API: Support protocol handlers

[joaomoreno]

From #45685

Problem

There is no current way for extension to react on URLs being open with the vscode: scheme. Allowing extensions to do so can enable many different scenarios. A specific one is to provide Clone URLs in Github/TFS which would simply invoke VS Code to clone a URL, using its git extension.

Security Concerns

Allowing extensions to handle a URL doesn't seem to open any additional security concerns, since it's already possible to run arbitrary code by having a user click a URL while taking advantage of the * activation event.

There is no added API for this to happen, since we can leverage commands to handle URLs and the contributes manifest field to expose a URL handler.

An example URL format would be:

vscode://pub.name/...

Proposal

I suggest to keep it simple and allow a single URL handler per extension:

  "contributes": {
    "urlHandler": "handleURLCommand"
  }
}

The handleURLCommand would be invoked with a single instance of vscode.Uri as a single argument:

commands.registerCommand('handleURLCommand', (uri: vscode.Uri) => { /* ... */ });

[jrieken]

There is no added API for this to happen, since we can leverage commands to handle URLs

Yeah, technically that allows to implement many things but also a little sneaky Why not explicit API like an activation event and some registerUriHandler-function?

[mjbvz]

For security, the main risk is not that the extension itself would be malicious but that it may unknowingly expose commands that could be used in a dangerous way. The classic example would be a friends list extension that wants to let people easily friend each other by clicking on a link. I then come along and craft a url such as: vscode:extensions/super.friendo?addFriendo=TotallyNotEvilAnalytica and trick you into clicking on it.

Just something to be aware of. At the very least, we should document that extensions must prompt before performing potentially dangerous operations. We could also prompt ourselves, something like: "Are you sure you want to activate extension X?"

[joaomoreno]

Feedback from API discussion:

• Use an activation event and registerUrlHandler() API instead
• Always have VS Code prompt whether the user wants to open the URL

[joaomoreno]

Proposed API pushed!

Created #48426 to promote it to public soon.