Shark: Crash when reading HeapInstance's fields from multiple threads

[DSteve595]

Description

Given a HeapInstance, calling readFields().any() in two threads simultaneous results in this error:

Exception in thread "Thread-0" java.util.NoSuchElementException: Key 0 is missing in the map.
        at kotlin.collections.MapsKt__MapWithDefaultKt.getOrImplicitDefaultNullable(MapWithDefault.kt:24)
        at kotlin.collections.MapsKt__MapsKt.getValue(Maps.kt:344)
        at shark.internal.ClassFieldsReader.skipStaticFields(ClassFieldsReader.kt:87)
        at shark.internal.ClassFieldsReader.classDumpFields(ClassFieldsReader.kt:55)
        at shark.HprofHeapGraph.classDumpFields$shark_graph(HprofHeapGraph.kt:207)
        at shark.HeapObject$HeapClass.readRecordFields(HeapObject.kt:259)
        at shark.HeapObject$HeapInstance$readFields$1.invoke(HeapObject.kt:450)
        at shark.HeapObject$HeapInstance$readFields$1.invoke(HeapObject.kt:315)
        at kotlin.sequences.FlatteningSequence$iterator$1.ensureItemIterator(Sequences.kt:315)
        at kotlin.sequences.FlatteningSequence$iterator$1.hasNext(Sequences.kt:303)
        at kotlin.sequences.SequencesKt___SequencesKt.any(_Sequences.kt:1179)

Steps to Reproduce

Sample project.

Run ./gradlew run to repro.

Expected behavior: [What you expect to happen]

Version Information

Shark 2.6.

[pyricau]

Thanks! We definitely should look at making LeakCanary thread safe yet efficient. Here the fix might be to make readFields() leverage a thread local.

[pyricau]

ClassFieldsReader holds the metadata for all class fields as one giant immutable byte array, so that's thread safe. However, it has a var position that is reset every time we read fields from one class and then moves forward as we read. We need to make that thread safe.