Propagate __exit__ call to underlying filestream

[ddelange]

Title

Propagate __exit__ call to underlying filestream.

No more need for tweak_close function: the double wrapping will now cause three calls to __exit__, which in turn will call close, first on the outermost filestream, working inward.

If a third party compression handler calls close on the inner filestream, the inner __exit__ call will do a second call to close on the inner filestream, which will be ignored.

Motivation

Currently, the compression mechanic has a side-effect that __exit__ is no longer called on the original filestream. Consequently, eg boto3 abort multipart upload is not called, causing lingering blobs that will be billed.

• Supercedes Fix #684 Abort S3 MultipartUpload if exception is raised #752
• Fixes S3 Multipart uploads comitted on exception when using compression via context manager #684

Tests

✅

Work in progress

Checklist

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• Picked a concise, informative and complete title
• Clearly explained the motivation behind the PR
• Linked to any existing issues that your PR will be solving
• Included tests for any new functionality
• Checked that all unit tests pass

Workflow

Please avoid rebasing and force-pushing to the branch of the PR once a review is in progress.
Rebasing can make your commits look a bit cleaner, but it also makes life more difficult from the reviewer, because they are no longer able to distinguish between code that has already been reviewed, and unreviewed code.

[ddelange]

terminate() called before a multipart upload was initiated:

self = <smart_open.tests.test_smart_open.S3OpenTest testMethod=test_write_bad_encoding_strict>

    @moto.mock_s3
    def test_write_bad_encoding_strict(self):
        """Should open the file for writing with the correct encoding."""
        s3 = _resource('s3')
        s3.create_bucket(Bucket='bucket')
        key = "s3://bucket/key.txt"
        text = u'欲しい気持ちが成長しすぎて'
    
        with self.assertRaises(UnicodeEncodeError):
            with smart_open.open(key, 'w', encoding='koi8-r', errors='strict') as fout:
>               fout.write(text)

[mpenkov]

LGTM, thank you