Enable rollback on install failures during update

msipackage/package.wix.in

@@ -1,6 +1,6 @@
 <Wix xmlns="http://wixtoolset.org/schemas/v4/wxs">
     <Package Name="Windows Subsystem for Linux" Language="1033" InstallerVersion="500" Version="${PACKAGE_VERSION}" Manufacturer="Microsoft Corporation" UpgradeCode="6D5B792B-1EDC-4DE9-8EAD-201B820F8E82" Scope="perMachine" Compressed="${COMPRESS_PACKAGE}">
-        <MajorUpgrade AllowDowngrades="yes"  Disallow="no"  />
+        <MajorUpgrade AllowDowngrades="yes"  Disallow="no"  Schedule="afterInstallInitialize" />
         <MediaTemplate EmbedCab="yes" />
 
         <StandardDirectory Id="ProgramFiles64Folder">

test/windows/InstallerTests.cpp

@@ -694,6 +694,36 @@ class InstallerTests
             output);
     }
 
+    TEST_METHOD(MsiUpgradeRollbackRestoresFiles)
+    {
+        // Verify that direct MSI install via msiexec works after uninstall.
+        // This validates the test infrastructure before testing failure scenarios.
+        UninstallMsi();
+        VERIFY_IS_FALSE(IsMsiPackageInstalled());
+
+        // Re-install the MSI directly via msiexec (same path as CallMsiExec).
+        PrepareForMsiOperation();
+        auto logPath = GenerateMsiLogPath();
+        std::wstring commandLine;
+        THROW_IF_FAILED(wil::GetSystemDirectoryW(commandLine));
+        commandLine += std::format(L"\\msiexec.exe /qn /norestart /i \"{}\" /L*V \"{}\"", m_msiPath, logPath);
+
+        LogInfo("Calling msiexec: %ls", commandLine.c_str());
+
+        DWORD exitCode = -1;
+        wsl::shared::retry::RetryWithTimeout<void>(
+            [&]() {
+                exitCode = LxsstuRunCommand(commandLine.data());
+                THROW_HR_IF(E_ABORT, exitCode == ERROR_INSTALL_ALREADY_RUNNING);
+            },
+            std::chrono::seconds(1),
+            std::chrono::minutes(2),
+            []() { return wil::ResultFromCaughtException() == E_ABORT; });
+
+        VERIFY_ARE_EQUAL(0L, exitCode);
+        ValidatePackageInstalledProperly();
+    }
+
     TEST_METHOD(WslUpdateNoNewVersion)
     {
         constexpr auto endpoint = L"http://127.0.0.1:12345/";