encoding/xml: Encoder duplicates namespace tags

[gopherbot]

by seanerussell:

== What does 'go version' print?

go version go1.2.1 darwin/amd64

== What steps reproduce the problem?

http://play.golang.org/p/3_oUruPYhq

== What happened?

Encoder.EncodeToken duplicates namespace attributes.

== What should have happened instead?

The encoded document should have had a single namespace attribute.

== Please provide any additional information below.

Attribute names on an element must be unique; this is a well-formedness constraint per
the XML 1.0 specification (http://www.w3.org/TR/xml/#uniqattspec). Per the
specification, both validating and non-validating parsers must report well-formedness
violations (http://www.w3.org/TR/xml/#sec-conformance).

Encoding and decoding XML documents should be idempotent and produce equivalent
documents.  This issue means that not only that decoding and encoding the result
produces a non-equivalent document, but that the document it generates is
not-well-formed.

This issue only occurs with namespaces.  Normal attributes are handled correctly.

[ianlancetaylor]

Comment 1:

Labels changed: added repo-main, release-none.

[rogpeppe]

Comment 2:

Here's another example: http://play.golang.org/p/GTjuLNxE-d
This lack of encode/decode idempotency makes things awkward when trying to test for
expected output, as well as the lack of well formedness.

[gopherbot]

Comment 3:

CL https://golang.org/cl/179540043 mentions this issue.

[mikioh]

See #11841

[rsc]

Blocked on #13400.

[pdw-mb]

I would expect Token to strip xmlns attributes: if you want them, use RawToken.

The change below does that and appears to fix both of the above examples:

https://code.blinkace.com/go/xml/commit/bded824c18c5a2595e750c920ea5e7437607900c

The code base above also exposes the current set of namespace bindings on Decoder, which is generally more useful that having the xmlns attributes themselves (see #12406)

[alexellis]

I wanted to know if there is a work around for this yet (duplication of namespaces)?

I want to use a decoder/encoder combination with Token() to selectively reconstitute an XML document (.NET csproj format)

	for {
		token, _ := decoder.Token()

		encoder.EncodeToken(token)
		if token == nil {
			break
		}
     }

Creating and maintaining all the structs to demarshal into an object is not a suitable solution.

Input:

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="4.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup>
    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
    <ProductVersion>9.0.30729</ProductVersion>
    <SchemaVersion>2.0</SchemaVersion>
    <ProjectGuid>{153CB7F7-EB7B-44F2-B53E-F157288E3F19}</ProjectGuid>
    <OutputType>Library</OutputType>
    <AppDesignerFolder>Properties</AppDesignerFolder>

Output:

<?xml version="1.0" encoding="utf-8"?>
<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003" ToolsVersion="4.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
    <Configuration xmlns="http://schemas.microsoft.com/developer/msbuild/2003" Condition=" &#39;$(Configuration)&#39; == &#39;&#39; ">Debug</Configuration>
    <Platform xmlns="http://schemas.microsoft.com/developer/msbuild/2003" Condition=" &#39;$(Platform)&#39; == &#39;&#39; ">AnyCPU</Platform>
    <ProductVersion xmlns="http://schemas.microsoft.com/developer/msbuild/2003">9.0.30729</ProductVersion>
    <SchemaVersion xmlns="http://schemas.microsoft.com/developer/msbuild/2003">2.0</SchemaVersion>
    <ProjectGuid xmlns="http://schemas.microsoft.com/developer/msbuild/2003">{153CB7F7-EB7B-44F2-B53E-F157288E3F19}</ProjectGuid>
    <OutputType xmlns="http://schemas.microsoft.com/developer/msbuild/2003">Library</OutputType>
    <AppDesignerFolder xmlns="http://schemas.microsoft.com/developer/msbuild/2003">Properties</AppDesignerFolder>