloud-go/spanner/value.go

Line 289 in 70e40db

payload, err := trimDoubleQuotes(payload)

Seems that the code didn't handle this case and will have different behavior between string and spanner.NullString when unmarshal from json and some escape characters is in the string.