PR Summary

Fix #25203
Fix #26228

Symptom

Running the following commands on Windows will hang indefinitely today.

• Invoke-Command -HostName "test" -UserName "test" -ScriptBlock { 1 }
• New-PSSession -HostName "test" -UserName "test"

The reported hang happens on Windows only. On Linux and macOS, both commands return to the prompt with an error written out.

Root Cause

What happens on Windows is -- the ssh.exe process already exited, but the stdin writer and stdout/stderr readers were not closed as expected. That led to the hang, as both the output and error reader threads were still blocked on the reader.ReadLine call.

This is because when creating the ssh.exe using the Win32 function CreateProcess, we don't close the client handles of the pipes (stdInPipeClient, stdOutPipeClient, and stdErrPipeClient) after they get inherited by the child process (see code).

When inherited by a child process, the parent side of those handles remain valid and unchanged. Windows duplicates that handle into the child process's handle table.

• The parent keeps its original handle.
• The child gets a new handle value (different numeric value) that refers to the same underlying kernel object.
• Both processes now have separate references to that object, each with its own handle entry.

So, when the child process ssh.exe exits, the pipe client handles in the child process are automatically closed. However, the pipe client handles in the parent process powershell are still open, so the pipes will remain functional and thus the pipe server handles (i.e. the readers used in the stdout/stderr reading threads) are still open. Therefore, the hang happens.

Fix Changes

To fix the issue, we need to close the parent side pipe client handles right after creating the ssh.exe process, so, only the ssh.exe process holds the handles of client side of the pipes. Then later, when ssh.exe exits, the client side of the pipe will be closed, which will, as expected, lead the server side of the pipes to be closed too.

The main changes of the fix are:

• Close the client pipe handles after creating the child process (by adding lpStartupInfo.Dispose() in the finally block).
• A bit cleanup of the redundant namespace-qualified usages of types (e.g., System.Diagnostics.Process, System.IO.Path, etc.)
• Added new tests to verify that PowerShell does not hang when attempting SSH connections to non-existent hosts.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge. If this PR is a work in progress, please open this as a Draft Pull Request and mark it as Ready to Review when it is ready to merge.
• Breaking changes
  • None
  • OR
  • Experimental feature(s) needed
    • Experimental feature name(s):
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed
    • Issue filed:
• Testing - New and feature
  • N/A or can only be tested interactively
  • OR
  • Make sure you've added a new test if existing tests do not effectively test the code changed