Fix New-Item -ItemType Hardlink to resolve target to absolute path and not allow link to itself (#18634)

src/System.Management.Automation/namespaces/FileSystemProvider.cs

@@ -2251,6 +2251,13 @@ namespace Microsoft.PowerShell.Commands
                         }
                         else
                         {
+                            // for hardlinks we resolve the target to an absolute path
+                            if (!IsAbsolutePath(strTargetPath))
+                            {
+                                // there is already a check before here so that strTargetPath should only resolve to 1 path
+                                strTargetPath = SessionState.Path.GetResolvedPSPathFromPSPath(strTargetPath).FirstOrDefault()?.Path;
+                            }
+
                             exists = GetFileSystemInfo(strTargetPath, out isDirectory) != null;
                         }
                     }
@@ -2293,6 +2300,13 @@ namespace Microsoft.PowerShell.Commands
 
                     if (Force)
                     {
+                        if (itemType == ItemType.HardLink && string.Equals(path, strTargetPath, StringComparison.OrdinalIgnoreCase))
+                        {
+                            string message = StringUtil.Format(FileSystemProviderStrings.NewItemTargetIsSameAsLink, path);
+                            WriteError(new ErrorRecord(new InvalidOperationException(message), "TargetIsSameAsLink", ErrorCategory.InvalidOperation, path));
+                            return;
+                        }
+
                         try
                         {
                             if (!isSymLinkDirectory && symLinkExists)

src/System.Management.Automation/resources/FileSystemProviderStrings.resx

@@ -328,7 +328,7 @@
     <value>Maximum size for drive has been exceeded: {0}.</value>
   </data>
   <data name="SymlinkItemExists" xml:space="preserve">
-    <value>Cannot create symbolic link because the path {0} already exists.</value>
+    <value>Cannot create link because the path already exists: {0}.</value>
   </data>
   <data name="AlreadyListedDirectory" xml:space="preserve">
     <value>Skip already-visited directory {0}.</value>
@@ -336,4 +336,7 @@
   <data name="TargetCannotBeSubdirectoryOfSource" xml:space="preserve">
     <value>Destination path cannot be a subdirectory of the source: {0}.</value>
   </data>
+  <data name="NewItemTargetIsSameAsLink" xml:space="preserve">
+    <value>The target and path cannot be the same.</value>
+  </data>
 </root>

test/powershell/Modules/Microsoft.PowerShell.Management/FileSystem.Tests.ps1

@@ -600,6 +600,24 @@ Describe "Hard link and symbolic link tests" -Tags "CI", "RequireAdminOnWindows"
             New-Item -ItemType Junction -Path $junctionToDir -Value $realDir > $null
             Test-Path $junctionToDir | Should -BeTrue
         }
+
+        It 'New-Item can create hardlink with relative path' {
+            try {
+                Push-Location $TestDrive
+                1 > 1.txt
+                New-Item -ItemType HardLink -Path 2.txt -Target 1.txt -ErrorAction Stop
+                $hl = Get-Item -Path .\2.txt -ErrorAction Stop
+                $hl.LinkType | Should -BeExactly "HardLink"
+            }
+            finally {
+                Pop-Location
+            }
+        }
+
+        It 'New-Item will fail to forcibly create hardlink to itself' {
+            $i = New-Item -ItemType File -Path "$TestDrive\file.txt" -Force -ErrorAction Ignore
+            { New-Item -ItemType HardLink -Path $i -Target $i -Force -ErrorAction Stop } | Should -Throw -ErrorId "TargetIsSameAsLink,Microsoft.PowerShell.Commands.NewItemCommand"
+        }
     }
 
     Context "Get-ChildItem and symbolic links" {