Support migrating multi-install Chrome to single-install.

chrome/installer/setup/setup_main.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/installer/setup/setup_main.h"
 
 #include <windows.h>
 #include <msi.h>
 #include <shellapi.h>
 #include <shlobj.h>
@@ skipping 431 matching lines @@
       // A product other than Chrome is being installed in multi-install mode,
       // and Chrome is already present. Add Chrome to the set of products
       // (making it multi-install in the process) so that it is updated, too.
       scoped_ptr<Product> multi_chrome(new Product(
           BrowserDistribution::GetSpecificDistribution(
               BrowserDistribution::CHROME_BROWSER)));
       multi_chrome->SetOption(installer::kOptionMultiInstall, true);
       chrome = installer_state->AddProduct(&multi_chrome);
       VLOG(1) << "Upgrading existing Chrome browser in multi-install mode.";
     }
-  } else {
-    // This is a non-multi installation.
-
-    // Check for an existing installation of the product.
-    const ProductState* product_state = original_state.GetProductState(
-        system_level, products[0]->distribution()->GetType());
-    if (product_state != NULL) {
-      // Block downgrades from multi-install to single-install.
-      if (product_state->is_multi_install()) {
-        LOG(ERROR) << "Multi-install "
-                   << products[0]->distribution()->GetDisplayName()
-                   << " exists; aborting single install.";
-        *status = installer::MULTI_INSTALLATION_EXISTS;
-        installer_state->WriteInstallerResult(*status,
-            IDS_INSTALL_MULTI_INSTALLATION_EXISTS_BASE, NULL);
-        return false;
-      }
-    }
-  }
+  }  // else migrate multi-install Chrome to single-install.
 
   return true;
 }
 
 // Checks app host pre-install conditions, specifically that this is a
 // user-level multi-install.  When the pre-install conditions are not
 // satisfied, the result is written to the registry (via WriteInstallerResult),
 // |status| is set appropriately, and false is returned.
 bool CheckAppHostPreconditions(const InstallationState& original_state,
                                InstallerState* installer_state,
@@ skipping 841 matching lines @@
 // multi-install install or update. The operation is performed directly rather
 // than delegated to the existing install since there is no facility in older
 // versions of setup.exe to uninstall GCF without touching the binaries. The
 // binaries will be uninstalled during later processing if they are not in-use
 // (see UninstallBinariesIfUnused). |original_state| and |installer_state| are
 // updated to reflect the state of the world following the operation.
 void UninstallMultiChromeFrameIfPresent(const base::CommandLine& cmd_line,
                                         const MasterPreferences& prefs,
                                         InstallationState* original_state,
                                         InstallerState* installer_state) {
-  // Early exit if not installing or updating multi-install product(s).
-  if (installer_state->operation() != InstallerState::MULTI_INSTALL &&
-      installer_state->operation() != InstallerState::MULTI_UPDATE) {
+  // Early exit if not installing or updating.
+  if (installer_state->operation() == InstallerState::UNINSTALL)
     return;
-  }
 
   // Early exit if Chrome Frame is not present as multi-install.
   const ProductState* chrome_frame_state =
       original_state->GetProductState(installer_state->system_install(),
                                       BrowserDistribution::CHROME_FRAME);
   if (!chrome_frame_state || !chrome_frame_state->is_multi_install())
     return;
 
   LOG(INFO) << "Uninstalling multi-install Chrome Frame.";
   installer_state->UpdateStage(installer::UNINSTALLING_CHROME_FRAME);
@@ skipping 472 matching lines @@
     // Note that we allow the status installer::UNINSTALL_REQUIRES_REBOOT
     // to pass through, since this is only returned on uninstall which is
     // never invoked directly by Google Update.
     return_code = InstallUtil::GetInstallReturnCode(install_status);
   }
 
   VLOG(1) << "Installation complete, returning: " << return_code;
 
   return return_code;
 }