Support cancellation in DAP communication

[weinand]

Currently there is no way for the VS Code UI to cancel a long running request in the debug adapter.

Example:

VS Code sends out a variables request which takes very long to complete because the variable is a huge array.
User sees the spinner in the variables view and presses "Step over".
This action should cancel the original variables request, however currently there is no way for VS Code to tell the debug adapter to stop what it is currently doing.

Related issues:

• DAP: Support cancellation debug-adapter-protocol#76
• DAP library: Support cancel request vscode-debugadapter-node#186

Cancellation in LSP:
https://microsoft.github.io/language-server-protocol/specification#cancelRequest

[weinand]

@isidorn I've added cancelation support to the "aweinand/cancelation" branch of mock-debug.
Please note: this branch does not contain the fake cancelation via the setVariable request.

[isidorn]

There is already some infrastrucutre for canceling pending requests here
I plan to use that method, and change the logic if the adapter supportsCancelRequest.
Also currently we only call cancelPending when the debug session terminates. I will change that to always call cancelPending when the session continues.

[weinand]

@isidorn just be careful that you do not cancel too much, because that method blindingly cancels all pending requests.

As an experiment you could call that method before the "continue" request even without checking for "supportsCancelRequest".

[isidorn]

@weinand I went through all the requests and the only ones which make sense to survive between steps is all the setBreakpoint requests.
So to start I will cancel all requests on step. In practice I do not think we will hit issues with this approach, though we shall see if I am wrong.

[isidorn]

I have pushed the initial support for this. Whenever we step, or a session is stopped we check if the adapter supports cancel request:

• if yes we send a cancel request for all the pending requests.
• if not we create a fake cancel response after 500 ms - our old strategy

Try it out and let me know what you think.
It would also be great to have some debug adapter that supports this.
@weinand should we ping our debug extension authors about this addition?

[weinand]

What is missing:

• thread specific cancellation: continuing one thread should only cancel requests for that thread abut leave alone requests for other threads.
• eval completion in debug console: typing another character should cancel pending CompletionsRequests.

[isidorn]

We have tackled all the missing issues as mentioned by @weinand
Though not sure how to verify this via verification-needed since we not have clients that support cancelation. Any ideas @weinand ?

[isidorn]

Adding verified label since I have no good ideas on how a tester can nicely verify this. I also tested this.
We should get more feedback once clients start adopting this, and then it will be easier to verify this approach.