Direct Clear-Host output to terminal

[iSazonov]

PR Summary

Fix #10181

Clear-Host on Unix calls clear command which issues escapes. Before the change we write them to PowerShell output stream and it could be unwantedly intercepted. After the change we write the escapes directly to console as suggested by @mklement0 .

Added test is formal and we need manual check on Linux and MacOs too.

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[SteveL-MSFT]

Should we use Write-Host here instead? There may be cases where there isn't a console?

[mklement0]

@SteveL-MSFT: In principle, $host.UI.Write() (or Write-Host) is better than [Console]::Write(), but note that we're calling a terminal-specific utility anyway - see also: #10181 (comment)

[SteveL-MSFT]

@iSazonov can you validate if this works remotely?

[mklement0]

@SteveL-MSFT: No, it doesn't (see linked comment), but it also didn't work before. However, if we're willing to assume an xterm terminal (which sounds like a safe bet), we can fix it as follows before calling clear: if (-not $env:TERM) { $env:TERM='xterm' } - probably better to reset the variable afterwards.

[SteveL-MSFT]

Ok, we can defer the remote scenario since it doesn't work today (and I think there's a issue on that already). We can take this with the current change to address the specific problem of the VT codes emitted to the pipeline.

[mklement0]

@SteveL-MSFT: Couldn't find an issue relating to remoting specifically.

As it turns out, my suggested $env:TERM='xterm' workaround for remoting only works with $host.UI.Write() (Write-Host -NoNewline), not with [console]::Write(), because the escape sequences must be sent to the caller's terminal.

In light of that - if you don't want to incorporate the full workaround yet - it's better to switch to $host.UI.Write().

For the record, here's the complete workaround (from my personal notes):

---

To fix the remoting problem, $env:TERM can be set to xterm temporarily, under the assumption that pretty much all terminals are Xterm-compatible these days - and that it is the caller that uses such a terminal, because the escape sequences must be printed to the caller's terminal:

  function Clear-Host {
    if ($noTerm = -not $env:TERM) { $env:TERM = 'xterm' }
    $host.UI.Write((
      & (Get-Command -CommandType Application clear | Select-Object -First 1).Definition
    ))
    if ($noTerm) { $env:TERM = $null }
  }

[SteveL-MSFT]

I like the suggestion to switch to $host.UI.Write(). Not so sure about the assumption that terminals are xterm compatible. The issue I was thinking about is #8606 where remoting was discussed but wasn't the focus.

[iSazonov]

I will look $host.UI.Write() today. I hope we could implement this step-by-step (first for local host. then remote one that would be more tricky)

[mklement0]

Thanks for the link, @SteveL-MSFT, but resolving #8608 will not address the remoting problem; while there is a single comment - #8606 (comment) - that touches on the need to support non-console hosts, a separate issue should really be opened for that.

As for assuming xterm: Anecdotally, I'd say it's a pretty safe bet, but the better solution would be to extend the $PSSenderInfo object to include the caller's $env:TERM value, if any, and have Clear-Host consult that.

[SteveL-MSFT]

Ok with the changes scoped to the specific problem being fixed. We can defer the other issues as they are existing issues.

[ghost]

🎉v7.0.0-preview.5 has been released which incorporates this pull request.:tada:

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