Check process tree when evaluating whether the shell supporting clearing

[Tyriar]

Follow up from #48146 (comment)

Running a simple bash script like while sleep 1; do echo 'a'; done and then pressing cmd+k will print ^L to the screen. We should check to see if any child processes are there and if so revert back to the standard clear all except the cursor line.

[cedric05]

any plans? or can i work on this?

[Tyriar]

@cedric05 you can give it a try to see if the performance hit is alright. Here's how I see the implementation working.

Basically what we want to do is that when the process title changes, that is this event for macOS/Linux https://github.com/Microsoft/vscode/blob/0c5da80ab2f8af0e94f29af5ffdbfb7b10bdf653/src/vs/workbench/parts/terminal/electron-browser/terminalInstance.ts#L604, shell out to get the process tree under the shell process (this._processManager.shellProcessId). Note that the listener would need to be permanent, not just based on whether name is set.

For Windows we're already getting the process tree here https://github.com/Microsoft/vscode/blob/0c5da80ab2f8af0e94f29af5ffdbfb7b10bdf653/src/vs/workbench/parts/terminal/node/windowsShellHelper.ts#L119, we just need to inform TerminalInstance of whether the shell has children or not.

Some notes:

• This work will also directly contribute at getting Terminal confirmOnExit should only warn when the terminal shell is running something #23808 fixed.

• We want to check the tree before clear or shutdown happens so that we're not blocked on IO when they happen as we don't have that much time.

• We want to scan the process tree as opposed to checking the process title (which we already have) such that cases like this are handled:

  bash -l
    bash some_script.sh
  

  I think it's a fine compromise to fallback on the old Terminal: Clear implementation if you're running a sub-shell.

[Tyriar]

I'm going to revert this feature, see #49162 (comment)