Two bugs in Python SDK S3 filesystem support

Hi

There seem to be 2 bugs in the S3 filesystem support.

I tried to use S3 storage for a simple wordcount demo with DirectRunner.

The demo script:

def main():
options = PipelineOptions().view_as(StandardOptions)
 options.runner = 'DirectRunner'
pipeline = beam.Pipeline(options = options)
(
 pipeline
 | ReadFromText("s3://mx-machine-learning/panwenhai/beam_test/test_data")
 | "extract_words" >> beam.FlatMap(lambda x: re.findall(r" [A-Za-z\']+", x))
 | beam.combiners.Count.PerElement()
 | beam.MapTuple(lambda word, count: "%s: %s" % (word, count))
 | WriteToText("s3://mx-machine-learning/panwenhai/beam_test/output")
 )
result = pipeline.run()
 result.wait_until_finish()
return

Error message 1:

apache_beam.io.filesystem.BeamIOError: Match operation failed with exceptions {'s3://mx-machine-learning/panwenhai/beam_test/output-*-of-00001': BeamIOError("List operation failed with exceptions {'s3://mx-machine-learning/panwenhai/beam_test/output-': S3ClientError('Tried to list nonexistent S3 path: s3://mx-machine-learning/panwenhai/beam_test/output-', 404)}")} [while running 'WriteToText/Write/WriteImpl/PreFinalize'] with exceptions None

After digging into the code, it seems the Boto3 client's list function will raise an exception when trying to list a nonexistent S3 path (beam/sdks/pythonapache_beam/io/aws/clients/s3/boto3_client.py line 111). And the S3IO class does not handle this exception in list_prefix function (beam/sdks/python/apache_beam/io/aws/s3io.py line 121).

When the runner tries to list and delete the existing output file, if there is no existing output file, it will try to list a nonexistent S3 path and will trigger the exception.

This should not be an issue here. I think we can ignore this exception safely in the S3IO list_prefix function.

Error Message 2:

File "/Users/wenhai.pan/venvs/tfx/lib/python3.7/site-packages/apache_beam-2.19.0.dev0-py3.7.egg/apache_beam/io/aws/s3filesystem.py", line 272, in delete
exceptions = {path: error for (path, error) in results
File "/Users/wenhai.pan/venvs/tfx/lib/python3.7/site-packages/apache_beam-2.19.0.dev0-py3.7.egg/apache_beam/io/aws/s3filesystem.py", line 272, in <dictcomp>
exceptions = {path: error for (path, error) in results
ValueError: too many values to unpack (expected 2) [while running 'WriteToText/Write/WriteImpl/FinalizeWrite']

When the runner tries to delete the temporary output directory, it will trigger this exception. This exception is caused by parsing (path, error) directly from the "results" which is a dict (beam/sdks/python/apache_beam/io/aws/s3filesystem.py line 272). I think we should use results.items() here.

I have submitted a patch for these 2 bugs: https://github.com/apache/beam/pull/10459

Thank you.