Untitled files aren't well supported in custom editors

[lramos15]

The custom editor flow is normally as follows:

1. User calls open with / the editor is the default editor
2. The custom editor is passed a URI, normally a file URI but sometimes can be the untitled file
3. The custom editor then utilizes various fs APIs to perform actions on that file URI.

This flow is of course broken when the URI passed in isn't a file URI as the custom editor can no longer utilize the fs APIs. We currently don't have a good story on how to handle untitled files in custom editors. Taking the hex editor for example, if the user calls reopen editor with -> hex editor on an untitled file the custom editor will crash due to the fs APIs failing. The current workaround is:

1. Check the URI passed in to your custom editor to see if it is an untitled file
2. Find the matching URI in vscode.workspace.textDocuments.
3. Call .getText() on the matching text document.
4. Convert that text into a Uint8Array as that is the form of data vscode.workspace.fs.readFile(uri) would normally return.
5. When the user attempts to save, always trigger a saveAs.

[jrieken]

@bpasero what's the deal with untitled files? Those are always text document, right? Who is owning them and disposing them?

[bpasero]

Can we have a meeting rather than Q&A in this issue please with everyone involved, I am not up to speed with the issues around untitled and custom editors and would like to get a intro + discussion in person.

[mjbvz]

Marking as a duplicate of #115631