"No leak found" for an actual leak

[pyricau]

• LeakCanary version: 1.6.2 / master (cefc33d)
• Device: Emulator Nexus 5 API 26

Create the following activity:

public class MainActivity extends Activity {

  static class Lol {
    static Object foo;
  }

  @Override protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    Lol.foo = this;

  }
}

Start the activity, then press back to finish it. The activity is now leaking. LeakCanary triggers as expected, but then concludes that there is no actual leak. That's incorrect.

[pyricau]

Gathering facts:

• On API 26, no leak is detected
• On API 21, the leak is detected
• Running tests parsing the heap dumps on a JVM confirmed that this has to do with the heap dumps themselves.
• YourKit 2018 is perfectly able to find the path from leaked instance to GC roots of the API 26 heap dump
• On an API 26 heap dump, the LeakCanary code (running on a JVM) never reaches the MainActivity.Lol class when we explore nodes starting from GC Roots. That's the case both for the LeakCanary shortest path impl and the default impl in perflib.

[pyricau]

When parsing the heapdumps with HAHA:

• The API 21 heap dump lists the MainActivity and MainActivity.Lol as GC roots (of type "system class")
• The API 26 heap dump does not list the MainActivity and MainActivity.Lol classes as GC Roots
• In all cases those are represented as ClassObj and exist, but in the API 21 case, for each class we find a RootObj where RootObj.getReferredInstance() returns the ClassObj.

[pyricau]

• In the API 26 heap dump, there are 16 RootObject of type system class where RootObj.getReferredInstance() returns null, ie "mHeap.mSnapshot.findClass(this.mId)" returns null. That doesn't happen in the API 21 heap dump.

[pyricau]

Problem and fix found!

Snapshot.getGCRoots() only returns the GC Roots from the first heap. In both API 21 and API 26, we had 4 heaps, however on API 21 only the first heap has GC Roots, while on API 26 the first 3 heaps had GC Roots. The missing gc roots lived in subsequent heaps.