encoding/xml: incompatible changes in the Go 1.21.0

[xuri]

What version of Go are you using (go version)?

Go1.21.0

Does this issue reproduce with the latest release?

Yes

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

darwin/amd64

What did you do?

There are some incompatible changes in the Go 1.21.0 encoding/xml library. Please take a look at following example:

package main

import (
    "encoding/xml"
    "fmt"
)

func main() {
    type B struct {
        XMLName xml.Name `xml:"b"`
        C       bool     `xml:"c"`
    }

    type A struct {
        XMLName xml.Name `xml:"http://example.com a"`
        B       B
    }

    var a A

    input := `<a xmlns="http://example.com"><b><c>true</c></b></a>`

    if err := xml.Unmarshal([]byte(input), &a); err != nil {
        fmt.Print(err)
    }

    output, err := xml.Marshal(&a)
    if err != nil {
        fmt.Println(err)
    }

    fmt.Printf("input:  %s\r\noutput: %s", input, string(output))
}

What did you expect to see?

Output by Go 1.20.7 and previous Go released version

input:  <a xmlns="http://example.com"><b><c>true</c></b></a>
output: <a xmlns="http://example.com"><b><c>true</c></b></a>

What did you see instead?

Output by Go 1.21.0

input:  <a xmlns="http://example.com"><b>true</b></a>
output: <a xmlns="http://example.com"><b xmlns=""><c>true</c></b></a>

There are new empty XML namespace xmlns attributes in the serialized output, which made the input and output XML content inconsistent without any modify.

I created a patch for it and still waiting for a reply, follow up to https://go.dev/cl/466295. Relates to #58401, and external Excelize project issues #1465, #1595 and #1603.

Thanks.

[gopherbot]

Change https://go.dev/cl/466295 mentions this issue: encoding/xml: overriding by empty namespace when no new name declaration

[mknyszek]

CC @rsc via https://dev.golang.org/owners

[bcmills]

Seems like this was probably introduced by https://go.dev/cl/108796 (for #7113)?

(attn @iwdgo @ianlancetaylor)

[bcmills]

Per #56986, though, it seems like this change it behavior should at least have been guarded by a new GODEBUG value. 🤔

[Hr0bar]

Is the milestone Go 1.22 going to be kept ? Shouldnt this be fixed in 1.21 minor version ?

[jfesler]

This has me in a bind. :(. I'm having to advise people using my code to copy go1.20's encoding/xml on top of go1.21 as a stopgap.

Extra annoying this wasn't in the release notes.

[bcmills]

@gopherbot, please backport to Go 1.21. Per #56986 this should at least have a release note and a GODEBUG guard.

[gopherbot]

Backport issue(s) opened: #62051 (for 1.21).

Remember to create the cherry-pick CL(s) as soon as the patch is submitted to master, according to https://go.dev/wiki/MinorReleases.