Fix minio handling of buckets in rare cases

[ponyisi]

Sometimes listing the bucket for an in-progress transformation gives objects that look like directories, not files. These have no file size reported and would raise an exception due to the coercion in the ResultFile constructor. Avoid these problematic objects.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.02%. Comparing base (2dfb069) to head (49cf33d).
Report is 1 commits behind head on 3.0_develop.

Additional details and impacted files

@@             Coverage Diff              @@
##           3.0_develop     #458   +/-   ##
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  Coverage        82.02%   82.02%           
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  Files               26       26           
  Lines             1363     1363           
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  Hits              1118     1118           
  Misses             245      245           

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[gordonwatts]

This looks fine with me. The only thing that comes to mind is if this gets combined with a "transform done" - so a file gets missed from the list of files reported to the client.

However, I know that is a more general problem and this fixes something specific. So lets approve this, and get it in, and then we'll address that bug when we actually see it.

[ponyisi]

Yeah - in any case this is strictly better behavior than we had before, because we would have just crashed and lost everything. I'm not 100% what circumstances lead to this but we should in any case validate the full number of expected files in the bucket at the end of transforms.