Native crash on disposed FilterEngine access

[asmirnov]

Environment

Access to disposed filter engine native crash.

How to reproduce

1. Create java filter engine instance (and waitForReady if it's created asynchronously)
2. Remember filter engine instance ptr
3. Dispose adblock engine (and filter engine)
4. Call any filter engine object method (eg. getHostURL)

...

Observed behaviour

Native crash

Expected behaviour

Don't access released filter engine, no native crash happens

[asmirnov]

This can be reproduce with the following code:

AdblockHelper.get().waitForReady();
FilterEngine filterEngine = AdblockHelper.get().getEngine().getFilterEngine();
AdblockHelper.get().release();
String host = filterEngine.getHostFromURL("http://somehost.com/somepage.html"); // crash here

[asmirnov]

reproduced crash stacktrace

[asmirnov]

One could check refs count using AdblockHelper.get().getCounter():
​https://github.com/adblockplus/libadblockplus-android/blob/master/libadblockplus-android-settings/src/org/adblockplus/libadblockplus/android/settings/AdblockHelper.java#L244

[asmirnov]

The crash happens if accessing filter engine after it's disposed.

It's up to developer to check that dispose() count is equal to release() jus like in C++ with new and delete. If using AdblockHelper you can use getCounter() to check instances count. Also AdblockHelper.get().getAdblockEngine() returns null after it's disposed.

I don't think we should do anything special to make it more safe (like check if filter engine is disposed every time when accessing).
If this happens in AdblockWebView this should be fixed in separate issue.

[asmirnov]

BTW AdblockWebView doesn't dispose external Adblock Engine instance:
​https://github.com/adblockplus/libadblockplus-android/blob/master/libadblockplus-android-webview/src/org/adblockplus/libadblockplus/android/webview/AdblockWebView.java#L258

So in use case where multiple AdblockWebviews are created in Activity it's up to activity to create AdblockEngine instance in the very beginning, set it to each AdblockWebView and dispose it in the very end of Activity lifecycle (so every AdblockWebView does not release it).