fix: capture cmdlet warnings to prevent stdout contamination breaking DSC JSON parsing (#2000)

* fix: capture cmdlet warnings to prevent stdout contamination breaking DSC JSON parsing

* Fix Copilot remark

Author: Gijsreyn

src/dsc/psresourceget.ps1

@@ -586,7 +586,11 @@ function SetPSResourceList {
     if ($resourcesToUninstall.Count -gt 0) {
         Write-Trace -message "Uninstalling resources: $($resourcesToUninstall | ForEach-Object { "$($_.Name) - $($_.Version)" })" -level debug
         $resourcesToUninstall | ForEach-Object {
-            Uninstall-PSResource -Name $_.Name -Scope $scope -ErrorAction Stop
+            $cmdWarnings = $null
+            Uninstall-PSResource -Name $_.Name -Scope $scope -ErrorAction Stop -WarningVariable cmdWarnings
+            foreach ($w in $cmdWarnings) {
+                Write-Trace -message ([string]$w) -level warn
+            }
         }
         $resourcesChanged = $true
     }
@@ -614,9 +618,12 @@ function SetPSResourceList {
             $version = $_.Version
 
             try {
-                Install-PSResource -Name $_.Name -Version $_.Version -Scope $scope -Repository $repositoryName -ErrorAction Stop -TrustRepository:$inputObj.trustedRepository -Prerelease:$usePrerelease -Reinstall
-            }
-            catch {
+                $cmdWarnings = $null
+                Install-PSResource -Name $_.Name -Version $_.Version -Scope $scope -Repository $repositoryName -ErrorAction Stop -TrustRepository:$inputObj.trustedRepository -Prerelease:$usePrerelease -Reinstall -WarningVariable cmdWarnings
+                foreach ($w in $cmdWarnings) {
+                    Write-Trace -message ([string]$w) -level warn
+                }
+            } catch {
                 Write-Trace -level error -message "Failed to install resource '$name' with version '$version'. Error: $($_.Exception.Message)"
                 $installErrors += $_.Exception.Message
             }
@@ -825,6 +832,10 @@ if ($null -eq (Get-Module -Name Microsoft.PowerShell.PSResourceGet)) {
     Import-Module -Name $path -Force -ErrorAction Stop
 }
 
+# Suppress warnings from PSResourceGet cmdlets to prevent them from reaching stdout and
+# breaking DSC's JSON parsing. Warnings should be captured on individual cmdlets
+$WarningPreference = 'SilentlyContinue'
+
 switch ($Operation.ToLower()) {
     'get' { return (GetOperation -ResourceType $ResourceType) }
     'set' { return (SetOperation -ResourceType $ResourceType) }

test/DscResource/PSResourceGetDSCResource.Tests.ps1

@@ -243,6 +243,37 @@ Describe "PSResourceList Resource Tests" -Tags 'CI' {
         $setResult.afterState.resources[1].version | Should -Be '5.0.0'
     }
 
+    It 'Set operation stdout contains only valid JSON and is not contaminated by warning messages' {
+        # Simple regression test as it is hard to predict a warning message but we want to ensure they do not break DSC's JSON parsing. This test does not verify that warnings are emitted when expected, 
+        # only that if they are emitted they do not reach stdout.
+        Uninstall-PSResource -Name $script:testModuleName -ErrorAction SilentlyContinue
+
+        $psResourceListParams = @{
+            repositoryName    = $script:localRepo
+            trustedRepository = $true
+            resources         = @(
+                @{
+                    name    = $script:testModuleName
+                    version = '1.0.0'
+                }
+            )
+        }
+
+        $resourceInput = $psResourceListParams | ConvertTo-Json -Depth 5
+
+        # Capture only stdout; stderr carries DSC trace messages and is intentionally discarded
+        $stdoutLines = & $script:dscExe resource set --resource Microsoft.PowerShell.PSResourceGet/PSResourceList --input $resourceInput -o json 2>$null
+
+        # No stdout line should contain warning text or ANSI escape sequences
+        $stdoutLines | Where-Object { $_ } | ForEach-Object {
+            $_ | Should -Not -Match 'WARNING:'
+            $_ | Should -Not -Match '\x1b\['
+        }
+
+        # stdout must be parseable as JSON without error
+        { $stdoutLines | ConvertFrom-Json -ErrorAction Stop } | Should -Not -Throw
+    }
+
     It 'Can test a PSResourceList resource instance with resources' {
         $psResourceListParams = @{
             repositoryName = $script:localRepo