Add option to only allow a single instance of the Electron app

[mcgordonite]

What it does

Added an option to the backend config: singleInstance. If true and in Electron mode, only one instance of the application is allowed to run at a time. If a second is started, it terminates itself and the original instance's window is focused.

See: https://electronjs.org/docs/api/app#apprequestsingleinstancelock.

How to test

Add the following to the "theia" section of the electron example's package.json: https://github.com/eclipse-theia/theia/blob/master/examples/electron/package.json#L6.

    "backend": {
      "config": {
        "singleInstance": true
      }
    }

Then build the example.

With singleInstance set to true, only one instance of the application can be started. A second instance will exit immediately and the first instance will receive focus.

With singleInstance false or not set, any number of instances can be run (as before).

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

Is there a way you can clean up the code?

There are a lot of changes and it is hard to follow if actual functionality was removed as part of the changes. I was able to extract only the part for singleInstance in my local changes and came under 20 lines.

With the changes locally, I can see that if another electron instance is attempted to be started with the singleInstance config set to true, it doesn't open and the previous window is focused so it works correctly.

[mcgordonite]

@vince-fugnitto Good point, the indent change made it very hard to read the diff. I've updated now. Thanks for the review!

[vince-fugnitto]

It looks much better now thank you! 👍
The code works and does what it intends. I tested the following cases:

1. if no backend config with singleInstance is defined, the behavior remains the same as today.
2. if singleInstance is false, it is possible to spawn multiple Electron instances.
3. if singleInstance is true, it is no longer possible to spawn multiple Electron instances.
   (the sole instance is always revealed - ex: if it is hidden behind a window, if it is minimized)

[vince-fugnitto]

I'll give others a chance to review as well :)
If no objections I'll merge for you on Monday.

[mcgordonite]

@vince-fugnitto do you still think this is ready be merged?

[vince-fugnitto]

@vince-fugnitto do you still think this is ready be merged?

Yes, I was just hoping for others to get a chance to review as well but they might be busy.

[thegecko]

LGTM