Whatsapp ANR的一个分析，MediaProvider的 synchronized 和 beginTransaction 死锁导致的奇葩问题

最近收到一个whatsapp ANR的问题，其trace大是这样的：

"main" prio=5 tid=1 Blocked
| group="main" sCount=1 dsCount=0 obj=0x75543b80 self=0xa5a05400
| sysTid=24292 nice=-10 cgrp=default sched=0/0 handle=0xa8ed0534
| state=S schedstat=( 1047561789 813894852 1943 ) utm=60 stm=44 core=0 HZ=100
| stack=0xbe351000-0xbe353000 stackSize=8MB
| held mutexes=
at com.whatsapp.data.h.Q(MessageStore.java:6069)
- waiting to lock <0x06cb743e> (a com.whatsapp.data.d) held by thread 41
at com.whatsapp.data.h.s(MessageStore.java:6060)
at com.whatsapp.kj$e.a(ConversationsFragment.java:1305)
at com.whatsapp.kj$e.getView(ConversationsFragment.java:1120)
at android.widget.HeaderViewListAdapter.getView(HeaderViewListAdapter.java:220)
at android.widget.AbsListView.obtainView(AbsListView.java:2497)
at android.widget.ListView.makeAndAddView(ListView.java:2012)
at android.widget.ListView.fillDown(ListView.java:721)
at android.widget.ListView.fillFromTop(ListView.java:782)
at android.widget.ListView.layoutChildren(ListView.java:1772)
at android.widget.AbsListView.onLayout(AbsListView.java:2255)
at com.whatsapp.observablelistview.ObservableListView.onLayout(ObservableListView.java:242)
at android.view.View.layout(View.java:17969)
.
.
.

"WhatsApp Worker #2" prio=5 tid=41 Native
| group="main" sCount=1 dsCount=0 obj=0x2ac0a700 self=0x8a7c4000
| sysTid=24337 nice=10 cgrp=default sched=0/0 handle=0x89325920
| state=S schedstat=( 488869957 208541146 1891 ) utm=27 stm=21 core=3 HZ=100
| stack=0x89223000-0x89225000 stackSize=1038KB
| held mutexes=
at android.os.BinderProxy.transactNative(Native method)
at android.os.BinderProxy.transact(Binder.java:622)
at android.content.ContentProviderProxy.delete(ContentProviderNative.java:542)
at android.content.ContentResolver.delete(ContentResolver.java:1424)
at com.whatsapp.data.h.b(MessageStore.java:4770)
at com.whatsapp.data.h.a(MessageStore.java:3092)
- locked <0x06cb743e> (a com.whatsapp.data.d)
at com.whatsapp.data.h.v(MessageStore.java:9276)
- locked <0x06cb743e> (a com.whatsapp.data.d)
at com.whatsapp.aio.run(unavailable:-1)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1133)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:607)
at com.whatsapp.util.br$2.a(WhatsAppWorkers.java:53)
at com.whatsapp.util.bt.run(unavailable:-1)
at java.lang.Thread.run(Thread.java:761)

04-07 10:40:04.431
7022 12063 W SQLiteConnectionPool: The connection pool for database '/data/user/0/com.android.providers.media/databases/external.db' has been unable to grant a connection to thread 494 (MediaScannerService) with flags 0x2 for 6510.3594 seconds.
04-07 10:40:04.431
7022 12063 W SQLiteConnectionPool: Connections: 0 active, 1 idle, 3 available.

04-07 13:21:36.643
7022
7022 W SQLiteConnectionPool: The connection pool for database '/data/user/0/com.android.providers.media/databases/internal.db' has been unable to grant a connection to thread 1 (main) with flags 0x6 for 750.02905 seconds.
04-07 13:21:36.643
7022
7022 W SQLiteConnectionPool: Connections: 0 active, 1 idle, 1 available.

可是，为会么Mediaprovider无法从池中取得数据库连接呢？  很不好的是，我看到的trace中没有提供MediaProvider的stack。

由于此问题重现路径未知，设想了各种可能性后，最大的能是死锁。  在比较了当前代码和原生Android代码后，发现MTK在MeidaProvider.delete的synchronized (sGetTableAndWhereParam) 中多加了个db.beginTransaction()

public int delete(Uri uri, String userWhere, String[] whereArgs)
{
...
synchronized (sGetTableAndWhereParam){
db.beginTransaction();
}
....
}

public ContentProviderResult[] applyBatch(ArrayList<ContentProviderOperation> operations){}
idb.beginTransaction();
edb.beginTransaction();
//然后这里可能间接的调用 delete() . . . }

至此，死锁的条件形成：

资源1： db.beginTransaction();

资源2：synchronized (sGetTableAndWhereParam)

线程1： MediaProvider.delete(),   占有：sGetTableAndWhereParam，申请beginTransaction

线程2：MediaProvider.applyBatch()-->delete()，占有beginTransaction，申请sGetTableAndWhereParam

一个模拟此死锁的代码：  synchronized 和 beginTransaction 死锁的一个例子

最后

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