proposal: encoding/xml: reject ill-formed XML

[DemiMarie]

Proposal Details

encoding/xml has multiple problems:

1. It does not check for XML well-formedness constraints: encoding/xml: allows a colon before or after an XML name #68294, encoding/xml: does not reject duplicate attribute names #68295
2. It does not check for XML namespace constraints: encoding/xml: does not check namespace constraints that do not require keeping extra state #68296, encoding/xml: does not reject multiple attributes with the same namespace URL and local part #68297
3. Its handling of XML namespaces is known to be buggy.

This proposal covers all issues linked by #68293. These can all be fixed internally to encoding/xml, without changes to the API. However, there will be new API on xml.Decoder:

const (
	AllowLeadingColons = 1 << iota
	AllowTrailingColons
	AllowDuplicateAttributes
	// possibly other flags
)
	
/*
	Sets whether the parser allows ill-formed XML.
	Prior to 1.22, the parser always allowed ill-formed XML.
	Starting in 1.23, ill-formed XML is not allowed by default,
	but it can be re-enabled by calling decoder.AllowIllFormed(-1).
	The Allow* flags can be used for more fine-grained control.
*/
func (d *xml.Decoder) AllowIllFormed(flags int64)

and a GODEBUG flag allow-ill-formed-xml=<bitmask> for course-grained global control.

In the future, xml.Decoder will reject ill-formed XML. If it is found to accept ill-formed XML, this will be considered a bug and fixed, with a new flag so that applications can opt-in to the old behavior.

Debug flags for encoding/xml may be removed with not less than two major versions notice.

My understanding is that this change is enough to guarantee round-trip stability for RawToken users (not for Token users). I believe that is possible to implement a namespace-aware, round-trip-stable parser correctly on top of the RawToken API, but the standard library does not currently implement such a parser.

This is essentially “merge #48641 + debug flags”. this will require additional changes beyond this PR.

[gabyhelp]

Related Issues

• encoding/xml: accepts various ill-formed XML #68293
• proposal: encoding/xml: Return a SyntaxError for unmatched start elements at EOF #11405 (closed)
• encoding/xml: does not check namespace constraints that do not require keeping extra state #68296
• encoding/xml: fix name spaces #13400
• proposal: encoding/xml: ignore invalid characters when Decoder.Strict is false #62217
• encoding/xml: does not reject multiple attributes with the same namespace URL and local part #68297

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