Lazily initialize MessageLoop for faster thread startup

net/android/network_change_notifier_android.cc

Index: net/android/network_change_notifier_android.cc
diff --git a/net/android/network_change_notifier_android.cc b/net/android/network_change_notifier_android.cc
index 36ebda679660abfeb2a4a0f8cf79a0f1a559eab3..889fe050ef0af17aa6a552b56010b3838f8db774 100644
--- a/net/android/network_change_notifier_android.cc
+++ b/net/android/network_change_notifier_android.cc
@@ -59,6 +59,9 @@
 
 #include "net/android/network_change_notifier_android.h"
 
+#include "base/memory/weak_ptr.h"
+#include "base/single_thread_task_runner.h"
+#include "base/thread_task_runner_handle.h"
 #include "base/threading/thread.h"
 #include "net/base/address_tracker_linux.h"
 #include "net/dns/dns_config_service_posix.h"
@@ -73,12 +76,16 @@ class NetworkChangeNotifierAndroid::DnsConfigServiceThread
  public:
   DnsConfigServiceThread(const DnsConfig* dns_config_for_testing)
       : base::Thread("DnsConfigService"),
+        calling_task_runner_(base::ThreadTaskRunnerHandle::Get()),
         dns_config_for_testing_(dns_config_for_testing),
         creation_time_(base::Time::Now()),
         address_tracker_(base::Bind(base::DoNothing),
                          base::Bind(base::DoNothing),
                          // We're only interested in tunnel interface changes.
-                         base::Bind(NotifyNetworkChangeNotifierObservers)) {}
+                         base::Bind(NotifyNetworkChangeNotifierObservers)),
+        weak_ptr_factory_(this) {
+    weak_ptr_ = weak_ptr_factory_.GetWeakPtr();
+  }
 
   ~DnsConfigServiceThread() override {
     NetworkChangeNotifier::RemoveNetworkChangeObserver(this);
@@ -98,6 +105,9 @@ class NetworkChangeNotifierAndroid::DnsConfigServiceThread
     dns_config_service_->WatchConfig(
         base::Bind(&DnsConfigServiceThread::DnsConfigChangeCallback,
                    base::Unretained(this)));
+    calling_task_runner_->PostTask(
+        FROM_HERE,
+        base::Bind(&DnsConfigServiceThread::InitAfterStart, weak_ptr_));

[pauljensen, 2015/05/01 11:54:44]

This is not good.  This will allow changes to escape the DnsConfigService.
The tiny and fast code in InitAfterStart() must block creation of
NetworkChangeNotifierAndroid.
I think we could just get rid of the DCHECK(IsRunning()) in InitAfterStart(),
which is probably causing the crash you mention.  The code in OnNetworkChanged
may need to change however.  Is task_runner() guaranteed to return non-null
after Thread()?

[kinuko, 2015/05/01 13:58:05]

Ok thanks, I suspected it might be the case. I can modify this part block.
No, the reason it was crashing was
NetworkChangeNotifier::AddNetworkChangeObserver(this) in InitAfterStart() gets
called and OnNetworkChanged() is called by the notifier before we initialize
dns_config_service_ in Init(), so the PostTask on line 132 tries to run
DnsConfigServicePosix::OnNetworkChanged on nullptr.
Yes.

   }
 
   void CleanUp() override { dns_config_service_.reset(); }
@@ -125,12 +135,17 @@ class NetworkChangeNotifierAndroid::DnsConfigServiceThread
                    base::Unretained(dns_config_service_.get()), type));
   }
 
+  scoped_refptr<base::SingleThreadTaskRunner> calling_task_runner_;
   const DnsConfig* dns_config_for_testing_;
   const base::Time creation_time_;
   scoped_ptr<internal::DnsConfigServicePosix> dns_config_service_;
   // Used to detect tunnel state changes.
   internal::AddressTrackerLinux address_tracker_;
 
+  // Used only on the calling thread that created this thread.
+  base::WeakPtr<DnsConfigServiceThread> weak_ptr_;
+  base::WeakPtrFactory<DnsConfigServiceThread> weak_ptr_factory_;
+
   DISALLOW_COPY_AND_ASSIGN(DnsConfigServiceThread);
 };
 
@@ -172,7 +187,6 @@ NetworkChangeNotifierAndroid::NetworkChangeNotifierAndroid(
   delegate_->AddObserver(this);
   dns_config_service_thread_->StartWithOptions(
       base::Thread::Options(base::MessageLoop::TYPE_IO, 0));
-  dns_config_service_thread_->InitAfterStart();
 }
 
 // static