Expose terminal process ID through API

[Tyriar]

Part of #11422

[Tyriar]

Extension authors want to get at the process ID of the shell so they can track what they run, at least on a low level. The problem is that due to a bug in electron (electron/electron#38) there is another process in between the renderer thread and the shell which launches the shell, meaning it's an async operation to grab the PID after the terminal is created.

Therefore, in order to maintain backwards compatibility I think this will need to be exposed as a promise:

// Returns the shell PID
getProcessID(): Thenable<number>;

Alternatively we could expose the in-between process synchronously, it would probably mean more work and confusion for extension authors though:

// Returns the PID of the process that launches the shell
getProcessID(): number;

@jrieken does the Thenable version look reasonable given the situation?

[jrieken]

You could just expose the tenable property? Like

interface Terminal {
 // ...
 pid: Thenable<number>
}

[Tyriar]

If that's preferable 👍

[jrieken]

Yeah - I think that more JavaScript'ish

[Tyriar]

FYI @weinand for the debug stuff we discussed #9957

[Tyriar]

I ended up expanding to processId for clarity:

interface Terminal {
  processId: Thenable<number>
}

FYI @daviwil