Add support for VT52 emulation

[j4james]

Summary of the Pull Request

This PR adds support for the core VT52 commands, and implements the DECANM private mode sequence, which switches the terminal between ANSI mode and VT52-compatible mode.

References

PR #2017 defined the initial specification for VT52 support.
PR #4044 removed the original VT52 cursor ops that conflicted with VT100 sequences.

PR Checklist

• Closes Investigate ESC D defined as IND vs CUB #976
• CLA signed. If not, go over here and sign the CLA
• Tests added/passed
• Requires documentation to be updated
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: First draft of a spec for VT52 escape sequences #2017

Detailed Description of the Pull Request / Additional comments

Most of the work involves updates to the parsing state machine, which behaves differently in VT52 mode. CSI, OSC, and SS3 sequences are not applicable, and there is one special-case escape sequence (Direct Cursor Address), which requires an additional state to handle parameters that come after the final character.

Once the parsing is handled though, it's mostly just a matter of dispatching the commands to existing methods in the ITermDispatch interface. Only one new method was required in the interface to handle the Identify command.

The only real new functionality is in the TerminalInput class, which needs to generate different escape sequences for certain keys in VT52 mode. This does not yet support all of the VT52 key sequences, because the VT100 support is itself not yet complete. But the basics are in place, and I think the rest is best left for a follow-up issue, and potentially a refactor of the TerminalInput class.

I should point out that the original spec called for a new Graphic Mode character set, but I've since discovered that the VT terminals that emulate VT52 just use the existing VT100 Special Graphics set, so that is really what we should be doing too. We can always consider adding the VT52 graphic set as a option later, if there is demand for strict VT52 compatibility.

Validation Steps Performed

I've added state machine and adapter tests to confirm that the DECANM mode changing sequences are correctly dispatched and forwarded to the ConGetSet handler. I've also added state machine tests that confirm the VT52 escape sequences are dispatched correctly when the ANSI mode is reset.

For fuzzing support, I've extended the VT command fuzzer to generate the different kinds of VT52 sequences, as well as mode change sequences to switch between the ANSI and VT52 modes.

In terms of manual testing, I've confirmed that the Test of VT52 mode in Vttest now works as expected.

[zadjii-msft]

This seems fine to me, and there's a lot of good work in here. Sorry we took so long :P

[DHowett]

@msftbot make sure @miniksa signs off on this

[ghost]

Hello @DHowett!

Because you've given me some instructions on how to help merge this pull request, I'll be modifying my merge approach. Here's how I understand your requirements for merging this pull request:

• I'll only merge this pull request if it's approved by @miniksa

If this doesn't seem right to you, you can tell me to cancel these instructions and use the auto-merge policy that has been configured for this repository. Try telling me "forget everything I just told you".

[miniksa]

Excellent, @j4james. Very glad to have this. Again and as usual lately, sorry for the delay.

[ghost]

Hello @zadjii-msft!

Because this pull request has the AutoMerge label, I will be glad to assist with helping to merge this pull request once all check-in policies pass.

p.s. you can customize the way I help with merging this pull request, such as holding this pull request until a specific person approves. Simply @mention me (@msftbot) and give me an instruction to get started! Learn more here.

[DHowett]

I read this post-merge and just wanted to say it's stellar as always.

[ghost]

🎉Windows Terminal Preview v1.1.1671.0 has been released which incorporates this pull request.:tada:

Handy links:

• Release Notes
• Store Download

[DHowett]

🎉 Once again, thanks for the contribution!

This pull request was included in a set of conhost changes that was just
released with Windows Insider Build 20161.