Stage locked Windows binary updates

install.ps1

@@ -315,15 +315,43 @@ function Install-Binary($Source, $Destination) {
     $dir = Split-Path -Parent $Destination
     $name = Split-Path -Leaf $Destination
     $tmp = Join-Path $dir ".$name.tmp.$PID"
-    Copy-Item $Source $tmp -Force
+    Copy-Item -LiteralPath $Source -Destination $tmp -Force
     try {
-        Move-Item $tmp $Destination -Force
+        Move-Item -LiteralPath $tmp -Destination $Destination -Force
     }
     catch {
-        Remove-Item $tmp -Force -ErrorAction SilentlyContinue
+        $pending = "$Destination.pending.$PID"
+        try {
+            Move-Item -LiteralPath $tmp -Destination $pending -Force
+            Start-DeferredBinaryReplacement $pending $Destination
+            Write-Warning "binary is in use; staged replacement for when Dosh exits: $Destination"
+        }
+        catch {
+            Remove-Item -LiteralPath $tmp -Force -ErrorAction SilentlyContinue
+            Remove-Item -LiteralPath $pending -Force -ErrorAction SilentlyContinue
             throw
         }
     }
+}
+
+function Start-DeferredBinaryReplacement($Pending, $Destination) {
+    $payload = @{ Pending = $Pending; Destination = $Destination } | ConvertTo-Json -Compress
+    $payload64 = [Convert]::ToBase64String([System.Text.Encoding]::UTF8.GetBytes($payload))
+    $script = @"
+`$ErrorActionPreference = 'SilentlyContinue'
+`$payload = [System.Text.Encoding]::UTF8.GetString([System.Convert]::FromBase64String('$payload64')) | ConvertFrom-Json
+for (`$i = 0; `$i -lt 3600; `$i++) {
+    if (-not (Test-Path -LiteralPath `$payload.Pending)) { exit 0 }
+    Move-Item -LiteralPath `$payload.Pending -Destination `$payload.Destination -Force
+    if (`$?) { exit 0 }
+    Start-Sleep -Seconds 1
+}
+exit 1
+"@
+    $encoded = [Convert]::ToBase64String([System.Text.Encoding]::Unicode.GetBytes($script))
+    $ps = [System.Diagnostics.Process]::GetCurrentProcess().MainModule.FileName
+    Start-Process -FilePath $ps -ArgumentList @("-NoProfile", "-EncodedCommand", $encoded) -WindowStyle Hidden | Out-Null
+}
 
 function Normalize-PathForCompare($Path) {
     [System.IO.Path]::GetFullPath($Path).TrimEnd(
@@ -440,7 +468,7 @@ function Install-Prebuilt {
         if ($bench) {
             Install-Binary $bench.FullName (Join-Path $bindir "dosh-bench.exe")
         }
-        Install-Binary (Join-Path $bindir "dosh-client.exe") (Join-Path $bindir "dosh.exe")
+        Install-Binary $client.FullName (Join-Path $bindir "dosh.exe")
         return $true
     }
     catch {

tests/release_scripts.rs

@@ -298,12 +298,30 @@ fn windows_prebuilt_install_requires_client_but_not_bench() {
     assert!(ps1.contains("throw \"prebuilt archive missing dosh-client.exe\""));
     assert!(ps1.contains("-Filter \"dosh-bench.exe\""));
     assert!(ps1.contains("if ($bench)"));
+    assert!(ps1.contains("Install-Binary $client.FullName (Join-Path $bindir \"dosh.exe\")"));
+    assert!(!ps1.contains("Install-Binary (Join-Path $bindir \"dosh-client.exe\")"));
     assert!(
         !ps1.contains("throw \"prebuilt archive missing dosh-bench.exe\""),
         "Windows prebuilt install should not fall back to source just because dosh-bench is absent"
     );
 }
 
+#[test]
+fn windows_installer_stages_locked_binary_replacements() {
+    let ps1 = include_str!("../install.ps1");
+    assert!(ps1.contains("function Start-DeferredBinaryReplacement"));
+    assert!(ps1.contains("$pending = \"$Destination.pending.$PID\""));
+    assert!(ps1.contains("Move-Item -LiteralPath $tmp -Destination $pending -Force"));
+    assert!(ps1.contains("Start-DeferredBinaryReplacement $pending $Destination"));
+    assert!(ps1.contains("binary is in use; staged replacement"));
+    assert!(ps1.contains("for (`$i = 0; `$i -lt 3600; `$i++)"));
+    assert!(ps1.contains(
+        "Move-Item -LiteralPath `$payload.Pending -Destination `$payload.Destination -Force"
+    ));
+    assert!(ps1.contains("[System.Text.Encoding]::Unicode.GetBytes($script)"));
+    assert!(ps1.contains("Start-Process -FilePath $ps"));
+}
+
 #[test]
 fn windows_installer_adds_user_path_idempotently() {
     let ps1 = include_str!("../install.ps1");