encoding/xml: add flag for stricter XML char parsing

[maceonthompson]

Proposal Details

Background

As reported by @DemiMarie

The encoding/xml package does not properly validate that the characters within comments, processing instructions, or directives are properly within the CharData range as defined by the XML specification.

Proposal

Add a godebug flag, xmlvalidatechars=1, which enables more strict validation of characters within comments, processing instructions, and directives. It is my understanding that changing XML behavior can sometimes lead to unexpected behavior/breaking changes, but I have tested what would happen if this flag were enabled by default internally and ran into zero issues.

[gabyhelp]

Related Issues and Documentation

• proposal: encoding/xml: ignore invalid characters when Decoder.Strict is false #62217

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[DemiMarie]

I’d prefer if strict validation were on by default, at least eventually if not initially. Would this be possible, or would it break the Go 1 stability guarantee?

[ianlancetaylor]

Making encoding/xml do strict validation by default would be OK with the Go 1 compatibility guarantee. Adding the GODEBUG would give people an easy way to back off.

That said: why would strict validation be a good default? Who would that help in practice?

[DemiMarie]

Making encoding/xml do strict validation by default would be OK with the Go 1 compatibility guarantee. Adding the GODEBUG would give people an easy way to back off.

That said: why would strict validation be a good default? Who would that help in practice?

XML with malformed characters is ill-formed, so the Principle of Least Astonishment suggests that it should be rejected.

[ianlancetaylor]

XML with malformed characters is ill-formed, so the Principle of Least Astonishment suggests that it should be rejected.

If we were starting from scratch, I would certainly agree. But we aren't. Making this change will most likely break some existing working code, which in an ecosystem that stresses backward compatibility is astonishing in a different way.

[DemiMarie]

XML with malformed characters is ill-formed, so the Principle of Least Astonishment suggests that it should be rejected.

If we were starting from scratch, I would certainly agree. But we aren't. Making this change will most likely break some existing working code, which in an ecosystem that stresses backward compatibility is astonishing in a different way.

Would it be better to create a new XML parsing entry point and mark the old entry points as deprecated? There are a ton of problems in encoding/xml and absolutely strict well-formedness checking might break a lot of code.

[ianlancetaylor]

I think that we should have a plan for encoding/xml/v2. It's clear that there are many problems with encoding/xml. But that is a big undertaking.

In the meantime, we need to decide about this proposal, which is not about v2.

[DemiMarie]

I think that we should have a plan for encoding/xml/v2. It's clear that there are many problems with encoding/xml.

👍

But that is a big undertaking.

💯

In the meantime, we need to decide about this proposal, which is not about v2.

Fair!

You have much more experience than I do when it comes to the Go ecosystem, so I think it would be better for you to make this decision. Other options include adding an extra flag field or setter method.

[aclements]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.