perf(download): use zero-copy NTFS hardlinks for cached payload extraction

lib/download.ps1

@@ -49,6 +49,32 @@ function Invoke-ScoopDownload ($app, $version, $manifest, $bucket, $architecture
     return $urls.ForEach({ url_filename $_ })
 }
 
+function Link-OrCopyFile($Source, $Destination) {
+    if (Test-Path $Destination) {
+        Remove-Item $Destination -Force
+    }
+
+    # Only hardlink archives that will be extracted and removed.
+    # Standalone executables, scripts, or payloads that remain in the app directory
+    # are copied so in-place mutations cannot corrupt the shared cache.
+    $isArchive = $Source -match '\.(zip|7z|tar|gz|tgz|bz2|tbz2|xz|txz|zst|lzma|cab|iso|msi)$'
+
+    if ($isArchive) {
+        $sourceDrive = [System.IO.Path]::GetPathRoot([System.IO.Path]::GetFullPath($Source))
+        $destDrive = [System.IO.Path]::GetPathRoot([System.IO.Path]::GetFullPath($Destination))
+
+        if ($sourceDrive -and ($sourceDrive -eq $destDrive)) {
+            try {
+                $null = New-Item -ItemType HardLink -Path $Destination -Target $Source -Force -ErrorAction Stop
+                return
+            } catch {
+                # Fallback to copy if hardlink is unsupported on volume
+            }
+        }
+    }
+    Copy-Item $Source $Destination -Force
+}
+
 ## [System.Net] downloader
 
 function Invoke-CachedDownload ($app, $version, $url, $to, $cookies = $null, $use_cache = $true) {
@@ -62,7 +88,7 @@ function Invoke-CachedDownload ($app, $version, $url, $to, $cookies = $null, $us
 
     if (!($null -eq $to)) {
         if ($use_cache) {
-            Copy-Item $cached $to
+            Link-OrCopyFile $cached $to
         } else {
             Move-Item $cached $to -Force
         }

test/Scoop-Download.Tests.ps1

@@ -47,3 +47,41 @@ Describe 'url_remote_filename' -Tag 'Scoop' {
         url_remote_filename 'http://example.org/foo-v2.zip#/foo.zip' | Should -Be 'foo-v2.zip'
     }
 }
+
+Describe 'Link-OrCopyFile' -Tag 'Scoop' {
+    BeforeAll {
+        $testDir = Join-Path $env:TEMP ("ScoopLinkTest_" + [guid]::NewGuid().ToString('N'))
+        New-Item -ItemType Directory -Path $testDir -Force | Out-Null
+        $cacheArchive = Join-Path $testDir 'app.zip'
+        Set-Content -Path $cacheArchive -Value 'test archive payload'
+        $cacheScript = Join-Path $testDir 'app.ps1'
+        Set-Content -Path $cacheScript -Value 'test script payload'
+    }
+    AfterAll {
+        Remove-Item -Recurse -Force $testDir -ErrorAction SilentlyContinue
+    }
+
+    It 'hardlinks archive payloads on the same volume' {
+        $stagedZip = Join-Path $testDir 'staged.zip'
+        Link-OrCopyFile $cacheArchive $stagedZip
+        Test-Path $stagedZip | Should -BeTrue
+        (Get-Item $stagedZip).LinkType | Should -Be 'HardLink'
+    }
+
+    It 'preserves cache file when staged archive link is deleted' {
+        $stagedZip = Join-Path $testDir 'staged_delete.zip'
+        Link-OrCopyFile $cacheArchive $stagedZip
+        Remove-Item $stagedZip -Force
+        Test-Path $cacheArchive | Should -BeTrue
+        (Get-Content $cacheArchive) | Should -Be 'test archive payload'
+    }
+
+    It 'copies non-archive payloads to prevent in-place cache mutation' {
+        $stagedScript = Join-Path $testDir 'staged.ps1'
+        Link-OrCopyFile $cacheScript $stagedScript
+        Test-Path $stagedScript | Should -BeTrue
+        Set-Content -Path $stagedScript -Value 'modified script'
+        (Get-Content $cacheArchive) | Should -Be 'test archive payload'
+        (Get-Content $cacheScript) | Should -Be 'test script payload'
+    }
+}