Issue 5790 - Get ready for integration into Chromium

libadblockplus-android-settings/src/org/adblockplus/libadblockplus/android/settings/AdblockHelper.java

Index: libadblockplus-android-settings/src/org/adblockplus/libadblockplus/android/settings/AdblockHelper.java
diff --git a/libadblockplus-android-settings/src/org/adblockplus/libadblockplus/android/settings/AdblockHelper.java b/libadblockplus-android-settings/src/org/adblockplus/libadblockplus/android/settings/AdblockHelper.java
index 41592ace84661ace8e97c007e5a9caec66dc8e37..4f4929e64574a5ab784cddf685a6db4d9d451dea 100644
--- a/libadblockplus-android-settings/src/org/adblockplus/libadblockplus/android/settings/AdblockHelper.java
+++ b/libadblockplus-android-settings/src/org/adblockplus/libadblockplus/android/settings/AdblockHelper.java
@@ -239,11 +239,16 @@ public class AdblockHelper
    * @param asynchronous If `true` engines will be created in background thread without locking of
    *                     current thread. Use waitForReady() before getEngine() later.
    *                     If `false` locks current thread.
+   * @return if a new instance is allocated
    */
-  public synchronized void retain(boolean asynchronous)
+  public synchronized boolean retain(boolean asynchronous)

[sergei, 2017/09/28 08:29:03]

Could you please point to where and how it's actually used. I would like to
check that the approach is not racy.

[anton, 2017/09/28 09:08:39]

Java's `synchronized` token excludes mutiple thread to access the block
concurrently. The second thread will be just blocked.
Also `referenceCounter` is `Atomic` and it excludes race here too.

https://bitbucket.org/4ntoine/chrome4sdp/commits/3bcbeadc50affdf2592d35fdb93c...

```
        // synchronously (blocks the UI (providing bad UX) but allows to get
pointer)
        // improvement: do it async and wait for engine to be created and passed
when URL is about to load
        if (AdblockHelper.get().retain(false)) {
            // pass FilterEngine instance pointer to C++ side
            long ptr =
AdblockHelper.get().getEngine().getFilterEngine().getFilterEnginePtr(); // not
just getPointer(); !
            Log.w(TAG, "Adblock: Notify C++ FilterEngine is created (passing
pointer) " + ptr + " in thread " + Thread.currentThread());
            sendFilterEnginePointer(ptr);
        }
```

[sergei, 2017/09/28 09:33:05]

BTW, I plan to add an asynchronous version of creation of FilterEngine in Java
because it's what should actually be used and it's already available in C++,
will it be more convenient here?
I cannot evaluate it now because it's not clear how the initAdblock and
deinitAdblock are called. JIC, please take into account that if they are called
from different threads then it's possible that between "Log.w(TAG, "Adblock:
Notify C++ FilterEngine is created (passing pointer) " + ptr + " in thread " +
Thread.currentThread());" and "sendFilterEnginePointer(ptr);" we execute
`deinitAdblock` and afterwards continue with "sendFilterEnginePointer(ptr);". I
just hope that initAdblock and deinitAdblock are called only from the same
thread and, although it's not important maybe merely once at the start of the
app and at the finishing of the app.

I would like to come back to the topic later.

   {
+    boolean firstInstance = false;
+
     if (referenceCounter.getAndIncrement() == 0)
     {
+      firstInstance = true;
+
       if (!asynchronous)
       {
         createAdblock();
@@ -266,15 +271,21 @@ public class AdblockHelper
         }).start();
       }
     }
+    return firstInstance;
   }
 
   /**
    * Unregister AdblockHelper engine client
+   * @return `true` if the last instance is destroyed
    */
-  public synchronized void release()
+  public synchronized boolean release()
   {
+    boolean lastInstance = false;
+
     if (referenceCounter.decrementAndGet() == 0)
     {
+      lastInstance = true;
+
       if (engineCreated != null)
       {
         // retained asynchronously
@@ -290,5 +301,6 @@ public class AdblockHelper
         disposeAdblock();
       }
     }
+    return lastInstance;
   }
 }