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unreachableObjects.md

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Garbage collection errors

NVDA's garbageHandler.py monitors Python's cyclic garbage collector and reports on objects that are unreachable. Cyclic references are typically a symptom of bad design, and can cause major problems for certain objects. For instance, cyclic references involving COM objects may cause a deadlock if the garbage collector happens to break the cycle and release the COM object in the wrong thread.

How to know about a cyclic reference?

The log may contain errors like the following.

WARNING - garbageHandler.notifyObjectDeletion (10:45:23.171) - MainThread (21820):
Garbage collector has found one or more unreachable objects. See further warnings for specific objects.
...
WARNING - garbageHandler.notifyObjectDeletion (10:45:23.171) - MainThread (21820):
Deleting unreachable object <eventHandler._EventExecuter object at 0x1AC15350>
ERROR - garbageHandler._collectionCallback (10:45:23.172) - MainThread (21820):
Found at least 1 unreachable objects in run

How to debug a cyclic reference?

Once you can reliably reproduce the log error, you can tell the garbage collector to save all unreachable objects. After an unreachable object is detected the references to the unreachable object can be inspected via the python console. Inspecting this should give you a fair idea of where the issue is occurring.

  1. Open the NVDA Python console NVDA+control+z

  2. Enable saving all objects:

    import gc
gc.set_debug(gc.DEBUG_SAVEALL)

  3. Reproduce the unreachable object error.

  4. If garbage collection errors have not yet been logged, force a collect by calling:

    gc.collect()

  5. All unreachable objects will now be stored in gc.garbage. It may be a very large list. Some tricks for narrowing this list down:

    • From the log, you can get the memory address (id) of the object. Then use: 
      memoryAddress = 0xabcd123
obj = None
for o in gc.garbage:
	if memoryAddress == id(o)
		obj = o
    • Listing the types collected, look for the type(s) matching the log message: 
      for index, o in enumerate(gc.garbage):
	print(index, type(o))

  6. Once you have a reference (obj) to the unreachable object, see what other objects refer to an object you can call. You can do this by using gc.get_referrers. The python console has a reference to obj, there may be a lot of output. You can reduce this by looking at the types and following the most relevant.

    for index, o in enumerate(gc.get_referrers(obj)):
	print(index, type(o))

  7. Continue following the references to build a picture of the cycle.

Typical problems

Some examples of common issues:

  • Exceptions caught and assigned to a local variable.