Investigation: do not restart Extension host when first folder changes

[bpasero]

Today we restart the extension host whenever the first folder changes because workspace.rootPath is always set to that first folder and it never used to change after the extension host started once.

If we would not restart the extension host anymore an extension:

• could use workspace.rootPath as before, it would still point to the first folder always (which can be undefined if the workspace contains no folders)
• could subscribe to the onDidChangeWorkspaceFolders event to get notified about updates
• would see the value of workspace.rootPath changing during runtime whenever the first folder changes

//cc @sandy081 @jrieken

[roblourens]

This is affecting Live Share because they want to be able to track workspace changes during a live share session. If the EH reloads, then the host will have to restart their session.

If we wanted to more aggressively deprecate rootPath, we could make it undefined whenever a workspace is open. I wonder how many extensions still rely on it.

FYI @cait1inhennessy

[roblourens]

As a workaround, they can keep a dummy folder in the workspace during a session, but the user needs to know to not touch that folder, so it's not a very good workaround.

[bpasero]

@roblourens

This is affecting Live Share because they want to be able to track workspace changes during a live share session. If the EH reloads, then the host will have to restart their session.

To be clear: the transition from an empty window or single-folder workspace to a multi-root workspace will always require an extension host restart because the workspace ID changes and thus the state location for example. Please check with them if they really start from an empty workspace and then add a folder.

[roblourens]

Yeah, we realized that, and they are able to start from an actually empty workspace. I think that if you cause a window reload, it will at least be obvious why your liveshare session terminated.

[bpasero]

I am all up for making this change, but we have to decide if:

• rootPath is undefined in multi-root
• rootPath changes in multi-root based on what is the first folder

I feel the former is a change with more impact on todays extensions compared to the latter.

From the API

/**
 * ~~The folder that is open in the editor. `undefined` when no folder
 * has been opened.~~
 *
 * @deprecated Use [`workspaceFolders`](#workspace.workspaceFolders) instead.
 */
export const rootPath: string | undefined;

I think we need to bring this up in an API call to decide, but I will not have cycles. This needs a champion to drive.

[roblourens]

I think we need an idea of how many extensions this would affect. @sandy081 I am using your tool but it seems to be showing me old versions of extensions that used this api, how do I limit it to newer versions?

I think the first option would actually be less impactful. It would mean that some extensions can't work in a multiroot workspace, but they should handle the case of no folder already for an empty window. The second option would break an assumption that those extensions may be making, that rootPath will not change, and we don't know what that would do.

I can bring it up in an API call.

@cait1inhennessy we will discuss it some more because I think this is the right change to make, but don't count on us changing it this month, you will probably have to plan to use the workaround with a dummy folder in the short term.