encoding/xml: disallow attributes named xmlns:*

[gopherbot]

by opennota:

The Encoder will add namespaced to the elements, even if there are the same namespaces
in the parent element:
http://play.golang.org/p/LDRJCxUJHX

// Output:
// <p xmlns="http://www.gribuser.ru/xml/fictionbook/2.0";
xmlns:l="http://www.w3.org/1999/xlink";><a
xmlns="http://www.gribuser.ru/xml/fictionbook/2.0";
xmlns:xlink="http://www.w3.org/1999/xlink";
xlink:href="http://google.com";>google.com</a></p>

I expect it to not repeat the namespaces:

// <p xmlns="http://www.gribuser.ru/xml/fictionbook/2.0";
xmlns:l="http://www.w3.org/1999/xlink";><a
l:href="http://google.com";>google.com</a></p>

[ianlancetaylor]

Comment 1:

Labels changed: added repo-main, release-go1.4.

[gopherbot]

Comment 2 by glen.newton:

Output XML is correct and equivalent to input XML (thus idempotent transformation).
And - unfortunately - larger.
That said, as this issue does not result in incorrect and/or non-equivalent XML, I would
suggest first fixing higher priority bugs that either produce incorrect and/or
non-equivalent XML (or just break), such as #7535, #6800, #8068, #8535, #7113

[rsc]

Comment 3:

It's not intended that you can even emit xmlns: attributes that way. The encoder does
not know about them and may step on you (or vice versa). If you want to put the name
space definition in an outer element, the easiest thing to do is to use it in an
attribute on that element: http://play.golang.org/p/2-Y-CrhaP7, which will let the
Encoder define it instead.
The test program in the initial report may be just invalid for trying to define an
attribute named xmlns:l. Perhaps we should disallow all attributes named xmlns:* in a
future version of Go. Not for 1.4 though.

Labels changed: added release-go1.5, removed release-go1.4.

Status changed to Accepted.

[bradfitz]

CL is here 4 days ago: https://go-review.googlesource.com/#/c/12351/ but lacked the magic syntax, so this bug wasn't updated.

[gopherbot]

CL https://golang.org/cl/12351 mentions this issue.