Add support for Checkpoint requests

[iAbadia]

Hi, I would like to present this PR to add support in the DAP for Checkpoints. The use case I find for this feature are GDB's Checkpoints.

I'm working on a PR for microsoft/vscode-cpptools to add GUI support for Checkpoints in GDB. I have it working using an EvaluateRequest but it would be ideal to use a standard DAP request instead. If this PR goes ahead I'll add the subsequent PR on microsoft/MIEngine to make use of it 🙂

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[iAbadia]

@microsoft-github-policy-service agree

[connor4312]

I think representing checkpointed processes as Threads might make sense. That allows individual addressability and lifetime management (and UI reuse for consumers) without a ton of new checkpoint-specific calls. What do you think?

[iAbadia]

Thank for the reply @connor4312 !

I love the idea of getting UI figured out "for free" but I'm concerned it may not be the best solution in this situation given the fundamental difference between Threads and Checkpoints. Since my use case is GDB in Linux, all this is Linux-specific:

While C++ threads are created as Linux threads (they all share a PID and are given individual SPIDs or TIDs, can share resources like memory and file descriptors), GDB handles Checkpoints as different Processes (they each have their own PID, men space, file descriptors, etc.). This makes it very easy to spawn and kill Checkpoints, while with Threads is not so trivial and rarely desired.

On the other hand it's true that Checkpoints and Threads are mutually exclusive (See bug 12628), which would work nicely UI-wise (If Threads: Show thread options; else show Checkpoint options) but Checkpoints seems like too much of a niche use case (C/C++ on Linux) to make a convincing argument for increasing the complexity of the common case (Any lang on any platform).

[connor4312]

Checkpoints seems like too much of a niche use case (C/C++ on Linux) to make a convincing argument for increasing the complexity of the common case (Any lang on any platform).

Yea, that's why I'm trying to see if we can roll it into Threads so client's don't have to do a lot of work (or, realistically, choose not to do the work) to support the niche.

While C++ threads are created as Linux threads (they all share a PID and are given individual SPIDs or TIDs, can share resources like memory and file descriptors), GDB handles Checkpoints as different Processes (they each have their own PID, men space, file descriptors, etc.). This makes it very easy to spawn and kill Checkpoints, while with Threads is not so trivial and rarely desired.

The resource usage and sharing is not something we really care about in the DAP specification, as far as I know. For example, while there is representation of memory, this is given on a variable-by-variable basis and not required to be contiguous for a program. And we do support thread termination already.

My question is, are there things that DAP can't express with threads that we'd want to do with checkpoints, and are there things DAP can do with threads that wouldn't be applicable for checkpoints?

[iAbadia]

are there things that DAP can't express with threads that we'd want to do with checkpoints, and are there things DAP can do with threads that wouldn't be applicable for checkpoints?

The only thing that can't be done with the current DAP spec is the creation of checkpoints. Checkpoint deletion can be done as Thread termination (a limitation would be that the main or Checkpoint 0 can't be terminated). Checkpoints can be represented as threads (they both have ID and a stack trace). Checkpoint resume can be done as Thread resume (thanks to supportsSingleThreadExecutionRequests since only one checkpoint can run at a time).

I'm thinking that an option would be to add Checkpoint Creation as Thread Spawn, and to go with the thread support. supportsThreadSpawn or similar would effectively mean what supportsCheckpointRequests does in my proposal.

[connor4312]

I'm okay keeping the verbiage as "Create Checkpoint", but it sounds like it's okay for the return value of that to be a thread ID, and then for clients to interact with that thread normally (or, if they want to, they could have special UI handling.) Then we only need a single new request for that.

cc @roblourens for thoughts too

[gregg-miskelly]

@iAbadia, in case this isn't obvious -- if your goal is just to support this in MIEngine with UI the C++ tools VS Code extension, it doesn't make sense for this to be in the standard, and doing so probably makes your task more difficult.

The reasons to add this to the standard are:

• Multiple DAP clients (ex: VS Code, vim) are interested in adding support for the snapshot management UI
  -and/or-
• Multiple debug adapters are interested in supporting these interfaces and at least one DAP client (ex: VS Code) is interested in supporting it

[puremourning]

I would be interested in adding support to Vimspector, but using the threads abstraction seems on the face of it odd to me. It feels like a lot of special casing would be required client side to surface this set of functions in a reasonable and discoverable way.

I would be happy perhaps to work with the cpptools team on a prototype to see how the UX turns out. But I suspect that something more like breakpoints is a better model, though I could be wrong.

[gregg-miskelly]

I have never used checkpoints in GDB, but in the general concept, checkpoints can have multiple threads, so I would agree with @puremourning that I don't think it makes sense to treat them like threads.

[iAbadia]

I would be happy perhaps to work with the cpptools team on a prototype to see how the UX turns out. But I suspect that something more like breakpoints is a better model, though I could be wrong.

I have an almost finished version of this. It adds a view like the Breakpoints View on the debug tab. The last bit pending is a way to ask the Watch and Variables view to refresh after loading a Checkpoint. Unfortunately the VSCode Extension API doesn't offer a way to do this and I was going to add support on MIEngine for a InvalidateRequest that would trigger an InvalidateEvent (which those Views listen for and refresh). The API exposes the DebugSession so te extension can issue the InvalidateRequest.

I opened this PR as it's tangent to that other piece of work and I thought: why not?

[roblourens]

The last bit pending is a way to ask the Watch and Variables view to refresh after loading a Checkpoint. Unfortunately the VSCode Extension API doesn't offer a way to do this and I was going to add support on MIEngine for a InvalidateRequest that would trigger an InvalidateEvent (which those Views listen for and refresh). The API exposes the DebugSession so te extension can issue the InvalidateRequest.

I think I don't follow why the invalidation is driven by the view, wouldn't the Checkpoint be loaded by the debug adapter anyway, and it would know that it needs to then fire an InvalidateEvent?

[iAbadia]

@roblourens After loading the checkpoint the debugger state has changed but the UI views don't know it has so they won't refresh. I asked in vscode-discussions#535 and Extensions can't just reach into the Views and ask them to refresh, which would be ideal. This is why initially I decided to go down to the debug adapter and fire an event from there since these views will receive it. From your comment I see how I could bundle sending an InvalidateEvent after receiving a LoadCheckpoint, which would save having to send an InvalidateRequest. I like this