Install CRX updates one at a time.

chrome/browser/extensions/crx_installer.cc

Index: chrome/browser/extensions/crx_installer.cc
diff --git a/chrome/browser/extensions/crx_installer.cc b/chrome/browser/extensions/crx_installer.cc
index 19285453ed3cc6c2797febdc69710957ed460acc..cde1514786a9ce57ab05c7fd0998baf4329e194a 100644
--- a/chrome/browser/extensions/crx_installer.cc
+++ b/chrome/browser/extensions/crx_installer.cc
@@ -113,9 +113,8 @@ bool CrxInstaller::ClearWhitelistedInstallId(const std::string& id) {
 
 CrxInstaller::CrxInstaller(base::WeakPtr<ExtensionService> frontend_weak,
                            ExtensionInstallUI* client)
-    : install_directory_(frontend_weak->install_directory()),
-      install_source_(Extension::INTERNAL),
-      extensions_enabled_(frontend_weak->extensions_enabled()),
+    : install_source_(Extension::INTERNAL),
+      extensions_enabled_(false),  // Will be set again in body of this method.
       delete_source_(false),
       is_gallery_install_(false),
       create_app_shortcut_(false),
@@ -124,6 +123,13 @@ CrxInstaller::CrxInstaller(base::WeakPtr<ExtensionService> frontend_weak,
       apps_require_extension_mime_type_(false),
       allow_silent_install_(false),
       install_cause_(extension_misc::INSTALL_CAUSE_UNSET) {
+
+  // |frontend_weak| can be NULL at this point in unit tests.

[Matt Perry, 2011/05/31 21:34:25]

Can we instead pass in a mock ExtensionService in the test? Allowing NULL for
unit tests is problematic. It can become confusing when required interfaces can
be NULL.

[Sam Kerner (Chrome), 2011/05/31 23:16:15]

There is ExtensionServiceInterface, which allows easy mocking.  Sadly,
CrxInstaller uses many methods not in that interface, and adding them all makes
the interface a lot bigger and harder to reason about.  I started down this
road, and gave up.

I could make the mock service used in the test inherit from the real extension
service.  This brings in a bunch of behavior the existing tests don't want.

I created an extension service on the testing profile, and used it to generate
some CrxInstallers.  Any action the CrxInstallers take is on the testing
profile's extensions service, but I don't want them to take an action.

[Matt Perry, 2011/05/31 23:41:38]

But I'm confused.. The rest of the code does not NULL-check frontend_weak. That
seems to indicate it's not actually getting used.

Also, we don't set install_directory_ or extensions_enabled_. What is the
behavior of CrxInstaller if those are unset?

[Sam Kerner (Chrome), 2011/06/01 00:15:11]

Not sure what you mean:  There are checks for frontend_weak before every user
other than the constructor.  They are in the start of
CrxInstaller::ConfirmInstall() and CrxInstaller::ReportSuccessFromUIThread(). 
No check is done in the constructor because construction is done by
ExtensionService::MakeCrxInstaller(), and if you can get there the extension
service is not null.
I reverted all changes to this file after the first round of reviews, so there
is no case where these two variables will be unset.  We will dereference a null
pointer while setting them if frontend_weak is null, but that can't happen in
the constructor.

[Matt Perry, 2011/06/01 00:20:20]

Ah, of course, sorry. That makes sense.
I didn't realize you reverted the changes. Carry on, then!

+  if (!frontend_weak.get())
+    return;
+
+  install_directory_ = frontend_weak->install_directory();
+  extensions_enabled_ = frontend_weak->extensions_enabled();
 }
 
 CrxInstaller::~CrxInstaller() {