Fix for SSH remoting when banner is enabled on SSHD endpoint (#16205)

src/System.Management.Automation/engine/ExperimentalFeature/ExperimentalFeature.cs

@@ -23,6 +23,7 @@ namespace System.Management.Automation
         internal const string EngineSource = "PSEngine";
         internal const string PSNativeCommandArgumentPassingFeatureName = "PSNativeCommandArgumentPassing";
         internal const string PSNativeCommandErrorActionPreferenceFeatureName = "PSNativeCommandErrorActionPreference";
+        internal const string PSRemotingSSHTransportErrorHandling = "PSRemotingSSHTransportErrorHandling";
         internal const string PSCleanBlockFeatureName = "PSCleanBlock";
 
         #endregion
@@ -127,6 +128,9 @@ namespace System.Management.Automation
                 new ExperimentalFeature(
                     name: PSNativeCommandErrorActionPreferenceFeatureName,
                     description: "Native commands with non-zero exit codes issue errors according to $ErrorActionPreference when $PSNativeCommandUseErrorActionPreference is $true"),
+                new ExperimentalFeature(
+                    name: PSRemotingSSHTransportErrorHandling,
+                    description: "Removes the SSH remoting transport stdErr stream message handling as terminating errors, and instead just writes error messages to console."),
                 new ExperimentalFeature(
                     name: PSCleanBlockFeatureName,
                     description: "Add support of a 'Clean' block to functions and script cmdlets for easy resource cleanup"),

src/System.Management.Automation/engine/remoting/fanin/OutOfProcTransportManager.cs

@@ -1762,7 +1762,39 @@ namespace System.Management.Automation.Remoting.Client
 
                 while (true)
                 {
-                    string error = ReadError(reader);
+                    string error;
+
+                    if (ExperimentalFeature.IsEnabled(ExperimentalFeature.PSRemotingSSHTransportErrorHandling))
+                    {
+                        // Blocking read from StdError stream
+                        error = reader.ReadLine();
+
+                        if (error == null)
+                        {
+                            // Stream is closed unexpectedly.
+                            throw new PSInvalidOperationException(RemotingErrorIdStrings.SSHAbruptlyTerminated);
+                        }
+
+                        if (error.Length == 0)
+                        {
+                            // Ignore
+                            continue;
+                        }
+
+                        try
+                        {
+                            // Messages in error stream from ssh are unreliable, and may just be warnings or
+                            // banner text.
+                            // So just report the messages but don't act on them.
+                            System.Console.WriteLine(error);
+                        }
+                        catch (IOException)
+                        { }
+
+                        continue;
+                    }
+
+                    error = ReadError(reader);
 
                     if (error.Length == 0)
                     {