Race condition in ObjectMorphology filters

[makarovNick]

GlobalDefaultNumberOfThreads affects result Applying ErodeObjectMorphologyImageFilter on binary image produces different result with GlobalDefaultNumberOfThreads > 1

To Reproduce Steps to reproduce the behavior:

1. Operating system, version, and architecture

   • OS: [Ubuntu 18.4] Architecture: x86_64 CPU(s): 16

2. Programming language: Python 3.9.14]

3. Version of SimpleITK: tested on 2.1.0, 2.2.0, 2.3.1

4. How was SimpleITK installed?

   • binary distribution [e.g. python -m pip install SimpleITK]

5. A minimal working example which causes the error. data - shape.zip

   
   import SimpleITK as sitk
   img = sitk.ReadImage("shape.nrrd")

sitk.ProcessObject.SetGlobalDefaultNumberOfThreads(1) eroded_img = sitk.ErodeObjectMorphology(img) result_volume = sitk.GetArrayFromImage(eroded_img).sum() print("Expecting volume: ", result_volume)

sitk.ProcessObject.SetGlobalDefaultNumberOfThreads(16) for _ in range(10): eroded_img = sitk.ErodeObjectMorphology(img) volume = sitk.GetArrayFromImage(eroded_img).sum() if result_volume != volume: print("Wrong result! Got", volume)

Got:

Expecting volume: 6039 Wrong result! Got 6213 Wrong result! Got 6109 Wrong result! Got 6046 Wrong result! Got 6119 Wrong result! Got 6212 Wrong result! Got 6042 Wrong result! Got 6183 Wrong result! Got 6212 Wrong result! Got 6044

**Expected behavior**
Expecting eroded image to be the same

Came here from SimpleITK [issue](https://github.com/SimpleITK/SimpleITK/issues/2095) as @blowekamp advised 

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[blowekamp]

edited Not clear to me what the race conditions are causing the varying results after my initial investigation.