Fix restoration of R and Python sessions in server mode (for Workbench)

[jmcphers]

This change fixes some issues around reconnecting to Python and R after various actions that unload the UI (browser refresh, loading in a new tab, closing a tab and coming back, reloading the desktop UI, etc).

The underlying issue is that we remember which Python and R sessions are running by saving a list of active sessions into VS Code's workspace storage. Unfortunately, workspace storage is not a suitable place to put these things, because it is too durable -- it hangs around even when Positron is closed -- and, in browser mode, it can also be shared among workspaces when switching folders. Previous iterations of this change tried to work around these problems by adding additional metadata or clearing out the storage in some situations, but we were not able to find a solution that covered all the edge cases.

To address this holistically we need a place to store the list of sessions that is unique to the workspace and the session -- that is, it has the same lifetime as the server process that hosts Python and R.

This change adds that, in the form of a new service called an "Ephemeral State Service". This service provides a way to store state that is ephemeral (compared to the durable IStorageService) but needs to persist across a user session, through browser refreshes and disconnects/reconnects.

flowchart TD
subgraph "Positron Desktop"
f[Frontend Process] -- IPC Channel --> e[Electron Main]
e -- IPC Channel --> f
e --> m1[(In-Memory Ephemeral Store)]
end

subgraph "Positron Server"
b[Browser] -- Remote Agent Channel --> s[Server]
s -- Remote Agent Channel --> b
s --> m2[(In-Memory Ephemeral Store)]
end

We store the list of active sessions in this new service, so that the session list has the same scope as the sessions themselves.

Addresses https://github.com/posit-dev/positron/issues/5267.

QA Notes

This affects the way we remember which interpreter sessions are running across all of Positron's various configurations. We don't have to test every member of this matrix, but it wouldn't hurt to try to get a representative sample:

Lifecycles:

• Close and reopen
• Reload page
• Exit / new session

Environments:

• Dev build
• Release build

Targets:

• Positron Desktop (Electron)
• Posit Workbench
• Remote SSH
• Positron Server (dev only)

The main class of errors will be either (a) attempts to reconnect to sessions that don't exist, and (b) failure to reconnect to sessions that do exist. In either case the most helpful logs are from the Developer Tools as that's where most of this code logs.

[sharon-wang]

Positron Server Web

Tested with a local dev build of Positron Server Web on Mac. Video shows R session outside of a folder, but I also tested within a project folder and with Python.

• ✅ Session variable persisted across tabs
• 🤔 There's some weirdness when using the same interpreter in the Console in multiple tabs, but I'm not sure we expect that to work? From 0:36 - 0:43, I'm pressing the Enter key but the Console doesn't respond or indicate that I can't perform the action. This occurs for both Python and R.

https://github.com/user-attachments/assets/fdcbad8d-5aff-46fb-b91f-bacba63b9a67

---

Waiting on a local build to test this in Workbench as well.

[jmcphers]

I'm not sure we expect that to work?

No, this definitely does not work today. We do need to fix this up, either by marking the session in the "old" tab as defunct, or by allowing two different IDEs to connect to the same session (I can file an issue). But this PR doesn't attempt to do either one.

(edit: filed as https://github.com/posit-dev/positron/issues/5374)

FWIW the Terminal tab has very nice behavior here -- if you try opening two different browser tabs with the same terminal, you'll see commands and output reflected in both!

[sharon-wang]

Positron on Workbench

Posit Workbench 2024.12.0-daily+364.pro11, “Kousa Dogwood” (9de4d019)

Video shows Python session outside of a folder, but I also tested within a project folder and with R.

• ✅ Session variable persisted across tabs

https://github.com/user-attachments/assets/fcb775bb-4c06-488e-b1a8-4d072adf4896

[sharon-wang]

There is some weirdness when switching folders. I didn't notice this previously, but I can now reproduce it in Positron Server Web as well.

This video is not very concise. You can skip to 1:30.

https://github.com/user-attachments/assets/20a29674-2422-4bce-8869-65882f60c1eb

[sharon-wang]

I was seeing duplicate interpreters in the Console and Interpreter dropdowns, but not anymore after the recent commits. This is what I was previously seeing:

I'm able to get out of a broken interpreter state much more easily now. I'm still hitting a case where switching folders causes a reconnect failure in my Consoles (skip to ~0:45), but I may be in a weird state after testing a bunch of things. If it isn't easily reproducible by others, then I think we're good to go on this!

https://github.com/user-attachments/assets/6bb4e6d7-1e98-4815-8f38-328cd5092b06

[sharon-wang]

Positron on Workbench @ 14951a8

Posit Workbench 2024.12.0-daily+375.pro4, “Kousa Dogwood” (123323c8)

Reloading and opening a recent folder

• reloading retains session state ✅
• switching to a recent folder causes us to try to reconnect to the session and fail, but clicking the Power button on the Console resolves the issue

https://github.com/user-attachments/assets/c2fafbab-6f5c-4178-be8e-034ba2783f6d

Opening a new Positron session

• the new Positron session with no workspace resumes the session from the other Positron session (the same vars are still set in the video, carried over from the session in the previous video)
  • this was a bit unexpected as I thought I'd get a new, empty session

https://github.com/user-attachments/assets/366e2fe0-d5cf-4e03-b63c-55dc06be8a8a